var publications = [{"numerical_label":"172","title":"Twisted bilayer graphene as a terahertz plasmonic crystal","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1103\/z7kr-y83w","authors":"Brian S. Vermilyea and Michael M. Fogler","journal":"Phys. Rev. B","volume":"113","issue_page":"205406","year":"2026","pdf":"bib\/Vermilyea2026TBG.pdf","view_link":"","abstract":"We study surface plasmons in minimally twisted gapped bilayer graphene that contains a triangular network of partial dislocations (or AB-BA domain walls) hosting topologically protected one-dimensional electronic states. We show that this system behaves as a plasmonic crystal and we calculate its band structure by solving classical equations of motion for charge dynamics on the network links with impedance boundary conditions at the network nodes. The plasmon dispersion exhibits several notable features such as multiple gapless branches, flat bands, and dissipationless modes at high-symmetry points. We compare our network-based formalism with the conventional random phase approximation and discuss when each approach is valid. Calculations of plasmon waves launched by local scatterers are presented to simulate terahertz nanoimaging experiments.","id":0},{"numerical_label":"171","title":"Diffusive and hydrodynamic magnetotransport around a density perturbation in a two-dimensional electron gas","category":"Graphene and 2D Materials; Miscellaneous","label":"","hyperlink":"https:\/\/doi.org\/10.1103\/5614-qb24","authors":"P. Shubham Parashar and Michael M. Fogler","journal":"Phys. Rev. B","volume":"113","issue_page":"155443","year":"2026","pdf":"bib\/Parashar2026DAH.pdf","view_link":"","abstract":"We study current flow around a density inhomogeneity in a two-dimensional electron gas in the presence of a strong magnetic field. The inhomogeneity is parametrized by a power-law tail with an exponent \ud835\udefd>2. We show that current and electrochemical potential are exponentially suppressed inside a surrounding area much larger than the geometric size of the perturbation. The corresponding \u201cno-go\u201d radius grows as a certain power of the magnetic field. Residual current and potential exhibit spiraling patterns inside the no-go region. Outside of it, they acquire corrections inversely proportional to the distance, which is known as the Landauer resistivity dipole. The Landauer dipole is rotated by the angle \ud835\udf0b(1\u22121\/\ud835\udefd) with respect to the average electric field. The rotation direction depends on whether the local density is raised or lowered. We also consider the effect of electron viscosity and show that the variation of the no-go radius with magnetic field becomes more rapid if viscosity is large enough. The Landauer dipole size is set by the Gurzhi length, which is much larger than the no-go radius, which is in turn much larger than the geometric size of the perturbation. Our results may be useful for interpreting nanoimaging of current distribution in graphene and other two-dimensional systems.","id":1},{"numerical_label":"170","title":"Application of deep neural networks for computing the renormalization group flow of the two-dimensional \u03d54 field theory","category":"Miscellaneous","label":"","hyperlink":"http:\/\/doi.org\/10.1088\/2632-2153\/ae493b","authors":"Y. Zhao, M. M. Fogler, Y.-Z. You","journal":"Mach. Learn.: Sci. Technol.","volume":"7","issue_page":"025064","year":"2026","pdf":"bib\/Zhao2026AOD.pdf","view_link":"","abstract":"We introduce RGFlow, a deep neural network\u2013based real-space renormalization group (RG) framework tailored for continuum scalar field theories. Leveraging generative capabilities of flow-based neural networks, RGFlow autonomously learns real-space RG transformations from data without prior knowledge of the underlying model. In contrast to conventional approaches, RGFlow is bijective (information-preserving) and is optimized based on the principle of minimal mutual information. We demonstrate the method on two examples. The first one is a one-dimensional Gaussian model, where RGFlow is shown to learn the classical decimation rule. The second is the two-dimensional \u03d54 theory, where the network successfully identifies a Wilson\u2013Fisher-like critical point and provides an estimate of the correlation-length critical exponent.","id":2},{"numerical_label":"169","title":"Nanoscale Imaging of Magnetotransport around a Circular p-n Junction in Graphene","category":"Graphene and 2D Materials; Miscellaneous","label":"","hyperlink":"https:\/\/doi.org\/10.1103\/3ml4-gr6h","authors":"Z. J. Krebs, W. A. Behn, K. J. Smith, M. A. Fortman, K. Watanabe, T. Taniguchi, P. S. Parashar, M. M. Fogler, V. W. Brar","journal":"Phys. Rev. Lett.","volume":"136","issue_page":"136301","year":"2026","pdf":"bib\/Krebs2026NIO.pdf","view_link":"","abstract":"Magnetoresistance studies of 2D systems are often shaped by the motion of electrons that occupy spatially confined wave functions, such as topological edge modes and disorder-induced bound states. Directly probing how such states form and behave in situ, under applied currents, provides a clear way of connecting microscopic physics to the macroscopic transport response. In this Letter, scanning tunneling potentiometry is used to probe the local, current-induced electrochemical potential of carriers in graphene near circular p-n junctions in an out-of-plane magnetic field ranging from 0 to 1.4 T. These measurements provide detailed information about the motion of carriers at the nanometer scale, revealing how it evolves with increasing field. The electrochemical potential displays distinct patterns, such as dipoles, spirals, and concentric disks in weak, moderate, and high fields, respectively. The size and orientation of these patterns can be used to understand how local carrier dynamics change in different transport regimes as well as to directly extract physical parameters such as the electron mean free path and cyclotron diameter.","id":3},{"numerical_label":"168","title":"Collective Modes in Multilayer Graphene\/\ud835\udefc\u2212RuCl3 Heterostructures","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1103\/rp3q-svws","authors":"Samuel L. Moore, Miguel S\u00e1nchez S\u00e1nchez, M.\u2009C. Strasbourg, Y. Shao, J. Pack, Y. Wang, D.\u2009J. Rizzo, B.\u2009S. Jessen, Matthew Cothrine, David G. Mandrus, Takashi Taniguchi, Kenji Watanabe, K.\u2009S. Burch, C.\u2009R. Dean, J. Hone, M. Fogler, A.\u2009J. Millis, A. Rubio, P.\u2009J. Schuck, T. Stauber, D.\u2009N. Basov","journal":"Phys. Rev. X","volume":"15","issue_page":"041011","year":"2025","pdf":"bib\/Moore2025CMI.pdf","view_link":"","abstract":"Collective modes in multilayer graphene, such as plasmons and phonons, exhibit sensitivity to displacement fields and interlayer coupling, distinguishing them from their counterparts in single-layer graphene. Here, we engineer collective modes in charge-transfer heterostructures composed of multilayer graphene and \ud835\udefc\u2212RuCl3. In heterostructures with a single \ud835\udefc\u2212RuCl3 interface, the charge transfer generates displacement fields up to 7 V\/nm at the interface between \ud835\udefc\u2212RuCl3 and the adjacent graphene layer\u2014the highest value achieved through charge-transfer methods. As a result of the broken inversion symmetry, we discover enhanced nonlinear optical response and modified phonon selection rules. Conversely, we find that multilayer graphene sandwiched between two \ud835\udefc\u2212RuCl3 flakes causes displacement fields to cancel. There, we achieve carrier densities as high as 8 \u00d71013\u2009\u2009cm\u22122 in multilayer graphene and restore the phonon selection rules to their unperturbed state. Meanwhile, we demonstrate that plasmonic properties derive from the depletion of multiple valence bands. As a result of the quasilinear band dispersion, these \u201cDirac multiband plasmons\u201d are relatively unaffected by displacement fields. On the other hand, the inverted heterostructure sequence\u2014two multilayer graphene sheets encapsulating \ud835\udefc\u2212RuCl3\u2014activates significant alteration of the plasmons via interlayer plasmon-plasmon coupling. Hence, multilayer graphene and \ud835\udefc\u2212RuCl3 heterostructures offer a gate-free platform for engineering collective modes derived from inversion symmetry and interlayer coupling.","id":4},{"numerical_label":"167","title":"Polaritonic quantum matter","category":"Near Field Optics; Miscellaneous","label":"","hyperlink":"https:\/\/doi.org\/10.1515\/nanoph-2025-0001","authors":"D.N. Basov, Ana Asenjo-Garcia, P. James Schuck, Xiaoyang Zhu, Angel Rubio, Andrea Cavalleri, Milan Delor, Michael M. Fogler, Mengkun Liu","journal":"Nanophotonics","volume":"14","issue_page":"3723","year":"2025","pdf":"bib\/Basov2025PQM.pdf","view_link":"","abstract":"Polaritons are quantum mechanical superpositions of photon states with elementary excitations in molecules and solids. The light\u2013matter admixture causes a characteristic frequency-momentum dispersion shared by all polaritons irrespective of the microscopic nature of material excitations that could entail charge, spin, lattice or orbital effects. Polaritons retain the strong nonlinearities of their matter component and simultaneously inherit ray-like propagation of light. Polaritons prompt new properties, enable new opportunities for spectroscopy\/imaging, empower quantum simulations and give rise to new forms of synthetic quantum matter. Here, we review the emergent effects rooted in polaritonic quasiparticles in a wide variety of their physical implementations. We present a broad portfolio of the physical platforms and phenomena of what we term polaritonic quantum matter. We discuss the unifying aspects of polaritons across different platforms and physical implementations and focus on recent developments in: polaritonic imaging, cavity electrodynamics and cavity materials engineering, topology and nonlinearities, as well as quantum polaritonics.","id":5},{"numerical_label":"166","title":"Current-driven nonequilibrium electrodynamics in graphene revealed by nano-infrared imaging","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1038\/s41467-025-58953-6","authors":"Y. Dong, Z. Sun, I. Y. Phinney, D. Sun, T. I. Andersen, L. Xiong, Y. Shao, S. Zhang, Andrey Rikhter, S. Liu, P. Jarillo-Herrero, P. Kim, C. R. Dean, A. J. Millis, M. M. Fogler, D. A. Bandurin, D. N. Basov","journal":"Nat. Commun.","volume":"16","issue_page":"3861","year":"2025","pdf":"bib\/Dong2025CDN.pdf","view_link":"","abstract":"Electrons in low-dimensional materials driven out of equilibrium by a strong electric field exhibit intriguing effects that have direct analogues in high-energy physics. In this work we demonstrate that two of these effects can be observed in graphene, leading to relevant implications for light-matter interactions at the nanoscale. For doped graphene, the Cherenkov emission of phonons caused by the fast flow of out-of-equilibrium electrons was found to induce direction-dependent asymmetric plasmon damping and an unexpected generation of photocurrent. For graphene close to charge neutrality, incident infrared photons were found to disrupt the creation-recombination balance of electron-hole pairs enabled by the condensed matter version of the Schwinger effect, resulting in an excess photocurrent that we term Schwinger photocurrent. Both Schwinger and Cherenkov photocurrents are different from other known light-to-current down conversions scenarios and thus expand the family of photoelectric effects in solid state devices. Through nano-infrared imaging methodology, we provide a more comprehensive view of current-driven nonequilibrium electrodynamics in graphene.","id":6},{"numerical_label":"165","title":"Plasmonic Polarization Sensing of Electrostatic Superlattice Potentials","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1103\/PhysRevX.15.011019","authors":"Shuai Zhang, Jordan Fonseca, Daniel Bennett, Zhiyuan Sun, Junhe Zhang, Ran Jing, Suheng Xu, Leo He, S.\u2009L. Moore, S.\u2009E. Rossi, Dmitry Ovchinnikov, David Cobden, Pablo Jarillo-Herrero, M.\u2009M. Fogler, Philip Kim, Efthimios Kaxiras, Xiaodong Xu, D.\u2009N. Basov","journal":"Phys. Rev. X","volume":"15","issue_page":"011019","year":"2025","pdf":"bib\/Zhang2025PPS.pdf","view_link":"","abstract":"Plasmon polaritons are formed by coupling light with delocalized electrons. The half-light and half-matter nature of plasmon polaritons endows them with unparalleled tunability via a range of parameters, such as dielectric environments and carrier density. Therefore, plasmon polaritons are expected to be tuned when in proximity to polar materials since the carrier density is tuned by an electrostatic potential; conversely, the plasmon polariton response might enable the sensing of polarization. Here, we use infrared nanoimaging and nanophotocurrent measurements to investigate heterostructures composed of graphene and twisted hexagonal boron nitride (t-BN), with alternating polarization in a triangular network of moir\u00e9 stacking domains. We observe that the carrier density and the corresponding plasmonic response of graphene are modulated by polar domains in t-BN. In addition, we demonstrate that the nanometer-wide domain walls of graphene moir\u00e9s superlattices, created by the polar domains of t-BN, provide momenta to assist the plasmonic excitations. Furthermore, our study establishes that the plasmon of graphene could function as a delicate sensor for polarization textures. The evolution of polarization textures in t-BN under uniform electric fields is tomographically examined via plasmonic imaging. Strikingly, no noticeable polarization switching is observed under applied electric fields up to 0.21 V\/nm, at variance with transport reports. Our nanoimages unambiguously reveal that t-BN with triangular domains acts like a ferrielectric rather than a ferroelectric as claimed by many previous studies.","id":7},{"numerical_label":"164","title":"Electronic interactions in Dirac fluids visualized by nano-terahertz spacetime interference of electron-photon quasiparticles","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1126\/sciadv.ado5553","authors":"Suheng Xu, Yutao Li, Rocco A. Vitalone, Ran Jing, Aaron J. Sternbach, Shuai Zhang, Julian Ingham, Milan Delor, James W. McIver, Matthew Yankowitz, Raquel Queiroz, Andrew J. Millis, Michael M. Fogler, Cory R. Dean, Abhay N. Pasupathy, James Hone, Mengkun Liu, D. N. Basov","journal":"Sci. Adv.","volume":"10","issue_page":"eado5553","year":"2024","pdf":"bib\/Xu2024EID.pdf","view_link":"","abstract":"Ultraclean graphene at charge neutrality hosts a quantum critical Dirac fluid of interacting electrons and holes. Interactions profoundly affect the charge dynamics of graphene, which is encoded in the properties of its electron-photon collective modes: surface plasmon polaritons (SPPs). Here, we show that polaritonic interference patterns are particularly well suited to unveil the interactions in Dirac fluids by tracking polaritonic interference in time at temporal scales commensurate with the electronic scattering. Spacetime SPP interference patterns recorded in terahertz (THz) frequency range provided unobstructed readouts of the group velocity and lifetime of polariton that can be directly mapped onto the electronic spectral weight and the relaxation rate. Our data uncovered prominent departures of the electron dynamics from the predictions of the conventional Fermi-liquid theory. The deviations are particularly strong when the densities of electrons and holes are approximately equal. The proposed spacetime imaging methodology can be broadly applied to probe the electrodynamics of quantum materials.","id":8},{"numerical_label":"163","title":"Charge Transfer Plasmonics in Bespoke Graphene\/\u03b1-RuCl3 Cavities","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1021\/acsnano.4c08441","authors":"Rocco A. Vitalone, Bjarke S Jessen, Ran Jing, Daniel J. Rizzo, Suheng Xu, Valerie Hsieh, Matthew Cothrine, David G. Mandrus, Lukas Wehmeier, G. Lawrence Carr, Valentina Bisogni, Cory R. Dean, James C. Hone, Mengkun Liu, Michael I. Weinstein, Michael M. Fogler, D. N. Basov","journal":"ACS Nano","volume":"18","issue_page":"29648-29657","year":"2024","pdf":"bib\/Vitalone2024CTP.pdf","view_link":"","abstract":"Surface plasmon polaritons (SPPs) provide a window into the nano-optical, electrodynamic response of their host material and its dielectric environment. Graphene\/\u03b1-RuCl3 serves as an ideal model system for imaging SPPs since the large work function difference between these two layers facilitates charge transfer that hole dopes graphene with n \u223c 1013 cm\u20132 free carriers. In this work, we study the emergent THz response of graphene\/\u03b1-RuCl3 heterostructures using our home-built cryogenic scanning near-field optical microscope. Using phase-resolved imaging, we clearly observe long wavelength, heavily damped THz SPPs in a series of variable-size graphene cavities. From this, we extract the plasmonic wavelength and scattering rate in the graphene\/\u03b1-RuCl3 heterostructures. We determine that the measured plasmon wavelength and electronic scattering rate match our heterostructures\u2019 theoretically predicted values. Our results demonstrate that shaping graphene into bespoke cavity structures enables observation and quantification of SPPs in heavily doped graphene that are largely not addressable with other experimental techniques. Moreover, the manifest lack of metallicity observed in the adjacent doped \u03b1-RuCl3 layer provides significant constraints on the nature of the interfacial charge transfer in this 2D heterostructure.","id":9},{"numerical_label":"162","title":"Excitonic Bose Polarons in Electron-Hole Bilayers","category":"Excitons","label":"","hyperlink":"https:\/\/doi.org\/10.1021\/acs.nanolett.4c03288","authors":"Erik A. Szwed, Brian Vermilyea, Darius J. Choksy, Zhiwen Zhou, Michael M. Fogler, Leonid V. Butov, Dmitry K. Efimkin, Kirk W. Baldwin, Loren Pfeiffer","journal":"Nano Lett.","volume":"24","issue_page":"13219-13223","year":"2024","pdf":"bib\/Szwed2024EBP.pdf","view_link":"","abstract":"Bose polarons are mobile particles of one kind dressed by excitations of the surrounding degenerate Bose gas of particles of another kind. These many-body objects have been realized in ultracold atomic gases and become a subject of intensive studies. In this work, we show that excitons in electron\u2013hole bilayers offer new opportunities for exploring polarons in strongly interacting, highly tunable bosonic systems. We found that Bose polarons are formed by spatially direct excitons immersed in degenerate Bose gases of spatially indirect excitons (IXs). We detected both attractive and repulsive Bose polarons by measuring photoluminescence excitation spectra. We controlled the density of IX Bose gas by optical excitation and observed an enhancement of the energy splitting between attractive and repulsive Bose polarons with increasing IX density, in agreement with our theoretical calculations.","id":10},{"numerical_label":"161","title":"Landau-phonon polaritons in Dirac heterostructures","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1126\/sciadv.adp3487","authors":"Lukas Wehmeier, Suheng Xu, Rafael A. Mayer, Brian Vermilyea, Makoto Tsuneto, Michael Dapolito, Rui Pu, Zengyi Du, Xinzhong Chen, Wenjun Zheng, Ran Jing, Zijian Zhou, Kenji Watanabe, Takashi Taniguchi, Adrian Gozar, Qiang Li, Alexey B. Kuzmenko, G. Lawrence Carr, Xu Du, Michael M. Fogler, D. N. Basov, Mengkun Liu","journal":"Sci. Adv.","volume":"10","issue_page":"eadp3487","year":"2024","pdf":"bib\/Wehmeier2024LPP.pdf","view_link":"","abstract":"Polaritons are light-matter quasiparticles that govern the optical response of quantum materials at the nanoscale, enabling on-chip communication and local sensing. Here, we report Landau-phonon polaritons (LPPs) in magnetized charge-neutral graphene encapsulated in hexagonal boron nitride (hBN). These quasiparticles emerge from the interaction of Dirac magnetoexciton modes in graphene with the hyperbolic phonon polariton modes in hBN. Using infrared magneto-nanoscopy, we reveal the ability to completely halt the LPP propagation in real space at quantized magnetic fields, defying the conventional optical selection rules. The LPP-based nanoscopy also tells apart two fundamental many-body phenomena: the Fermi velocity renormalization and field-dependent magnetoexciton binding energies. Our results highlight the potential of magnetically tuned Dirac heterostructures for precise nanoscale control and sensing of light-matter interaction.","id":11},{"numerical_label":"160","title":"\"The unreasonable effectiveness of mathematics\" in evading polaritonic losses","category":"Miscellaneous","label":"","hyperlink":"https:\/\/doi.org\/10.1038\/s41563-024-01843-x","authors":"D.N. Basov, M. M. Fogler","journal":"Nature Mat.","volume":"23","issue_page":"445-446","year":"2024","pdf":"bib\/Basov2024UEM.pdf","view_link":"","abstract":"Polaritonic losses, a root impediment to the many bounties of nanophotonics, may be evaded by resorting to the mathematics of synthetic frequencies offering \u2018virtual\u2019 gain.","id":12},{"numerical_label":"159","title":"Modeling of plasmonic and polaritonic effects in photocurrent nanoscopy","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1063\/5.0192814","authors":"A. Rikhter, D. N. Basov, M. M. Fogler","journal":"J. Appl. Phys.","volume":"135","issue_page":"103101 ","year":"2024","pdf":"bib\/Rikhter2024MOP.pdf","view_link":"","abstract":"We present a basic framework for modeling collective mode effects in photocurrent measurements performed on two-dimensional materials using nano-optical scanned probes. We consider photothermal, photovoltaic, and bolometric contributions to the photocurrent. We show that any one of these can dominate depending on frequency, temperature, applied bias, and sample geometry. Our model is able to account for periodic spatial oscillations (fringes) of the photocurrent observed near sample edges or inhomogeneities. For the case of a non-absorbing substrate, we find a direct relation between the spectra measured by the photocurrent nanoscopy and its parental scanning technique near-field optical microscopy.","id":13},{"numerical_label":"158","title":"Feshbach resonance of heavy exciton-polaritons","category":"Excitons","label":"","hyperlink":"https:\/\/doi.org\/10.1103\/PhysRevB.109.115401","authors":"B. Vermilyea and M. M. Fogler","journal":"Phys. Rev. B","volume":"109","issue_page":"115401","year":"2024","pdf":"bib\/Vermilyea2024FRO.pdf","view_link":"","abstract":"We study interactions between polaritons formed by hybridization of excitons in a two-dimensional semiconductor with surface optical phonons or plasmons. These quasiparticles have a high effective mass and can bind into bipolaritons near a Feshbach-like scattering resonance. We analyze the phase diagram of a many-body condensate of heavy polaritons and bipolaritons and calculate their absorption and luminescence spectra, which can be measured experimentally.","id":14},{"numerical_label":"157","title":"Visualizing moir\u00e9 ferroelectricity via plasmons and nano-photocurrent in graphene\/twisted-WSe_2 structures","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1038\/s41467-023-41773-x","authors":"Shuai Zhang, Yang Liu, Zhiyuan Sun, Xinzhong Chen, Baichang Li, S. L. Moore, Song Liu, Zhiying Wang, S. E. Rossi, Ran Jing, Jordan Fonseca, Birui Yang, Yinming Shao, Chun-Ying Huang, Taketo Handa, Lin, Xiong, Matthew Fu, Tsai-Chun Pan, Dorri Halbertal, Xinyi Xu, Wenjun Zheng, P.J. Schuck, A.N. Pasupathy, C.R. Dean, Xiaoyang Zhu, David H. Cobden, Xiaodong Xu, Mengkun Liu, M.M. Fogler, James C. Hone, D.N. Basov","journal":"Nature Comms.","volume":"14","issue_page":"6200","year":"2023","pdf":"bib\/Zhang2023VMF.pdf","view_link":"","abstract":"Ferroelectricity, a spontaneous and reversible electric polarization, is found in certain classes of van der Waals (vdW) material heterostructures. The discovery of ferroelectricity in twisted vdW layers provides new opportunities to engineer spatially dependent electric and optical properties associated with the configuration of moir\u00e9 superlattice domains and the network of domain walls. Here, we employ near-field infrared nano-imaging and nano-photocurrent measurements to study ferroelectricity in minimally twisted WSe2. The ferroelectric domains are visualized through the imaging of the plasmonic response in a graphene monolayer adjacent to the moir\u00e9 WSe2 bilayers. Specifically, we find that the ferroelectric polarization in moir\u00e9 domains is imprinted on the plasmonic response of the graphene. Complementary nano-photocurrent measurements demonstrate that the optoelectronic properties of graphene are also modulated by the proximal ferroelectric domains. Our approach represents an alternative strategy for studying moir\u00e9 ferroelectricity at native length scales and opens promising prospects for (opto)electronic devices.","id":15},{"numerical_label":"156","title":"Machine Learning for Optical Scanning Probe Nanoscopy","category":"Near Field Optics; Miscellaneous","label":"","hyperlink":"https:\/\/doi.org\/10.1002\/adma.202109171","authors":"X.Z. Chen, S. Xu, S. Shabani, Y. Zhao, M. Fu, A. J. Millis, M. M. Fogler, A. N. Pasupathy, M.K. Liu, D. N. Basov","journal":"Adv. Mat.","volume":"35","issue_page":"2109171","year":"2023","pdf":"bib\/Chen2023MLF.pdf","view_link":"","abstract":"The ability to perform nanometer-scale optical imaging and spectroscopy is key to deciphering the low-energy effects in quantum materials, as well as vibrational fingerprints in planetary and extraterrestrial particles, catalytic substances, and aqueous biological samples. These tasks can be accomplished by the scattering-type scanning near-field optical microscopy (s-SNOM) technique that has recently spread to many research fields and enabled notable discoveries. Herein, it is shown that the s-SNOM, together with scanning probe research in general, can benefit in many ways from artificial-intelligence (AI) and machine-learning (ML) algorithms. Augmented with AI- and ML-enhanced data acquisition and analysis, scanning probe optical nanoscopy is poised to become more efficient, accurate, and intelligent.","id":16},{"numerical_label":"155","title":"Infrared Nano-Imaging of Dirac Magnetoexcitons in Graphene","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1038\/s41565-023-01488-y","authors":"Michael Dapolito, Makoto Tsuneto, Wenjun Zheng, Lukas Wehmeier, Suheng Xu, Xinzhong Chen, Jiacheng Sun, Zengyi Du, Yinming Shao, Ran Jing, Shuai Zhang, Adrien Bercher, Yinan Dong, Dorri Halbertal, Vibhu Ravindran, Zijian Zhou, Mila Petrovic, Adrian Gozar, G. L. Carr, Qiang Li, Alexey B. Kuzmenko, Michael M. Fogler, D. N. Basov, Xu Du, Mengkun Liu","journal":"Nature Nano","volume":"18","issue_page":"1409-1415","year":"2023","pdf":"bib\/Dapolito2023INO.pdf","view_link":"","abstract":"Magnetic fields can have profound effects on the motion of electrons in quantum materials. Two-dimensional electron systems subject to strong magnetic fields are expected to exhibit quantized Hall conductivity, chiral edge currents and distinctive collective modes referred to as magnetoplasmons and magnetoexcitons. Generating these propagating collective modes in charge-neutral samples and imaging them at their native nanometre length scales have thus far been experimentally elusive. Here we visualize propagating magnetoexciton polaritons at their native length scales and report their magnetic-field-tunable dispersion in near-charge-neutral graphene. Imaging these collective modes and their associated nano-electro-optical responses allows us to identify polariton-modulated optical and photo-thermal electric effects at the sample edges, which are the most pronounced near charge neutrality. Our work is enabled by innovations in cryogenic near-field optical microscopy techniques that allow for the nano-imaging of the near-field responses of two-dimensional materials under magnetic fields up to 7\u2009T. This nano-magneto-optics approach allows us to explore and manipulate magnetopolaritons in specimens with low carrier doping via harnessing high magnetic fields.","id":17},{"numerical_label":"154","title":"Phase-resolved terahertz nanoimaging of WTe_2 microcrystals","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1103\/PhysRevB.107.155413","authors":"Ran Jing, Rocco A Vitalone, Suheng Xu, Chiu Fan Bowen Lo, Zaiyao Fei, Elliott Runburg, Yinming Shao, Xinzhong Chen, Fabian Mooshammer, Alexander S Mcleod, Mengkun Liu, Michael M Fogler, David H Cobden, Xiaodong Xu, D.N. Basov","journal":"Phys. Rev. B","volume":"107","issue_page":"155413","year":"2023","pdf":"bib\/Jing2023PRT.pdf","view_link":"","abstract":"The terahertz (THz) electrodynamics of few-layer WTe2 is dominated by the plasmon response. However, THz surface plasmons (SPs) with long wavelengths in two-dimensional exfoliated crystals are typically confined by the lateral geometry. Direct visualization of the plasmonic standing wave patterns is challenging due to the spatial confinement and low quality factor of the SP, especially for samples that are only a few monolayers thick. Here, we resolve subtle real-space features of the plasmonic response of WTe2 by augmenting more common scattering amplitude experiments with the phase contrast accomplished within the time-domain version of THz nanoimaging. Amplitude and phase images allow us to quantitatively evaluate the evolution of the plasmonic response at cryogenic temperatures in samples with variable thickness from 3 to 12 monolayers. The proposed imaging modality is universally applicable to the THz near-field nanoscopy of low-dimensional materials.","id":18},{"numerical_label":"153","title":"Polaritonic Probe of an Emergent 2D Dipole Interface","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1021\/acs.nanolett.3c01611","authors":"Rizzo, D.J., Zhang, J., Jessen, B.S., Ruta, F.L., Cothrine, M., Yan, J.Q., Mandrus, D.G., Nagler, S.E., Taniguchi, T., Watanabe, K., Fogler, M.M., Pasupathy, A.N., Millis, A.J., Rubio, A., Hone, J.C., Dean, C.R., Basov, D.N.","journal":"Nano Lett.","volume":"23","issue_page":"8426\u20138435","year":"2023","pdf":"bib\/Rizzo2023PPO.pdf","view_link":"","abstract":"The use of work-function-mediated charge transfer has recently emerged as a reliable route toward nanoscale electrostatic control of individual atomic layers. Using a-RuCl3 as a 2D electron acceptor, we are able to induce emergent nano-optical behavior in hexagonal boron nitride (hBN) that arises due to interlayer charge polarization. Using scattering-type scanning near-field optical microscopy (s-SNOM), we find that a thin layer of a-RuCl3 adjacent to an hBN slab reduces the propagation length of hBN phonon polaritons (PhPs) in significant excess of what can be attributed to intrinsic optical losses. Concomitant nano-optical spectroscopy experiments reveal a novel resonance that aligns energetically with the region of excess PhP losses. These experimental observations are elucidated by first-principles density-functional theory and near-field model calculations, which show that the formation of a large interfacial dipole suppresses out-of-plane PhP propagation. Our results demonstrate the potential utility of charge-transfer heterostructures for tailoring optoelectronic properties of 2D insulators.","id":19},{"numerical_label":"152","title":"Deep-learning-aided extraction of optical constants in scanning near-field optical microscopy","category":"Near Field Optics; Miscellaneous","label":"","hyperlink":"https:\/\/doi.org\/10.1063\/5.0139517","authors":"Y. Zhao,  X. Chen,  Z. Yao,  M. K. Liu, M. M. Fogler","journal":"J. Appl. Phys.","volume":"133","issue_page":"133105","year":"2023","pdf":"bib\/Zhao2023DLA.pdf","view_link":"","abstract":"Scanning near-field optical microscopy is one of the most effective techniques for spectroscopy of nanoscale systems. However, inferring optical constants from the measured near-field signal can be challenging because of a complicated and highly nonlinear interaction between the scanned probe and the sample. Conventional fitting methods applied to this problem often suffer from the lack of convergence or require human intervention. Here, we develop an alternative approach where the optical parameter extraction is automated by a deep learning network. The network provides an initial estimate that is subsequently refined by a traditional fitting algorithm. We show that this method demonstrates superior accuracy, stability against noise, and computational speed when applied to simulated near-field spectra.","id":20},{"numerical_label":"151","title":"Negative refraction in hyperbolic hetero-bicrystals","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"http:\/\/www.science.org\/doi\/10.1126\/science.adf1065","authors":"A. J. Sternbach, S. L. Moore, A. Rikhter, S. Zhang, R. Jing, Y. Shao, B. S. Y. Kim, S. Xu, S. Liu, J. H. Edgar, A. Rubio, C. Dean, J. Hone, M. M. Fogler, D. N. Basov","journal":"Science","volume":"379","issue_page":"555-557","year":"2023","pdf":"bib\/Sternbach2023NRI.pdf","view_link":"","abstract":"We visualized negative refraction of phonon polaritons, which occurs at the interface between two natural crystals. The polaritons\u2014hybrids of infrared photons and lattice vibrations\u2014form collimated rays that display negative refraction when passing through a planar interface between the two hyperbolic van der Waals materials: molybdenum oxide (MoO_3) and isotopically pure hexagonal boron nitride (h^{11}BN). At a special frequency \\omega_0, these rays can circulate along closed diamond-shaped trajectories. We have shown that polariton eigenmodes display regions of both positive and negative dispersion interrupted by multiple gaps that result from polaritonic-level repulsion and strong coupling.","id":21},{"numerical_label":"150","title":"Shape prediction on the basis of spectrum using neural networks","category":"Miscellaneous","label":"","hyperlink":"https:\/\/doi.org\/10.1103\/PhysRevResearch.5.013110","authors":"Y. Zhao and M. M. Fogler","journal":"Phys. Rev. Research","volume":"5","issue_page":"013110","year":"2023","pdf":"bib\/Zhao2023SPO.pdf","view_link":"","abstract":"We have developed a deep neural network that reconstructs the shape of a polygonal domain given the first hundred of its Dirichlet Laplacian eigenvalues. Having an encoder-decoder structure, the network maps input spectra to a latent space and then predicts the discretized image of the domain on a square grid. Tested on randomly generated pentagons, the predictions prove to be highly accurate for both concave and convex pentagons. Our analysis indicates that the network has discovered fundamental properties of the Laplacian operator, the scaling rule, and the continuous rotational symmetry. Additionally, the latent variables are strongly correlated with Weyl's parameters (area, perimeter, and a certain function of the angles) of the test polygons.","id":22},{"numerical_label":"149","title":"Infrared Plasmons Propagate through a Hyperbolic Nodal Metal","category":"Near Field Optics; Miscellaneous","label":"","hyperlink":"https:\/\/doi.org\/10.1126\/sciadv.add6169","authors":"Y. M. Shao, A. J. Sternbach, B. S. Y. Kim, A. A. Rikhter, X. Xu, U. De Giovannini, R. Jing, S. H. Chae, Z. Y. Sun, S. H. Lee, Y. L. Zhu, Z. Q. Mao, J. Hone, R. Queiroz, A. J. Millis, P. J. Schuck, A. Rubio, M. M. Fogler, D. N. Basov","journal":"Sci. Adv.","volume":"8","issue_page":"eadd6169","year":"2022","pdf":"bib\/Shao2022IPP.pdf","view_link":"","abstract":"Metals are canonical plasmonic media at infrared and optical wavelengths, allowing one to guide and manipulate light at the nano-scale. A special form of optical waveguiding is afforded by highly anisotropic crystals revealing the opposite signs of the dielectric functions along orthogonal directions. These media are classified as hyperbolic and include crystalline insulators, semiconductors and artificial metamaterials. Layered anisotropic metals are also anticipated to support hyperbolic waveguiding. Yet this behavior remains elusive, primarily because interband losses arrest the propagation of infrared modes. Here, we report on the observation of propagating hyperbolic waves in a prototypical layered nodal-line semimetal ZrSiSe. The observed waveguiding originates from polaritonic hybridization between near-infrared light and nodal-line plasmons. Unique nodal electronic structures simultaneously suppress interband loss and boost the plasmonic response, ultimately enabling the propagation of infrared modes through the bulk of the crystal.","id":23},{"numerical_label":"148","title":"Graphene as a source of entangled plasmons","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1103\/PhysRevResearch.4.023208","authors":"Z.Y. Sun, D. N. Basov, M. M. Fogler","journal":"Phys. Rev. Research","volume":"4","issue_page":"023208","year":"2022","pdf":"bib\/Sun2022GAS.pdf","view_link":"","abstract":"We analyze nonlinear optics schemes for generating pairs of quantum entangled plasmons in the terahertz-infrared range in graphene. We predict that high plasmonic field concentration and strong optical nonlinearity of monolayer graphene enables pair-generation rates much higher than those of conventional photonic sources. The first scheme we study is spontaneous parametric down conversion in a graphene nanoribbon. In this second-order nonlinear process a plasmon excited by an external pump splits into a pair of plasmons, of half the original frequency each, emitted in opposite directions. The conversion is activated by applying a dc electric field that induces a density gradient or a current across the ribbon. Another scheme is degenerate four-wave mixing where the counter-propagating plasmons are emitted at the pump frequency. This third-order nonlinear process does not require a symmetry-breaking dc field. We suggest nano-optical experiments for measuring position-momentum entanglement of the emitted plasmon pairs. We estimate the critical pump fields at which the plasmon generation rates exceed their dissipation, leading to parametric instabilities.","id":24},{"numerical_label":"147","title":"Ground and excited states of coupled exciton liquids in electron-hole quadrilayers","category":"Excitons","label":"","hyperlink":"https:\/\/doi.org\/10.1103\/PhysRevB.104.195430","authors":"C. Xu and M.M. Fogler","journal":"Phys. Rev. B","volume":"104","issue_page":"195430 ","year":"2021","pdf":"bib\/Xu2021GAE.pdf","view_link":"","abstract":"Interlayer excitons are bound states of electrons and holes confined in separate two-dimensional layers. Due to their repulsive dipolar interaction, interlayer excitons can form a correlated liquid. If another electron-hole bilayer is present, excitons from different bilayers can exhibit mutual attraction. We study such a quadrilayer system by a hypernetted chain formalism. We compute ground-state energies, pair correlation functions, and collective mode velocities as functions of the exciton densities. We estimate the critical density for the transition to a paired biexciton phase. For a strongly unbalanced (unequal density) system, the excitons in the more dilute bilayer behave as polarons. We compute energies and effective masses of such exciton-polarons.","id":25},{"numerical_label":"146","title":"Polaritonic Vortices with a Half-Integer Charge","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1021\/acs.nanolett.1c03175","authors":"L. Xiong, Y. T. Li, D. Halbertal, M. Sammon, Z. Y. Sun, S. Liu, J. H. Edgar, T. Low, M. M. Fogler, C. R. Dean, A. J. Millis, D. N. Basov","journal":"Nano Lett.","volume":"21","issue_page":"9256-9261","year":"2021","pdf":"bib\/Xiong2021PVW.pdf","view_link":"","abstract":"Topological spin textures are field arrangements that cannot be continuously deformed to a fully polarized state. In particular, merons are topological textures characterized by half-integer topological charge +\/- 1\/2 and vortex-like swirling patterns at large distances. Merons have been studied previously in the context of cosmology, fluid dynamics, condensed matter physics and plasmonics. Here, we visualized optical spin angular momentum of phonon polaritons that resembles nanoscale meron spin textures. Phonon polaritons, hybrids of infrared photons and phonons in hexagonal boron nitride, were excited by circularly polarized light incident on a ring-shaped antenna and imaged using infrared near-field techniques. The polariton field reveals a half-integer topological charge determined by the handedness of the incident beam. Our phonon polaritonic platform opens up new pathways to create, control, and visualize topological textures.","id":26},{"numerical_label":"145","title":"Terahertz response of monolayer and few-layer WTe_2 at the nanoscale","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1038\/s41467-021-23933-z","authors":"R. Jing, Y. M. Shao, Z. Y. Fei, C. F. B Lo, R. A. Vitalone, F. L. Ruta, J. Staunton, W. J. C. Zheng, A. S. Mcleod, Z. Y. Sun, B. Y. Jiang, X. Z. Chen, M. M. Fogler, A. J. Millis, M. K. Liu, D. H. Cobden, X. D. Xu, D. N. Basov","journal":"Nature Comms.","volume":"12","issue_page":"5594","year":"2021","pdf":"bib\/Jing2021TRO.pdf","view_link":"","abstract":"Tungsten ditelluride (WTe2) is an atomically layered transition metal dichalcogenide whose physical properties change systematically from monolayer to bilayer and few-layer versions. In this report, we use apertureless scattering-type near-field optical microscopy operating at Terahertz (THz) frequencies and cryogenic temperatures to study the distinct THz range electromagnetic responses of mono-, bi- and trilayer WTe2 in the same multi-terraced micro-crystal. THz nano-images of monolayer terraces uncovered weakly insulating behavior that is consistent with transport measurements. The near-field signal on bilayer regions shows moderate metallicity with negligible temperature dependence. Subdiffractional THz imaging data together with theoretical calculations involving thermally activated carriers favor the semimetal scenario with Delta  = -10meV over the semiconductor scenario for bilayer WTe2. Also, we observed clear metallic behavior of the near-field signal on trilayer regions. Our data are consistent with the existence of surface plasmon polaritons in the THz range confined to trilayer terraces in our specimens. Finally, data for microcrystals up to 12 layers thick reveal how the response of a few-layer WTe2 asymptotically approaches the bulk limit.","id":27},{"numerical_label":"144","title":"Hybrid Machine Learning for Scanning Near-Field Optical Spectroscopy","category":"Near Field Optics; Miscellaneous","label":"","hyperlink":"https:\/\/doi.org\/10.1021\/acsphotonics.1c00915","authors":"X. Z. Chen, Z. H. Yao, S. H. Xu, A. S. McLeod, S. N. G. Corder, Y. Zhao, M. Tsuneto, H. A. Bechtel, M. C. Martin, G. L. Carr, M. M. Fogler, S. G. Stanciu, D. N. Basov, M. K. Liu","journal":"ACS Photonics","volume":"8","issue_page":"2987-2996","year":"2021","pdf":"bib\/Chen2021HML.pdf","view_link":"","abstract":"The underlying physics behind an experimental observation often lacks a simple analytical description. This is especially the case for scanning probe microscopy techniques, where the interaction between the probe and the sample is nontrivial. Realistic modeling to include the exact details of the probe is widely acknowledged as a challenge. Due to various complexity constraints, the probe is often only approximated in a simplified geometry, leading to a source for modeling inconsistencies. On the other hand, a well-trained artificial neural network based on real data can grasp the hidden correlation between the signal and the sample properties, circumventing the explicit probe modeling process. In this work we show that, via a combination of model calculation and experimental data acquisition, a physics-infused hybrid neural network can predict the probe-sample interaction in the widely used scatteringtype scanning near-field optical microscope. This hybrid network provides a long-sought solution for accurate extraction of material properties from tip-specific raw data. The methodology can be extended to other scanning probe microscopy techniques as well as other data-oriented physical problems in general.","id":28},{"numerical_label":"143","title":"Fizeau drag in graphene plasmonics","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1038\/s41586-021-03640-x","authors":"Y. Dong, L. Xiong, I.Y. Phinney, Z. Sun, R. Jing, A.S. McLeod, S. Zhang, S. Liu, F.L. Ruta, H. Gao, Z. Dong, R. Pan, J.H. Edgar, P. Jarillo-Herrero, L.S. Levitov, A.J. Millis, M.M. Fogler, D.A. Bandurin, and D.N. Basov","journal":"Nature","volume":"594","issue_page":"513","year":"2021","pdf":"bib\/Dong2021FDI.pdf","view_link":"","abstract":"Dragging of light by moving media was predicted by Fresnel and verified by Fizeau\u2019s celebrated experiments with flowing water. This momentous discovery is among the experimental cornerstones of Einstein's special relativity theory and is well understood in the context of relativistic kinematics. By contrast, experiments on dragging photons by an electron flow in solids are riddled with inconsistencies and have so far eluded agreement with the theory. Here we report on the electron flow dragging surface plasmon polaritons (SPPs): hybrid quasiparticles of infrared photons and electrons in graphene. The drag is visualized directly through infrared nano-imaging of propagating plasmonic waves in the presence of a high-density current. The polaritons in graphene shorten their wavelength when propagating against the drifting carriers. Unlike the Fizeau effect for light, the SPP drag by electrical currents defies explanation by simple kinematics and is linked to the nonlinear electrodynamics of Dirac electrons in graphene. The observed plasmonic Fizeau drag enables breaking of time-reversal symmetry and reciprocity10 at infrared frequencies without resorting to magnetic fields or chiral optical pumping. The Fizeau drag also provides a tool with which to study interactions and nonequilibrium effects in electron liquids.","id":29},{"numerical_label":"142","title":"Programmable Bloch polaritons in graphene","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1126\/sciadv.abe8087","authors":"L. Xiong, Y. Li, M. Jung, C. Forsythe, S. Zhang, A. S. McLeod, Y. Dong, S. Liu, F. L. Ruta, C. Li, K. Watanabe, T. Taniguchi, M. M. Fogler, J. H. Edgar, G. Shvets, C. R. Dean, D. N. Basov","journal":"Sci. Adv.","volume":"7","issue_page":"eabe8087","year":"2021","pdf":"bib\/Xiong2021PBP.pdf","view_link":"","abstract":"Efficient control of photons is enabled by hybridizing light with matter. The resulting light-matter quasi-particles can be readily programmed by manipulating either their photonic or matter constituents. Here, we hybridized infrared photons with graphene Dirac electrons to form surface plasmon polaritons (SPPs) and uncovered a previously unexplored means to control SPPs in structures with periodically modulated carrier density. In these periodic structures, common SPPs with continuous dispersion are transformed into Bloch polaritons with attendant discrete bands separated by bandgaps. We explored directional Bloch polaritons and steered their propagation by dialing the proper gate voltage. Fourier analysis of the near-field images corroborates that this on-demand nano-optics functionality is rooted in the polaritonic band structure. Our programmable polaritonic platform paves the way for the much-sought benefits of on-the-chip photonic circuits.","id":30},{"numerical_label":"141","title":"Programmable hyperbolic polaritons in van der Waals semiconductors","category":"Near Field Optics; Graphene and 2D Materials; Excitons","label":"","hyperlink":"https:\/\/doi.org\/10.1126\/science.abe9163","authors":"A. J. Sternbach, S. H. Chae, S. Latini, A. A. Rikhter, Y. Shao, B. Li, D. Rhodes, B. Kim, P. J. Schuck, X. Xu, X.-Y. Zhu, R. D. Averitt, J. Hone, M. M. Fogler, A. Rubio, and D. N. Basov","journal":"Science","volume":"371","issue_page":"617","year":"2021","pdf":"bib\/Sternbach2021PHP.pdf","view_link":"","abstract":"Collective electronic modes or lattice vibrations usually prohibit propagation of electromagnetic radiation through the bulk of common materials over a frequency range associated with these oscillations. However, this textbook tenet does not necessarily apply to layered crystals. Highly anisotropic materials often display nonintuitive optical properties and can permit propagation of subdiffractional waveguide modes, with hyperbolic dispersion, throughout their bulk. Here, we report on the observation of optically induced electronic hyperbolicity in the layered transition metal dichalcogenide tungsten diselenide (WSe2). We used photoexcitation to inject electron-hole pairs in WSe2 and then visualized, by transient nanoimaging, the hyperbolic rays that traveled along conical trajectories inside of the crystal. We establish here the signatures of programmable hyperbolic electrodynamics and assess the role of quantum transitions of excitons within the Rydberg series in the observed polaritonic response.","id":31},{"numerical_label":"140","title":"Hyperbolic enhancement of photocurrent patterns in minimally twisted bilayer graphene","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"http:\/\/doi.org\/10.1038\/s41467-021-21792-2","authors":"S. S. Sunku, D. Halbertal, T. Stauber, S. Chen, A. S. McLeod, A. Rikhter, M. E. Berkowitz, C. F. B. Lo, D. E. Gonzalez-Acevedo, J. C. Hone, C. R. Dean, M. M. Fogler, D. N. Basov","journal":"Nature Comms.","volume":"12","issue_page":"1641","year":"2021","pdf":"bib\/Sunku2021HEO.pdf","view_link":"","abstract":"Quasi-periodic moir\u00e9 patterns and their effect on electronic properties of twisted bilayer graphene have been intensely studied. At small twist angle \u03b8, due to atomic reconstruction, the moir\u00e9 superlattice morphs into a network of narrow domain walls separating micronscale AB and BA stacking regions. We use scanning probe photocurrent imaging to resolve nanoscale variations of the Seebeck coefficient occurring at these domain walls. The observed features become enhanced in a range of mid-infrared frequencies where the hexagonal boron nitride substrate is optically hyperbolic. Our results illustrate the capabilities of the nano-photocurrent technique for probing nanoscale electronic inhomogeneities in twodimensional materials.","id":32},{"numerical_label":"139","title":"Hyperbolic Cooper-Pair Polaritons in Planar Graphene\/Cuprate Plasmonic Cavities","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1021\/acs.nanolett.0c03684","authors":"M. E. Berkowitz, B. S. Y. Kim, G. Ni, A. S. McLeod, C. Fan Bowen Lo, Z. Sun, G. Gu, K. Watanabe, T. Taniguchi, A. J. Millis, J. C. Hone, M. M. Fogler, R. D. Averitt, and D. N. Basov","journal":"Nano Lett.","volume":"21","issue_page":"308","year":"2021","pdf":"bib\/Berkowitz2021HCP.pdf","view_link":"","abstract":"Hyperbolic Cooper-pair polaritons (HCP) in cuprate superconductors are of fundamental interest due to their potential for providing insights into the nature of unconventional superconductivity. Here, we critically assess an experimental approach using near-field imaging to probe HCP in Bi2Sr2CaCu2O8+x (Bi-2212) in the presence of graphene surface plasmon polaritons (SPP). Our simulations show that inherently weak HCP features in the near-field can be strongly enhanced when coupled to graphene SPP in layered graphene\/hexagonal boron nitride (hBN)\/Bi-2212 heterostructures. This enhancement arises from our multilayered structures effectively acting as plasmonic cavities capable of altering collective modes of a layered superconductor by modifying its electromagnetic environment. The degree of enhancement can be selectively controlled by tuning the insulating spacer thickness with atomic precision. Finally, we verify the expected renormalization of room-temperature graphene SPP using near-field infrared imaging. Our modeling, augmented with data, attests to the validity of our approach for probing HCP modes in cuprate superconductors.","id":33},{"numerical_label":"138","title":"Attractive and repulsive dipolar interaction in bilayers of indirect excitons","category":"Excitons","label":"","hyperlink":"https:\/\/doi.org\/10.1103\/PhysRevB.103.045126","authors":"D. J. Choksy, Chao Xu, M. M. Fogler, L. V. Butov, J. Norman, and A. C. Gossard","journal":"Phys. Rev. B","volume":"103","issue_page":"045126","year":"2021","pdf":"bib\/Choksy2021AAR.pdf","view_link":"","abstract":"We explore attractive dipolar interaction in indirect excitons (IXs). For one layer of IXs in a single pair of coupled quantum wells (CQWs), the out-of-plane IX electric dipoles lead to repulsive dipolar interaction between IXs. The attractive dipolar interaction between IXs is realized in a two-CQW heterostructure with two IX layers in two separated CQW pairs. We found both in experimental measurements and theoretical simulations that increasing density of IXs in one layer causes a monotonic energy reduction for IXs in the other layer. We also found an in-plane shift of a cloud of IXs in one layer towards a cloud of IXs in the other layer. This behavior is qualitatively consistent with attractive dipolar interaction. The measured IX energy reduction and IX cloud shift are higher than the values given by the correlated liquid theory.","id":34},{"numerical_label":"137","title":"Charge-Transfer Plasmon Polaritons at Graphene\/\u03b1-RuCl_3 Interfaces","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/dx.doi.org\/10.1021\/acs.nanolett.0c03466","authors":"D. J. Rizzo, B. S. Jessen, Z. Sun, F. L. Ruta, J. Zhang, J.-Q. Yan, L. Xian, A. S. McLeod, M. E. Berkowitz, K. Watanabe, T. Taniguchi, S. E. Nagler, D. G. Mandrus, A. Rubio, M. M. Fogler, A. J. Millis, J. C. Hone, C. R. Dean, D. N. Basov","journal":"Nano Lett.","volume":"20","issue_page":"8338-8445","year":"2020","pdf":"bib\/Rizzo2020CTP.pdf","view_link":"","abstract":"Nanoscale charge control is a key enabling technology in plasmonics, electronic band structure engineering, and the topology of two-dimensional materials. By exploiting the large electron affinity of \u03b1-RuCl_3, we are able to visualize and quantify massive charge transfer at graphene\/\u03b1-RuCl_3 interfaces through generation of charge transfer plasmon polaritons (CPPs). We performed nanoimaging experiments on graphene\/\u03b1-RuCl_3 at both ambient and cryogenic temperatures and discovered robust plasmonic features in otherwise ungated and undoped structures. The CPP wavelength evaluated through several distinct imaging modalities offers a high-fidelity measure of the Fermi energy of the graphene layer: E_F = 0.6 eV (n = 2.7 x 10^13 cm^\u22122). Our first-principles calculations link the plasmonic response to the work function difference between graphene and \u03b1-RuCl3 giving rise to CPPs. Our results provide a novel general strategy for generating nanometer-scale plasmonic interfaces without resorting to external contacts or chemical doping.","id":35},{"numerical_label":"136","title":"Long-Lived Phonon Polaritons in Hyperbolic Materials","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1021\/acs.nanolett.1c01562","authors":"Ni, G.X., McLeod, A.S., Sun, Z.Y., Matson, J.R., Lo, C.F.B.W., Rhodes, D.A., Ruta, F.L., Moore, S.L., Vitalone, R.A., Cusco, R., Artus, L., Xiong, L., Dean, C.R., Hone, J.C., Millis, A.J., Fogler, M.M., Edgar, J.H., Caldwell, J.D., Basov, D.N.","journal":"Nano Lett.","volume":"21","issue_page":"5767-5773","year":"2021","pdf":"bib\/Ni2021LLP.pdf","view_link":"","abstract":"Natural hyperbolic materials with dielectric permittivities of opposite signs along different principal axes can confine long-wavelength electromagnetic waves down to the nanoscale, well below the diffraction limit. Confined electromagnetic waves coupled to phonons in hyperbolic dielectrics including hexagonal boron nitride (hBN) and \u03b1-MoO3 are referred to as hyperbolic phonon polaritons (HPPs). HPP dissipation at ambient conditions is substantial, and its fundamental limits remain unexplored. Here, we exploit cryogenic nanoinfrared imaging to investigate propagating HPPs in isotopically pure hBN and naturally abundant \u03b1-MoO3 crystals. Close to liquid-nitrogen temperatures, losses for HPPs in isotopic hBN drop significantly, resulting in propagation lengths in excess of 8 \u03bcm, with lifetimes exceeding 5 ps, thereby surpassing prior reports on such highly confined polaritonic modes. Our nanoscale, temperature-dependent imaging reveals the relevance of acoustic phonons in HPP damping and will be instrumental in mitigating such losses for miniaturized mid-infrared technologies operating at liquid-nitrogen temperatures.","id":36},{"numerical_label":"135","title":"Inductor coil of the highest possible Q","category":"Miscellaneous","label":"","hyperlink":"https:\/\/doi.org\/10.1038\/s41598-020-72308-9","authors":"A. Rikhter, M. M. Fogler","journal":"Sci. Rep.","volume":"10","issue_page":"15380","year":"2020","pdf":"bib\/Rikhter2020ICO.pdf","view_link":"","abstract":"The geometry of an inductor made of a long thin wire and having the highest possible Q-factor is found by numerical optimization. As frequency increases, the Q-factor first grows linearly and then according to a square-root law, while the cross-section of the optimal coil evolves from near-circular to sickle-shaped.","id":37},{"numerical_label":"134","title":"Imaging viscous flow of the Dirac fluid in graphene","category":"Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1038\/s41586-020-2507-2","authors":"M. J. H. Ku, T. X. Zhou, Q. Li, Y. J. Shin, J. K. Shi, C. Burch, L. E. Anderson, A. T. Pierce, Y. Xie, A. Hamo, U. Vool, H. Zhang, F. Casola, T. Taniguchi, K. Watanabe, M. M. Fogler, P. Kim, A. Yacoby, R. L. Walsworth","journal":"Nature","volume":"583","issue_page":"537\u2013541","year":"2020","pdf":"bib\/Ku2020IVF.pdf","view_link":"","abstract":"The electron\u2013hole plasma in charge-neutral graphene is predicted to realize a quantum critical system in which electrical transport features a universal hydrodynamic description, even at room temperature1,2. This quantum critical \u2018Dirac fluid\u2019 is expected to have a shear viscosity close to a minimum bound3,4, with an interparticle scattering rate saturating1 at the Planckian time, the shortest possible timescale for particles to relax. Although electrical transport measurements at finite carrier density are consistent with hydrodynamic electron flow in graphene5,6,7,8, a clear demonstration of viscous flow at the charge-neutrality point remains elusive. Here we directly image viscous Dirac fluid flow in graphene at room temperature by measuring the associated stray magnetic field. Nanoscale magnetic imaging is performed using quantum spin magnetometers realized with nitrogen vacancy centres in diamond. Scanning single-spin and wide-field magnetometry reveal a parabolic Poiseuille profile for electron flow in a high-mobility graphene channel near the charge-neutrality point, establishing the viscous transport of the Dirac fluid. This measurement is in contrast to the conventional uniform flow profile imaged in a metallic conductor and also in a low-mobility graphene channel. Via combined imaging and transport measurements, we obtain viscosity and scattering rates, and observe that these quantities are comparable to the universal values expected at quantum criticality. This finding establishes a nearly ideal electron fluid in charge-neutral, high-mobility graphene at room temperature4. Our results will enable the study of hydrodynamic transport in quantum critical fluids relevant to strongly correlated electrons in high-temperature superconductors9. This work also highlights the capability of quantum spin magnetometers to probe correlated electronic phenomena at the nanoscale.","id":38},{"numerical_label":"133","title":"Femtosecond exciton dynamics in WSe_2 optical waveguides","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1038\/s41467-020-17335-w","authors":"A.J. Sternbach, S. Latini, S. Chae, H. H\u00fcbener, U. De Giovannini, Y. Shao, L. Xiong, Z. Sun, N. Shi, P. Kissin, G. Ni, D. Rhodes, B. Kim, N. Yu, A. J. Millis, M. M. Fogler, P. J. Schuck, M. Lipson, X.-Y. Zhu, J. Hone, R. D. Averitt, A. Rubio, D. N. Basov","journal":"Nat. Commun.","volume":"11","issue_page":"3567","year":"2020","pdf":"bib\/Sternbach2020FED.pdf","view_link":"","abstract":"Van-der Waals (vdW) atomically layered crystals can act as optical waveguides over a broad range of the electromagnetic spectrum ranging from Terahertz to visible. Unlike common Si-based waveguides, vdW semiconductors host strong excitonic resonances that may be controlled using non-thermal stimuli including electrostatic gating and photoexcitation. Here, we utilize waveguide modes to examine photo-induced changes of excitons in the prototypical vdW semiconductor, WSe2, prompted by femtosecond light pulses. Using time-resolved scanning near-field optical microscopy we visualize the electric field profiles of waveguide modes in real space and time and extract the temporal evolution of the optical constants following femtosecond photoexcitation. By monitoring the phase velocity of the waveguide modes, we detect incoherent A-exciton bleaching along with a coherent optical Stark shift in WSe2.","id":39},{"numerical_label":"132","title":"Nano-photocurrent Mapping of Local Electronic Structure in Twisted Bilayer Graphene","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1021\/acs.nanolett.9b04637","authors":"S. S. Sunku, A. S. McLeod, T. Stauber, H. Yoo, D. Halbertal, G. Ni, A. Sternbach, B.-Y. Jiang, T. Taniguchi, K. Watanabe, P. Kim, M. M. Fogler, D. N. Basov","journal":"Nano Lett.","volume":"20","issue_page":"2958\u20132964","year":"2020","pdf":"bib\/Sunku2020NPM.pdf","view_link":"","abstract":"We report a combined nano-photocurrent and infrared nanoscopy study of twisted bilayer graphene (TBG) enabling access to the local electronic phenomena at length scales as short as 20 nm. We show that the photocurrent changes sign at carrier densities tracking the local superlattice density of states of TBG. We use this property to identify domains of varying local twist angle by local photothermoelectric effect. Consistent with the photocurrent study, infrared nanoimaging experiments reveal optical conductivity features dominated by twist-angle-dependent interband transitions. Our results provide a fast and robust method for mapping the electronic structure of TBG and suggest that similar methods can be broadly applied to probe electronic inhomogeneities of Moir\u00e9 superlattices in other van der Waals heterostructures.","id":40},{"numerical_label":"131","title":"Nanoscale Infrared Spectroscopy and Imaging of Catalytic Reactions in Cu_2O Crystals","category":"Near Field Optics","label":"","hyperlink":"https:\/\/doi.org\/10.1021\/acsphotonics.9b01704","authors":"G. X. Ni, S. Chen, S. S. Sunku, A. Sternbach, A. S. McLeod, L. Xiong, M. M. Fogler, G. Chen, and D. N. Basov","journal":"ACS Photonics","volume":"7","issue_page":"576","year":"2020","pdf":"bib\/Ni2020NIS.pdf","view_link":"","abstract":"Many of the existing electrochemical catalysts suffer from poor selectivity, instability, and low exchange current densities. These shortcomings call for a comprehensive exploration of the catalytic processes at the fundamental nanometer length scale levels. Here we exploit infrared (IR) nanoimaging and nanospectroscopy to directly visualize catalytic reactions on the surface of Cu2O polyhedral single crystals with nanoscale spatial resolution. Nano-IR data revealed signatures of this common catalyst after electrochemical reduction of carbon dioxides (CO2). We discuss the utility of nano-IR methods for surface\/facet engineering of efficient electrochemical catalysts.","id":41},{"numerical_label":"130","title":"Collective modes and THz near field response of superconductors","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/doi.org\/10.1103\/PhysRevResearch.2.023413","authors":"Z. Sun, M.M. Fogler, D.N. Basov, A.J. Millis","journal":"Phys. Rev. Research","volume":"2","issue_page":"023413","year":"2020","pdf":"bib\/Sun2020CMA.pdf","view_link":"","abstract":"We theoretically study the low-energy electromagnetic response of Bardeen-Cooper-Schrieffer\u2013type superconductors focusing on propagating collective modes that are observable with terahertz near-field optics. The interesting frequency and momentum range is \u03c9<2\u0394 and q<1\/\u03be, where \u0394 is the gap and \u03be is the coherence length. We show that it is possible to observe the superfluid plasmons, amplitude (Higgs) modes, Bardasis-Schrieffer modes, and Carlson-Goldman modes using the terahertz near-field technique, although none of these modes couple linearly to far-field radiation. Coupling of terahertz near-field radiation to the amplitude mode requires particle-hole symmetry breaking, while coupling to the Bardasis-Schrieffer mode does not and is typically stronger. For parameters appropriate to layered superconductors of current interest, the Carlson-Goldman mode appears in the near-field reflection coefficient as a weak feature in the subterahertz frequency range. In a system of two superconducting layers with nanometer-scale separation, an acoustic phase mode appears as the antisymmetric density fluctuation mode of the system. This mode produces well-defined resonance peaks in the near-field terahertz response and has strong anticrossings with the Bardasis-Schrieffer and amplitude modes, enhancing their response. In a slab consisting of many layers of quasi-two-dimensional superconductors, realized for example in samples of high-Tc cuprate compounds, many branches of propagating Josephson plasmon modes are found to couple to the terahertz near-field radiation.","id":42},{"numerical_label":"129","title":"Photonic crystal for graphene plasmons","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/www.nature.com\/articles\/s41467-019-12778-2","authors":"L. Xiong, C. Forsythe, M. Jung, A.S. McLeod, S.S. Sunku, Y.M. Shao, G.X. Ni, A.J. Sternbach, S. Liu, J.H. Edgar, E.J. Mele, M.M. Fogler, G. Shvets, C.R. Dean, D.N. Basov","journal":"Nat. Commun.","volume":"10","issue_page":"4780 ","year":"2019","pdf":"bib\/Xiong2019PCF.pdf","view_link":"","abstract":"Photonic crystals are commonly implemented in media with periodically varying optical properties. Photonic crystals enable exquisite control of light propagation in integrated optical circuits, and also emulate advanced physical concepts. However, common photonic crystals are unfit for in-operando on\/off controls. We overcome this limitation and demonstrate a broadly tunable two-dimensional photonic crystal for surface plasmon polaritons. Our platform consists of a continuous graphene monolayer integrated in a back-gated platform with nano-structured gate insulators. Infrared nano-imaging reveals the formation of a photonic bandgap and strong modulation of the local plasmonic density of states that can be turned on\/off or gradually tuned by the applied gate voltage. We also implement an artificial domain wall which supports highly confined one-dimensional plasmonic modes. Our electrostatically-tunable photonic crystals are derived from standard metal oxide semiconductor field effect transistor technology and pave a way for practical on-chip light manipulation.","id":43},{"numerical_label":"128","title":"Phonon Polaritons in Monolayers of Hexagonal Boron Nitride","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1002\/adma.201806603","authors":"S. Dai, W. Fang, N. Rivera, Y. Stehle, B.-Y. Jiang, J. Shen, R. Y. Tay, Ch. J. Ciccarino, Q. Ma, D. Rodan-Legrain, P. Jarillo-Herrero, E. H. Tong Teo, M. M. Fogler, P. Narang, J. Kong, D. N. Basov","journal":"Adv. Mat.","volume":"31","issue_page":"1806603","year":"2019","pdf":"bib\/Dai2019PPI.pdf","view_link":"","abstract":"Phonon polaritons in van der Waals materials reveal significant confinement accompanied with long propagation length: important virtues for tasks pertaining to the control of light and energy flow at the nanoscale. While previous studies of phonon polaritons have relied on relatively thick samples, here reported is the first observation of surface phonon polaritons in single atomic layers and bilayers of hexagonal boron nitride (hBN). Using antenna\u2010based near\u2010field microscopy, propagating surface phonon polaritons in mono\u2010 and bilayer hBN microcrystals are imaged. Phonon polaritons in monolayer hBN are confined in a volume about one million times smaller than the free\u2010space photons. Both the polariton dispersion and their wavelength\u2013thickness scaling law are altered compared to those of hBN bulk counterparts. These changes are attributed to phonon hardening in monolayer\u2010thick crystals. The data reported here have bearing on applications of polaritons in metasurfaces and ultrathin optical elements.","id":44},{"numerical_label":"127","title":"Soliton superlattices in twisted hexagonal boron nitride","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1038\/s41467-019-12327-x","authors":"G.X. Ni, H. Wang, B.-Y. Jiang, L. X. Chen, Y. Du, Z. Y. Sun, M. D. Goldflam, A. J. Frenzel, X. M. Xie, M. M. Fogler, D. N. Basov","journal":"Nat. Commun.","volume":"10","issue_page":"4360","year":"2019","pdf":"bib\/Ni2019SSI.pdf","view_link":"","abstract":"Properties of atomic van der Waals heterostructures are profoundly influenced by interlayer coupling, which critically depends on stacking of the proximal layers. Rotational misalignment or lattice mismatch of the layers gives rise to a periodic modulation of the stacking, the moir\u00e9 superlattice. Provided the superlattice period extends over many unit cells, the coupled layers undergo lattice relaxation, leading to the concentration of strain at line defects \u2013 solitons - separating large area commensurate domains. We visualize such long-range periodic superstructures in thin crystals of hexagonal boron nitride using atomic-force microscopy and nano-infrared spectroscopy. The solitons form sub-surface hexagonal networks with periods of a few hundred nanometers. We analyze the topography and infrared contrast of these networks to obtain spatial distribution of local strain and its effect on the infrared-active phonons of hBN.","id":45},{"numerical_label":"126","title":"Strong Metasurface-Josephson Plasma Resonance Coupling in Superconducting La_{2-x}Sr_{x}CuO_4","category":"Miscellaneous","label":"","hyperlink":"https:\/\/doi.org\/10.1002\/adom.201900712","authors":"J. S. Schalch, K. Post, G. Duan, X. Zhao, Y. D. Kim, J. Hone, M. M. Fogler, X. Zhang, D. N. Basov, R. D. Averitt","journal":"Adv. Optical Mater.","volume":"","issue_page":"1900712","year":"2019","pdf":"bib\/Schalch2019SMJ.pdf","view_link":"","abstract":"Terahertz spectroscopy of the c\u2010axis Josephson plasma resonance (JPR) in high\u2010temperature cuprates is a powerful probe of superconductivity, providing a route to couple to and interact with the condensate. Electromagnetic coupling between metasurface arrays of split ring resonators (SRRs) and the JPR of a La2\u2212xSrxCuO4 single crystal (Tc = 32 K) is investigated. The metasurface resonance frequency (\u03c9_MM), determined by the SRR geometry, is swept through the JPR frequency (\u03c9_JPR = 1.53 THz) using a series of interchangeable tapes applied to the same single crystal. Terahertz reflectivity measurements on the resulting hybrid superconducting metamaterials (HSMMs) reveal anticrossing behavior characteristic of strong coupling. The experimental results, validated with numerical simulations, indicate a normalized Rabi frequency of \u03a9_R = 0.29. Further, it is shown that HSMMs with \u03c9_MM > \u03c9_JPR provide a route to couple to hyperbolic waveguide modes in c\u2010axis cuprate samples. This work informs future possibilities for optimizing the coupling strength of HSMMs and investigating nonlinear superconductivity under high field terahertz excitation.","id":46},{"numerical_label":"125","title":"Exciton Gas Transport through Nanoconstrictions","category":"Excitons","label":"","hyperlink":"https:\/\/pubs.acs.org\/doi\/10.1021\/acs.nanolett.9b01877","authors":"C. Xu, J. R. Leonard, C. J. Dorow, L. V. Butov, M. M. Fogler, D. E. Nikonov, I. A. Young","journal":"Nano Lett.","volume":"19","issue_page":"5373-5379","year":"2019","pdf":"bib\/Xu2019EGT.pdf","view_link":"","abstract":"An indirect exciton is a bound state of an electron and a hole in spatially separated layers. Two-dimensional indirect excitons can be created optically in heterostructures containing double quantum wells or atomically thin semiconductors. We study theoretically the transmission of such bosonic quasiparticles through nanoconstrictions. We show that the quantum transport phenomena, for example, conductance quantization, single-slit diffraction, two-slit interference, and the Talbot effect, are experimentally realizable in systems of indirect excitons. We discuss similarities and differences between these phenomena and their counterparts in electronic devices.","id":47},{"numerical_label":"124","title":"Photo-enhanced metastable c-axis electrodynamics in stripe ordered cuprate La_{1.885}Ba_{0.115}CuO_4","category":"Near Field Optics","label":"","hyperlink":"https:\/\/doi.org\/10.1073\/pnas.1908368116","authors":"K. A. Cremin, J. Zhang, C. C. Homes, G. D. Gu, Z. Sun, M. M. Fogler, A. J. Millis, D. N. Basov, R. D. Averitt","journal":"PNAS","volume":"116","issue_page":"19875-19879","year":"2019","pdf":"bib\/Cremin2019PMC.pdf","view_link":"","abstract":"Quantum materials are amenable to nonequilibrium manipulation with light, enabling modification and control of macroscopic properties. Light-based augmentation of superconductivity is particularly intriguing. Copper-oxide superconductors exhibit complex interplay between spin order, charge order, and superconductivity, offering the prospect of enhanced coherence by altering the balance between competing orders. We utilize terahertz time-domain spectroscopy to monitor the c-axis Josephson plasma resonance (JPR) in La_{2\u2212x}Ba_{x}CuO_4 (x = 0.115) as a direct probe of superconductivity dynamics following excitation with near-infrared pulses. Starting from the superconducting state, c-axis polarized excitation with a fluence of 100 \\mu J\/cm^2 results in an increase of the far-infrared spectral weight by more than an order of magnitude as evidenced by a blueshift of the JPR, interpreted as resulting from nonthermal collapse of the charge order. The photoinduced signal persists well beyond our measurement window of 300 ps and exhibits signatures of spatial inhomogeneity. The electrodynamic response of this metastable state is consistent with enhanced superconducting fluctuations. Our results reveal that La2\u2212xBaxCuO4 is highly sensitive to nonequilibrium excitation over a wide fluence range, providing an unambiguous example of photoinduced modification of order-parameter competition.","id":48},{"numerical_label":"123","title":"Optical signatures of Dirac nodal lines in NbAs_2","category":"Miscellaneous","label":"","hyperlink":"https:\/\/doi.org\/10.1073\/pnas.1809631115","authors":"Y. Shao, Z. Sun, Y. Wang, C. Xu, R. Sankar, A.J. Breindel, C. Cao, M.M. Fogler, A.J. Millis, F. Chou, Z. Li, T. Timusk, M.B. Maple, D.N. Basov","journal":"PNAS","volume":"116","issue_page":"1168-1173","year":"2019","pdf":"bib\/Shao2018OSO.pdf","view_link":"","abstract":"Using polarized optical and magneto-optical spectroscopy, we have demonstrated universal aspects of electrodynamics associated with Dirac nodal lines that are found in several classes of unconventional intermetallic compounds. We investigated anisotropic electrodynamics of NbAs2 where the spin-orbit coupling (SOC) triggers energy gaps along the nodal lines. These gaps manifest as sharp steps in the optical conductivity spectra \u03c31(\u03c9). This behavior is followed by the linear power-law scaling of \u03c31(\u03c9) at higher frequencies, consistent with our theoretical analysis for dispersive Dirac nodal lines. Magneto-optics data affirm the dominant role of nodal lines in the electrodynamics of NbAs2.","id":49},{"numerical_label":"119","title":"Universal linear and nonlinear electrodynamics of a Dirac fluid","category":"Graphene and 2D Materials","label":"","hyperlink":"http:\/\/www.pnas.org\/content\/early\/2018\/03\/09\/1717010115.short","authors":"Z. Sun, D.N. Basov, M.M. Fogler","journal":"PNAS","volume":"115","issue_page":"3285-3289","year":"2018","pdf":"bib\/Sun2018ULA.pdf","view_link":"","abstract":"A general relation is derived between the linear and second-order nonlinear ac conductivities of an electron system in the hydrodynamic regime of frequencies below the interparticle scattering rate. The magnitude and tensorial structure of the hydrodynamic nonlinear conductivity are shown to differ from their counterparts in the more familiar kinetic regime of higher frequencies. Due to universality of the hydrodynamic equations, the obtained formulas are valid for systems with an arbitrary Dirac-like dispersion, ranging from solid-state electron gases to free-space plasmas, either massive or massless, at any temperature, chemical potential, or space dimension. Predictions for photon drag and second-harmonic generation in graphene are presented as one application of this theory.","id":50},{"numerical_label":"118","title":"Third-order optical conductivity of an electron fluid","category":"Graphene and 2D Materials","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.97.075432","authors":"Z. Sun, D.N. Basov, M.M. Fogler","journal":"Phys. Rev. B","volume":"97","issue_page":"75432","year":"2018","pdf":"bib\/Sun2018TOO.pdf","view_link":"","abstract":"We derive the nonlinear optical conductivity of an isotropic electron fluid at frequencies below the interparticle collision rate. In this regime, governed by hydrodynamics, the conductivity acquires a universal form at any temperature, chemical potential, and spatial dimension. We show that the nonlinear response of the fluid to a uniform field is dominated by the third-order conductivity tensor \u03c3 (3) whose magnitude and temperature dependence differ qualitatively from those in the conventional kinetic regime of higher frequencies. We obtain explicit formulas for \u03c3 (3) for Dirac materials such as graphene and Weyl semimetals. We make predictions for the third-harmonic generation, renormalization of the collective-mode spectrum, and the third-order circular magnetic birefringence experiments.","id":51},{"numerical_label":"117","title":"Indirect excitons in van der Waals heterostructures at room temperature","category":"Excitons; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/www.nature.com\/articles\/s41467-018-04293-7","authors":"E.V. Calman, M.M. Fogler, L.V. Butov, S. Hu, A. Mishchenko, A.K. Geim","journal":"Nat. Commun.","volume":"9","issue_page":"1895","year":"2018","pdf":"bib\/Calman2018IEI.pdf","view_link":"","abstract":"ndirect excitons (IXs) are explored both for studying quantum Bose gases in semiconductor materials and for the development of excitonic devices. IXs were extensively studied in III\u2013V and II\u2013VI semiconductor heterostructures where IX range of existence has been limited to low temperatures. Here, we present the observation of IXs at room temperature in van der Waals transition metal dichalcogenide (TMD) heterostructures. This is achieved in TMD heterostructures based on monolayers of MoS2 separated by atomically thin hexagonal boron nitride. The IXs we realize in the TMD heterostructure have lifetimes orders of magnitude longer than lifetimes of direct excitons in single-layer TMD and their energy is gate controlled. The realization of IXs at room temperature establishes the TMD heterostructures as a material platform both for a field of high-temperature quantum Bose gases of IXs and for a field of high-temperature excitonic devices.","id":52},{"numerical_label":"116","title":"Pancharatnam\u2013Berry phase in condensate of indirect excitons","category":"Excitons","label":"","hyperlink":"https:\/\/www.nature.com\/articles\/s41467-018-04667-x","authors":"J.R. Leonard, A.A. High, A.T. Hammack, M.M. Fogler, L.V. Butov, K.L. Campman, A.C. Gossard","journal":"Nat. Commun.","volume":"9","issue_page":"2158","year":"2018","pdf":"bib\/Leonard2018PBP.pdf","view_link":"","abstract":"The Pancharatnam\u2013Berry phase is a geometric phase acquired over a cycle of parameters in the Hamiltonian governing the evolution of the system. Here, we report on the observation of the Pancharatnam\u2013Berry phase in a condensate of indirect excitons (IXs) in a GaAs-coupled quantum well structure. The Pancharatnam\u2013Berry phase is directly measured by detecting phase shifts of interference fringes in IX interference patterns. Correlations are found between the phase shifts, polarization pattern of IX emission, and onset of IX spontaneous coherence. The evolving Pancharatnam\u2013Berry phase is acquired due to coherent spin precession in IX condensate and is observed with no decay over lengths exceeding 10\u2009\u03bcm indicating long-range coherent spin transport.","id":53},{"numerical_label":"115","title":"Nanoscale mapping and spectroscopy of nonradiative hyperbolic modes in hexagonal boron nitride nanostructures","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/pubs.acs.org\/doi\/abs\/10.1021\/acs.nanolett.7b04476","authors":"L.V. Brown, M. Davanco, Z. Sun, A. Kretinin, Y. Chen, J.R. Matson, I. Vurgaftman, N. Sharac, A.J. Giles, M.M. Fogler, T. Taniguchi, K. Watanabe, K.S. Novoselov, S.A. Maier, A. Centrone, J.D. Caldwell","journal":"Nano Lett.","volume":"18","issue_page":"1628-1636","year":"2018","pdf":"bib\/Brown2018NMA.pdf","view_link":"","abstract":"The inherent crystal anisotropy of hexagonal boron nitride (hBN) provides the ability to support hyperbolic phonon polaritons, that is, polaritons that can propagate with very large wave vectors within the material volume, thereby enabling optical confinement to exceedingly small dimensions. Indeed, previous research has shown that nanometer-scale truncated nanocone hBN cavities, with deep subdiffractional dimensions, support three-dimensionally confined optical modes in the mid-infrared. Because of optical selection rules, only a few of the many theoretically predicted modes have been observed experimentally via far-field reflection and scattering-type scanning near-field optical microscopy (s-SNOM). The photMiscellaneousmal induced resonance (PTIR) technique probes optical and vibrational resonances overcoming weak far-field emission by leveraging an atomic force \u2026","id":54},{"numerical_label":"114","title":"Ultralow-loss polaritons in isotopically pure boron nitride","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/www.nature.com\/articles\/nmat5047","authors":"A. Giles, S. Dai, I. Vurgaftman, T. Hoffman, S. Liu, L. Lindsay, C.T. Ellis, N. Assefa, I. Chatzakis, T.L. Reinecke, J.G. Tischler, M.M. Fogler, J.H. Edgar, D.N. Basov, J.D. Caldwel","journal":"Nat. Mater.","volume":"17","issue_page":"134","year":"2018","pdf":"bib\/Gilles2018ULP.pdf","view_link":"","abstract":"Conventional optical components are limited to size scales much larger than the wavelength of light, as changes to the amplitude, phase and polarization of the electromagnetic fields are accrued gradually along an optical path. However, advances in nanophotonics have produced ultrathin, so-called 'flat'optical components that beget abrupt changes in these properties over distances significantly shorter than the free-space wavelength 1, 2, 3, 4, 5, 6, 7, 8. Although high optical losses still plague many approaches 9, phonon polariton (PhP) materials have demonstrated long lifetimes for sub-diffractional modes 10, 11, 12, 13 in comparison to plasmon-polariton-based nanophotonics. We experimentally observe a threefold improvement in polariton lifetime through isotopic enrichment of hexagonal boron nitride (hBN). Commensurate increases in the polariton propagation length are \u2026","id":55},{"numerical_label":"113","title":"Split-gate device for indirect excitons","category":"Excitons","label":"","hyperlink":"https:\/\/aip.scitation.org\/doi\/abs\/10.1063\/1.5021488","authors":"C.J. Dorow, J.R. Leonard, M.M. Fogler, L.V. Butov, K.W. West, L.N. Pfeiffer","journal":"Appl. Phys. Lett.","volume":"112","issue_page":"183501","year":"2018","pdf":"bib\/Dorow2018SGD.pdf","view_link":"","abstract":"We present a concept and experimental proof of principle for split-gate devices for indirect excitons (IXs). The split-gate forms a narrow channel, a point contact, for IX current. Control of IX flow through the split-gate with both gate voltage and excitation power is demonstrated.","id":56},{"numerical_label":"112","title":"Manipulation and steering of hyperbolic surface polaritons in hexagonal boron nitride","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1002\/adma.201706358","authors":"S. Dai, M. Tymchenko, Y. Yang, Q. Ma, M. Pita-Vidal, K. Watanabe, T. Taniguchi, P. Jarillo-Herrero, M.M. Fogler, A. Al\u00f9, D.N. Basov","journal":"Adv. Mater.","volume":"30","issue_page":"1706358","year":"2018","pdf":"bib\/Dai2018MAS.pdf","view_link":"","abstract":"Hexagonal boron nitride (hBN) is a natural hyperbolic material that supports both volume-confined hyperbolic polaritons (HPs) and sidewall-confined hyperbolic surface polaritons (HSPs). In this work, we demonstrate effective excitation, control and steering of HSPs in hBN through engineering the geometry and orientation of hBN sidewalls. By combining infrared (IR) nano-imaging and numerical simulations, we investigate the reflection, transmission and scattering of HSPs at the hBN corners with various apex angles. We show that the sidewall-confined nature of HSPs enables a high degree of control over their propagation by designing the geometry of hBN nanostructures.","id":57},{"numerical_label":"111","title":"Artifact free time resolved near-field spectroscopy","category":"Near Field Optics","label":"","hyperlink":"https:\/\/www.osapublishing.org\/abstract.cfm?uri=oe-25-23-28589","authors":"A.J. Sternbach, J. Hinton, T. Slusar, A.S. McLeod, M.K. Liu, A. Frenzel, M. Wagner, R. Iraheta, F. Keilmann, A. Leitenstorfer, M.M. Fogler, H.T. Kim, R.D. Averitt, D.N. Basov","journal":"Opt. Express","volume":"25","issue_page":"28589-28611","year":"2017","pdf":"bib\/Sternbach2017AFT.pdf","view_link":"","abstract":"We report on the first implementation of ultrafast near field measurements carried out with the transient pseudoheterodyne detection method (Tr-pHD). This method is well suited for efficient and artifact free pump-probe scattering-type near-field optical microscopy with nanometer scale resolution. The Tr-pHD technique is critically compared to Miscellaneous data acquisition methods and found to offer significant advantages. Experimental evidence for the advantages of Tr-pHD is provided in the near-IR frequency range. Crucial factors involved in achieving proper performance of the Tr-pHD method with pulsed laser sources are analyzed and detailed in this work. We applied this novel method to femtosecond time-resolved and nanometer spatially resolved studies of the photo-induced effects in the insulator-to-metal transition system vanadium dioxide.","id":58},{"numerical_label":"110","title":"Plasmon reflections by topological electronic boundaries in bilayer graphene","category":"Graphene and 2D Materials; Near Field Optics","label":"","hyperlink":"http:\/\/pubs.acs.org\/doi\/abs\/10.1021\/acs.nanolett.7b03816","authors":"B.Y. Jiang, G. Ni, Z. Addison, J.K. Shi, X. Liu, S. Yang, F. Zhao, P. Kim, E.J. Mele, D.N. Basov, M.M. Fogler","journal":"Nano Lett.","volume":"17","issue_page":"7080-7085","year":"2017","pdf":"bib\/Jiang2017PRB.pdf","view_link":"","abstract":"Domain walls separating regions of AB and BA interlayer stacking in bilayer graphene have attracted attention as novel examples of structural solitons, topological electronic boundaries, and nanoscale plasmonic scatterers. We show that strong coupling of domain walls to surface plasmons observed in infrared nanoimaging experiments is due to topological chiral modes confined to the walls. The optical transitions among these chiral modes and the band continua enhance the local conductivity, which leads to plasmon reflection by the domain walls. The imaging reveals two kinds of plasmonic standing-wave interference patterns, which we attribute to shear and tensile domain walls. We compute the electronic structure of both wall varieties and show that the tensile wall contains additional confined bands which produce a structure-specific contrast of the local conductivity, in \u2026","id":59},{"numerical_label":"109","title":"Intrinsic Plasmon\u2013Phonon Interactions in Highly Doped Graphene: A Near-Field Imaging Study","category":"Near Field Optics","label":"","hyperlink":"http:\/\/pubs.acs.org\/doi\/abs\/10.1021\/acs.nanolett.7b01603","authors":"F.J. Bezares, A. De Sanctis, J.R.M. Saavedra, A. Woessner, P. Alonso-Gonz\u00e1lez, I. Amenabar, J.Chen, T.H. Bointon, S. Dai, M.M. Fogler, D.N. Basov, R. Hillenbrand, M.F. Craciun, F.J. Garci\u0301a de Abajo, S. Russo, F.H.L. Koppens","journal":"Nano Lett.","volume":"17","issue_page":"5908-5913","year":"2017","pdf":"bib\/Bezares2017IPP.pdf","view_link":"","abstract":"As a two-dimensional semimetal, graphene offers clear advantages for plasmonic applications over conventional metals, such as stronger optical field confinement, in situ tunability, and relatively low intrinsic losses. However, the operational frequencies at which plasmons can be excited in graphene are limited by the Fermi energy EF, which in practice can be controlled electrostatically only up to a few tenths of an electronvolt. Higher Fermi energies open the door to novel plasmonic devices with unprecedented capabilities, particularly at mid-infrared and shorter-wave infrared frequencies. In addition, this grants us a better understanding of the interaction physics of intrinsic graphene phonons with graphene plasmons. Here, we present FeCl3-intercalated graphene as a new plasmonic material with high stability under environmental conditions and carrier concentrations \u2026","id":60},{"numerical_label":"108","title":"Mechanical detection and imaging of hyperbolic phonon polaritons in hexagonal Boron Nitride","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"http:\/\/pubs.acs.org\/doi\/abs\/10.1021\/acsnano.7b02323","authors":"A. Ambrosio, L.A. Jauregui, S. Dai, K. Chaudhary, M. Tamagnone, M.M. Fogler, D.N. Basov, F. Capasso, P. Kim, W.L. Wilson","journal":"ACS Nano","volume":"11","issue_page":"8741-8746","year":"2017","pdf":"bib\/Ambrosio2017MDA.pdf","view_link":"","abstract":"Mid-infrared nanoimaging and spectroscopy of two-dimensional (2D) materials have been limited so far to scattering-type scanning near-field optical microscopy (s-SNOM) experiments, where light from the sample is scattered by a metallic-coated atomic force microscope (AFM) tip interacting with the material at the nanoscale. These experiments have recently allowed imaging of plasmon polaritons in graphene as well as hyperbolic phonon polaritons in hexagonal boron nitride (hBN). Here we show that the high mechanical sensitivity of an AFM cantilever can be exploited for imaging hyperbolic phonon polaritons in hBN. In our imaging process, the lattice vibrations of hBN micrometer-sized flakes are locally enhanced by the launched phonon polaritons. These enhanced vibrations are coupled to the AFM tip in contact with the sample surface and recorded during scanning. Imaging \u2026","id":61},{"numerical_label":"107","title":"Imaging the localized plasmon resonance modes in graphene nanoribbons","category":"Graphene and 2D Materials; Near Field Optics","label":"","hyperlink":"http:\/\/pubs.acs.org\/doi\/abs\/10.1021\/acs.nanolett.7b02029","authors":"F. Hu, Y. Luan, Z. Fei, I.Z. Palubski, M.D. Goldflam, S. Dai, J.S. Wu, K.W. Post, G.C.A.M. Janssen, M.M. Fogler, D.N. Basov","journal":"Nano Lett.","volume":"17","issue_page":"5423-5428","year":"2017","pdf":"bib\/Hu2017ITL.pdf","view_link":"","abstract":"We report a nanoinfrared (IR) imaging study of the localized plasmon resonance modes of graphene nanoribbons (GNRs) using a scattering-type scanning near-field optical microscope (s-SNOM). By comparing the imaging data of GNRs that are aligned parallel and perpendicular to the in-plane component of the excitation laser field, we observed symmetric and asymmetric plasmonic interference fringes, respectively. Theoretical analysis indicates that the asymmetric fringes are formed due to the interplay between the localized surface plasmon resonance (SPR) mode excited by the GNRs and the propagative surface plasmon polariton (SPP) mode launched by the s-SNOM tip. With rigorous simulations, we reproduce the observed fringe patterns and address quantitatively the role of the s-SNOM tip on both the SPR and SPP modes. Furthermore, we have seen real-space signatures of both the \u2026","id":62},{"numerical_label":"106","title":"Efficiency of launching highly confined polaritons by infrared light incident on a hyperbolic material","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"http:\/\/pubs.acs.org\/doi\/abs\/10.1021\/acs.nanolett.7b01587","authors":"S. Dai, Q. Ma, Y. Yang, J. Rosenfeld, M.D. Goldflam, A. McLeod, Z. Sun, T.I. Andersen, Z. Fei, M. Liu, Y. Shao, K. Watanabe, T. Taniguchi, M. Thiemens, F. Keilmann, P. Jarillo-Herrero, M.M. Fogler, D.N. Basov","journal":"Nano Lett.","volume":"17","issue_page":"5285-5290","year":"2017","pdf":"bib\/Dai2017EOL.pdf","view_link":"","abstract":"We investigated phonon\u2013polaritons in hexagonal boron nitride a naturally hyperbolic van der Waals material\ue5f8 by means of the scattering-type scanning near-field optical microscopy. Real-space nanoimages we have obtained detail how the polaritons are launched when the light incident on a thin hexagonal boron nitride slab is scattered by various intrinsic and extrinsic inhomogeneities, including sample edges, metallic nanodisks deposited on its top surface, random defects, and surface impurities. The scanned tip of the near-field microscope is itself a polariton launcher whose efficiency proves to be superior to all the Miscellaneous types of polariton launchers we studied. Our work may inform future development of polaritonic nanodevices as well as fundamental studies of collective modes in van der Waals materials.","id":63},{"numerical_label":"105","title":"Electrical detection of hyperbolic phonon-polaritons in heterostructures of graphene and boron nitride","category":"Graphene and 2D Materials; Near Field Optics","label":"","hyperlink":"https:\/\/www.nature.com\/articles\/s41699-017-0031-5","authors":"A. Woessner, R. Parret, D. Davydovskaya, Y. Gao, J.S. Wu, M.B. Lundeberg, S. Nanot, P. Alonso-Gonz\u00e1lez, K. Watanabe, T. Taniguchi, R. Hillenbrand, M.M. Fogler, J. Hone, F.H.L. Koppens","journal":"NPJ 2D Mater. and App.","volume":"1","issue_page":"25","year":"2017","pdf":"bib\/Woessner2017EDO.pdf","view_link":"","abstract":"Light properties in the mid-infrared can be controlled at a deep subwavelength scale using hyperbolic phonons-polaritons of hexagonal boron nitride. While propagating as waveguided modes hyperbolic phonons-polaritons can concentrate the electric field in a chosen nano-volume. Such a behavior is at the heart of many applications including subdiffraction imaging and sensing. Here we employ HPPs in heterostructures of hexagonal boron nitride and graphene as new nano-optoelectronic platform by uniting the benefits of efficient hot-carrier photoconversion in graphene and the hyperbolic nature of hexagonal boron nitride. We demonstrate electrical detection of hyperbolic phonons-polaritons by guiding them towards a graphene pn-junction. We shine a laser beam onto a gap in metal gates underneath the heterostructure, where the light is converted into hyperbolic \u2026","id":64},{"numerical_label":"104","title":"Plasmonic imaging is gaining momentum","category":"Near Field Optics","label":"","hyperlink":"http:\/\/science.sciencemag.org\/content\/357\/6347\/132.summary","authors":"D.N. Basov, M.M. Fogler","journal":"Science","volume":"357","issue_page":"132-133","year":"2017","pdf":"bib\/Basov2017PII.pdf","view_link":"","abstract":"High-temperature superconductivity, unconventional magnetism, and charge-ordered states are examples of the spectacular properties that arise in solids through many-body effects, a consequence of electrons strongly interacting with one anMiscellaneous and with the crystal lattice. In a seminal contribution, Landau introduced quasiparticles, objects that behave in many ways as free electrons but with velocities and masses altered or \u201crenormalized.\u201d Information about the renormalization is encoded, for example, in optical properties of materials (1). The majority of optical studies have focused on response functions that depend on frequency \u03c9, but dependence on momentum q, which is equally valuable, has remained beyond the reach of common spectroscopic tools. On page 187 of this issue, Lundeberg et al.(2) developed a means to probe the nonlocal or q-dependent electromagnetic response by \u2026","id":65},{"numerical_label":"103","title":"Nanoplasmonic Phenomena at Electronic Boundaries in Graphene","category":"Graphene and 2D Materials; Near Field Optics","label":"","hyperlink":"http:\/\/pubs.acs.org\/doi\/abs\/10.1021\/acsphotonics.7b00477","authors":"Z. Fei, G.X. Ni, B.Y. Jiang, M.M. Fogler, D.N. Basov","journal":"ACS Photonics","volume":"4","issue_page":"2971-2977","year":"2017","pdf":"bib\/Fei2017NPA.pdf","view_link":"","abstract":"We review recent discoveries of the intriguing plasmonic phenomena at a variety of electronic boundaries (EBs) in graphene including a line of charges in graphene induced by a carbon nanotube gate, grain boundaries in chemical vapor deposited graphene films, an interface between graphene and moire\u0301 patterned graphene, an interface between graphene and bilayer graphene, and Miscellaneouss. All these and Miscellaneous EBs cause plasmonic impedance mismatch at the two sides of the boundaries. Manifestations of this effect include plasmonic fringes that stem from plasmon reflections and interference. Quantitative analysis and modeling of these plasmonic fringes uncovered intriguing properties and underlying physics of the EBs. Potential plasmonic applications associated with these EBs are also briefly discussed.","id":66},{"numerical_label":"102","title":"Quantum materials: The quest for ultrafast plasmonics","category":"Miscellaneous","label":"","hyperlink":"https:\/\/www.nature.com\/articles\/nnano.2016.283","authors":"D.N. Basov, M.M. Fogler","journal":"Nat. Nanotechnol.","volume":"12","issue_page":"187-188","year":"2017","pdf":"bib\/Basov2017TQF.pdf","view_link":"","abstract":"Besides graphene, the family of single-or few-layer crystalline materials today includes members as diverse as semiconductors (MoS 2 and Miscellaneous transition-metal dichalcogenides, black phosphorus), superconductors (NbSe 2, FeSe), wide-gap insulators (hexagonal-BN), topological insulators (Bi 2 Se 3) and ferromagnets (CrI 3). Many physical properties of these van der Waals materials (so-called after the weak forces that hold together individual atomic planes in these crystals) can be dynamically controlled by an applied electric field and\/or photoexcitation, providing a versatile platform for investigating quantum phenomena and for applications in electronics and photonics.Of particular interest is the exploration of plasmon polaritons\u2014hybrid light-matter modes involving the collective oscillation of mobile charges 1. In graphene, plasmon polaritons have been exploited to confine and manipulate \u2026","id":67},{"numerical_label":"101","title":"Faraday Rotation Due to Surface States in the Topological Insulator (Bi_{1\u2013x}Sb_x)_2Te_3","category":"Miscellaneous","label":"","hyperlink":"http:\/\/pubs.acs.org\/doi\/abs\/10.1021\/acs.nanolett.6b04313","authors":"Y. Shao, K.W, Post, J.S. Wu, S. Dai, A.J. Frenzel, A.R. Richardella, J.S. Lee, N. Samarth, M.M. Fogler, A.V. Balatsky, D.E. Kharzeev, D.N. Basov","journal":"Nano Lett.","volume":"17","issue_page":"980-984","year":"2017","pdf":"bib\/Shao2017FRD.pdf","view_link":"","abstract":"Using magneto-infrared spectroscopy, we have explored the charge dynamics of (Bi, Sb) 2Te3 thin films on InP substrates. From the magneto-transmission data we extracted three distinct cyclotron resonance (CR) energies that are all apparent in the broad band Faraday rotation (FR) spectra. This comprehensive FR-CR data set has allowed us to isolate the response of the bulk states from the intrinsic surface states associated with both the top and bottom surfaces of the film. The FR data uncovered that electron-and hole-type Dirac Fermions reside on opposite surfaces of our films, which paves the way for observing many exotic quantum phenomena in topological insulators.","id":68},{"numerical_label":"100","title":"Polaritons in van der Waals materials","category":"Graphene and 2D Materials; Near Field Optics","label":"","hyperlink":"http:\/\/doi.org\/10.1126\/science.aag1992","authors":"D.N. Basov, M.M. Fogler, F.J.G. de Abajo","journal":"Science","volume":"354","issue_page":"aag1992","year":"2016","pdf":"bib\/Basov2016PIV.pdf","view_link":"","abstract":"Light trapped at the nanoscale, deep below the optical wavelength, exhibits an increase in the associated electric field strength, which results in enhanced light-matter interaction. This leads to strong nonlinearities, large photonic forces, and enhanced emission and absorption probabilities. A practical approach toward nanoscale light trapping and manipulation is offered by interfaces separating media with permittivities of opposite signs. Such interfaces sustain hybrid light-matter modes involving collective oscillations of polarization charges in matter, hence the term polaritons. Surface plasmon polaritons, supported by electrons in metals, constitute a most-studied prominent example. Yet there are many other varieties of polaritons, including those formed by atomic vibrations in polar insulators, excitons in semiconductors, Cooper pairs in superconductors, and spin resonances in (anti)ferromagnets. Together, they span a broad region of the electromagnetic spectrum, ranging from microwave to ultraviolet wavelengths. We discuss polaritons in van der Waals (vdW) materials: layered systems in which individual atomic planes are bonded by weak vdW attraction (see the figure). This class of quantum materials includes graphene and other two-dimensional crystals. In artificial structures assembled from dissimilar vdW atomic layers, polaritons associated with different constituents can interact to produce unique optical effects by design.","id":69},{"numerical_label":"099","title":"Ultraconfined Plasmonic Hotspots Inside Graphene Nanobubbles","category":"Graphene and 2D Materials; Near Field Optics","label":"","hyperlink":"http:\/\/pubs.acs.org\/doi\/abs\/10.1021\/acs.nanolett.6b04076","authors":"Z. Fei, J.J. Foley IV, W. Gannett, M.K. Liu, S. Dai, G.X. Ni, A. Zettl, M.M. Fogler, G.P. Wiederrecht, S.K. Gray, D.N. Basov","journal":"Nano Lett.","volume":"16","issue_page":"7842-7848","year":"2016","pdf":"bib\/Fei2016UPH.pdf","view_link":"","abstract":"We report on a nanoinfrared (IR) imaging study of ultraconfined plasmonic hotspots inside graphene nanobubbles formed in graphene\/hexagonal boron nitride (hBN) heterostructures. The volume of these plasmonic hotspots is more than one-million-times smaller than what could be achieved by free-space IR photons, and their real-space distributions are controlled by the sizes and shapes of the nanobubbles. Theoretical analysis indicates that the observed plasmonic hotspots are formed due to a significant increase of the local plasmon wavelength in the nanobubble regions. Such an increase is attributed to the high sensitivity of graphene plasmons to its dielectric environment. Our work presents a novel scheme for plasmonic hotspot formation and sheds light on future applications of graphene nanobubbles for plasmon-enhanced IR spectroscopy.","id":70},{"numerical_label":"098","title":"Tunable plasmonic reflection by bound 1D electron states in a 2D Dirac metal","category":"Graphene and 2D Materials; Near Field Optics","label":"","hyperlink":"https:\/\/doi.org\/10.1103\/PhysRevLett.117.086801","authors":"B.Y. Jiang, G.X. Ni, C. Pan, Z. Fei, B. Cheng, C.N. Lau, M. Bockrath, D.N. Basov, M.M. Fogler","journal":"Phys. Rev. Lett.","volume":"117","issue_page":"086801","year":"2016","pdf":"bib\/Jiang2016TPR.pdf","view_link":"","abstract":"We show that the surface plasmons of a two-dimensional Dirac metal such as graphene can be reflected by linelike perturbations hosting one-dimensional electron states. The reflection originates from a strong enhancement of the local optical conductivity caused by optical transitions involving these bound states. We propose that the bound states can be systematically created, controlled, and liquidated by an ultranarrow electrostatic gate. Using infrared nanoimaging, we obtain experimental evidence for the locally enhanced conductivity of graphene induced by a carbon nanotube gate, which supports this theoretical concept.","id":71},{"numerical_label":"097","title":"Adiabatic amplification of plasmons and demons in 2D systems","category":"Graphene and 2D Materials; Near Field Optics","label":"","hyperlink":"https:\/\/doi.org\/10.1103\/PhysRevLett.117.076805","authors":"Z. Sun, D.N. Basov, M.M. Fogler","journal":"Phys. Rev. Lett.","volume":"117","issue_page":"076805","year":"2016","pdf":"bib\/Sun2016AAO.pdf","view_link":"","abstract":"We theoretically investigate charged collective modes in a two-dimensional conductor with hot electrons where the instantaneous mode frequencies gradually increase or decrease with time. We show that the loss compensation or even amplification of the modes may occur. We apply our theory to two types of collective modes in graphene, the plasmons and the energy waves, which can be probed in optical pump-probe experiments.","id":72},{"numerical_label":"096","title":"Imaging of anomalous internal reflections of hyperbolic phonon-polaritons in hexagonal boron nitride","category":"Graphene and 2D Materials; Near Field Optics","label":"","hyperlink":"http:\/\/doi.org\/10.1021\/acs.nanolett.6b01341","authors":"A.J. Giles, S. Dai, O.J. Glembocki, A.V. Kretinin, Z. Sun, C.T. Ellis, J.G. Tischler, T. Taniguchi, K. Watanabe, M.M. Fogler, K.S. Novoselov, D.N. Basov, J.D. Caldwell","journal":"Nano Lett.","volume":"16","issue_page":"3858-3865","year":"2016","pdf":"bib\/Giles2016IOA.pdf","view_link":"","abstract":"We use scanning near-field optical microscopy to study the response of hexagonal boron nitride nanocones at infrared frequencies, where this material behaves as a hyperbolic medium. The obtained images are dominated by a series of \u201chot\u201d rings that occur on the sloped sidewalls of the nanocones. The ring positions depend on the incident laser frequency and the nanocone shape. Both dependences are consistent with directional propagation of hyperbolic phonon-polariton rays that are launched at the edges and zigzag through the interior of the nanocones, sustaining multiple internal reflections off the sidewalls. Additionally, we observe a strong overall enhancement of the near-field signal at discrete resonance frequencies. These resonances attest to low dielectric losses that permit coherent standing waves of the subdiffractional polaritons to form. We comment on potential applications of such shape-dependent resonances and the field concentration at the hot rings.","id":73},{"numerical_label":"095","title":"Ultrafast optical switching of infrared plasmon polaritons in high-mobility graphene","category":"Graphene and 2D Materials; Near Field Optics","label":"","hyperlink":"https:\/\/doi.org\/10.1038\/nphoton.2016.45","authors":"Z. Fei, E.G. Iwinski, G.X. Ni, L.M. Zhang, W. Bao, A.S. Rodin, Y. Lee, M. Wagner, M.K. Liu, S. Dai, M.D. Goldflam, M. Thiemens, F. Keilmann, C.N. Lau, A.H. Castro Neto, M.M. Fogler, D.N. Basov","journal":"Nat. Photon.","volume":"10","issue_page":"244","year":"2016","pdf":"bib\/Ni2016UOS.pdf","view_link":"","abstract":"The success of metal-based plasmonics for manipulating light at the nanoscale has been empowered by imaginative designs and advanced nano-fabrication. However, the fundamental optical and electronic properties of elemental metals, the prevailing plasmonic media, are difficult to alter using external stimuli. This limitation is particularly restrictive in applications that require modification of the plasmonic response at subpicosecond timescales. This handicap has prompted the search for alternative plasmonic media1\u20133, with graphene emerging as one of the most capable candidates for infrared wavelengths. Here we visualize and elucidate the properties of non-equilibrium photo-induced plasmons in a high-mobility graphene monolayer4. We activate plasmons with femtosecond optical pulses in a specimen of graphene that otherwise lacks infrared plasmonic response at equilibrium. In combination with static nano-imaging results on plasmon propagation, our infrared pump\u2013probe nano-spectroscopy investigation reveals new aspects of carrier relaxation in heterostructures based on high-purity graphene. Graphene plasmonics has progressed rapidly, propelled by the electrical tunability, high field confinement, potentially long lifetimes of plasmons and the strong light\u2013matter interactions in graphene. An earlier spectroscopic study has reported photoinduced alteration of the plasmonic response of graphene on optical pumping. In this work, we harnessed ultrafast optical pulses to generate mid-infrared (mid-IR) plasmons in a sample that lacks a plasmonic response at equilibrium.","id":74},{"numerical_label":"094","title":"Control of excitons in multi-layer van der Waals heterostructures","category":"Graphene and 2D Materials; Excitons","label":"","hyperlink":"https:\/\/doi.org\/10.1063\/1.4943204","authors":"E.V. Calman, C.J. Dorow, M.M. Fogler, L.V. Butov, S. Hu, A. Mishchenko, A.K. Geim","journal":"Appl. Phys. Lett.","volume":"108","issue_page":"101901","year":"2016","pdf":"bib\/Calman2016COE.pdf","view_link":"","abstract":"We report an experimental study of excitons in a double quantum well van der Waals heterostructure made of atomically thin layers of MoS2 and hexagonal boron nitride. The emission of neutral and charged excitons is controlled by gate voltage, temperature, and both the helicity and the power of optical excitation.","id":75},{"numerical_label":"093","title":"Generalized spectral method for near-field optical microscopy","category":"Near Field Optics","label":"","hyperlink":"http:\/\/aip.scitation.org\/doi\/abs\/10.1063\/1.4941343","authors":"B.Y. Jiang, L.M. Zhang, A.H. Castro Neto, D.N. Basov, M.M. Fogler","journal":"J. Appl. Phys.","volume":"119","issue_page":"054305","year":"2016","pdf":"bib\/Jiang2016GSM.pdf","view_link":"","abstract":"Electromagnetic interaction between a sub-wavelength particle (the \u201cprobe\u201d) and a material surface (the \u201csample\u201d) is studied theoretically. The interaction is shown to be governed by a series of resonances corresponding to surface polariton modes localized near the probe. The resonance parameters depend on the dielectric function and geometry of the probe as well as on the surface reflectivity of the material. Calculation of such resonances is carried out for several types of axisymmetric probes: spherical, spheroidal, and pear-shaped. For spheroids, an efficient numerical method is developed, capable of handling cases of large or strongly momentum-dependent surface reflectivity. Application of the method to highly resonant materials, such as aluminum oxide (by itself or covered with graphene), reveals a rich structure of multi-peak spectra and nonmonotonic approach curves, ie, the probe \u2026","id":76},{"numerical_label":"092","title":"Plasmons in graphene moir\u00e9 superlattices","category":"Graphene and 2D Materials; Near Field Optics","label":"","hyperlink":"https:\/\/www.nature.com\/articles\/nmat4425","authors":"G.X. Ni, H. Wang, J.S. Wu, Z. Fei, M.D. Goldflam, F. Keilmann, B. \u00d6zyilmaz, A.H. Castro Neto, X.M. Xie, M.M. Fogler, D.N. Basov","journal":"Nat. Mater.","volume":"14","issue_page":"1217","year":"2015","pdf":"bib\/Ni2015PIG.pdf","view_link":"","abstract":"Moir\u00e9 patterns are periodic superlattice structures that appear when two crystals with a minor lattice mismatch are superimposed. A prominent recent example is that of monolayer graphene placed on a crystal of hexagonal boron nitride. As a result of the moir\u00e9 pattern superlattice created by this stacking, the electronic band structure of graphene is radically altered, acquiring satellite sub-Dirac cones at the superlattice zone boundaries. To probe the dynamical response of the moir\u00e9 graphene, we use infrared (IR) nano-imaging to explore propagation of surface plasmons, collective oscillations of electrons coupled to IR light. We show that interband transitions associated with the superlattice mini-bands in concert with free electrons in the Dirac bands produce two additive contributions to composite IR plasmons in graphene moir\u00e9 superstructures. This novel form of collective \u2026","id":77},{"numerical_label":"091","title":"Topological insulators are tunable waveguides for hyperbolic polaritons","category":"Graphene and 2D Materials; Near Field Optics","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.92.205430","authors":"J.S. Wu, D.N. Basov, M.M. Fogler","journal":"Phys. Rev. B","volume":"92","issue_page":"205430","year":"2015","pdf":"bib\/Wu2015TIA.pdf","view_link":"","abstract":"We present a theoretical analysis showing that layered topological insulators, for example, Bi 2 Se 3 are optically hyperbolic materials in the range of terahertz (THz) frequencies. As such, these topological insulators possess deeply subdiffractional, highly directional collective modes: hyperbolic phonon polaritons. We predict that in thin crystals the dispersion of these modes is split into discrete subbands and is strongly influenced by electron surface states. If the surface states are doped, then hybrid collective modes result from coupling of the phonon polaritons with surface plasmons. The strength of the hybridization can be controlled by an external gate that varies the chemical potential of the surface states. We also show that the momentum dependence of the plasmon-phonon coupling leads to a polaritonic analog of the Goos-H\u00e4nchen effect. The directionality of \u2026","id":78},{"numerical_label":"090","title":"Edge and surface plasmons in graphene nanoribbons","category":"Graphene and 2D Materials; Near Field Optics","label":"","hyperlink":"http:\/\/pubs.acs.org\/doi\/abs\/10.1021\/acs.nanolett.5b03834","authors":"Z. Fei, M.D. Goldflam, J.S. Wu, S. Dai, M. Wagner, A.S. McLeod, M.K. Liu, K.W. Post, S. Zhu, G.C.A.M. Janssen, M.M. Fogler, D.N. Basov","journal":"Nano Lett.","volume":"15","issue_page":"8271-8276","year":"2015","pdf":"bib\/Fei2015EAS.pdf","view_link":"","abstract":"We report on nano-infrared (IR) imaging studies of confined plasmon modes inside patterned graphene nanoribbons (GNRs) fabricated with high-quality chemical-vapor-deposited (CVD) graphene on Al2O3 substrates. The confined geometry of these ribbons leads to distinct mode patterns and strong field enhancement, both of which evolve systematically with the ribbon width. In addition, spectroscopic nanoimaging in the mid-infrared range 850\u20131450 cm\u20131 allowed us to evaluate the effect of the substrate phonons on the plasmon damping. Furthermore, we observed edge plasmons: peculiar one-dimensional modes propagating strictly along the edges of our patterned graphene nanostructures.","id":79},{"numerical_label":"089","title":"Measurement of exciton correlations using electrostatic lattices","category":"Excitons","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.92.115311","authors":"M. Remeika, J.R. Leonard, C.J. Dorow, M.M. Fogler, L.V. Butov, M. Hanson, A.C. Gossard","journal":"Phys. Rev. B","volume":"92","issue_page":"115311","year":"2015","pdf":"bib\/Remeika2015MOE.pdf","view_link":"","abstract":"We present a method for determining correlations in a gas of indirect excitons in a semiconductor quantum well structure. The method involves subjecting the excitons to a periodic electrostatic potential that causes modulations of the exciton density and photoluminescence (PL). Experimentally measured amplitudes of energy and intensity modulations of exciton PL serve as an input to a theoretical estimate of the exciton correlation parameter and temperature. We also present a proof-of-principle demonstration of the method for determining the correlation parameter and discuss how its accuracy can be improved.","id":80},{"numerical_label":"088","title":"Sum-rule constraints on the surface state conductance of topological insulators","category":"Miscellaneous","label":"","hyperlink":"https:\/\/journals.aps.org\/prl\/abstract\/10.1103\/PhysRevLett.115.116804","authors":"K.W. Post, B.C. Chapler, M.K. Liu, J.S. Wu, H.T. Stinson, M.D. Goldflam, A.R. Richardella, J.S. Lee, A.A. Reijnders, K.S. Burch, M.M. Fogler, N. Samarth, D.N. Basov","journal":"Phys. Rev. Lett.","volume":"115","issue_page":"116804","year":"2015","pdf":"bib\/Post2015SRC.pdf","view_link":"","abstract":"We report the Drude oscillator strength D and the magnitude of the bulk band gap E g of the epitaxially grown, topological insulator (Bi, Sb) 2 Te 3. The magnitude of E g, in conjunction with the model independent f-sum rule, allows us to establish an upper bound for the magnitude of D expected in a typical Dirac-like system composed of linear bands. The experimentally observed D is found to be at or below this theoretical upper bound, demonstrating the effectiveness of alloying in eliminating bulk charge carriers. Moreover, direct comparison of the measured D to magnetoresistance measurements of the same sample supports assignment of the observed low-energy conduction to topological surface states.","id":81},{"numerical_label":"087","title":"Tunneling plasmonics in bilayer graphene","category":"Graphene and 2D Materials; Near Field Optics","label":"","hyperlink":"http:\/\/pubs.acs.org\/doi\/abs\/10.1021\/acs.nanolett.5b00912","authors":"Z. Fei, E.G. Iwinski, G.X. Ni, L.M. Zhang, W. Bao, A.S. Rodin, Y. Lee, M. Wagner, M.K. Liu, S. Dai, M.D. Goldflam, M. Thiemens, F. Keilmann, C.N. Lau, A.H. Castro Neto, M.M. Fogler, D.N. Basov","journal":"Nano Lett.","volume":"15","issue_page":"4973-4978","year":"2015","pdf":"bib\/Fei2015TPI.pdf","view_link":"","abstract":"We report experimental signatures of plasmonic effects due to electron tunneling between adjacent graphene layers. At subnanometer separation, such layers can form either a strongly coupled bilayer graphene with a Bernal stacking or a weakly coupled double-layer graphene with a random stacking order. Effects due to interlayer tunneling dominate in the former case but are negligible in the latter. We found through infrared nanoimaging that bilayer graphene supports plasmons with a higher degree of confinement compared to single-and double-layer graphene, a direct consequence of interlayer tunneling. Moreover, we were able to shut off plasmons in bilayer graphene through gating within a wide voltage range. Theoretical modeling indicates that such a plasmon-off region is directly linked to a gapped insulating state of bilayer graphene, yet anMiscellaneous implication of interlayer \u2026","id":82},{"numerical_label":"086","title":"Tuning and persistent switching of graphene plasmons on a ferroelectric substrate","category":"Graphene and 2D Materials; Near Field Optics","label":"","hyperlink":"http:\/\/pubs.acs.org\/doi\/abs\/10.1021\/acs.nanolett.5b00125","authors":"M.D. Goldflam, G.X. Ni, K.W. Post, Z. Fei, Y. Yeo, J.Y. Tan, A.S. Rodin, B.C. Chapler, B. O\u0308zyilmaz, A.H. Castro Neto, M.M. Fogler, D.N. Basov","journal":"Nano Lett.","volume":"15","issue_page":"4859-4864","year":"2015","pdf":"bib\/Goldflam2015TAP.pdf","view_link":"","abstract":"We characterized plasmon propagation in graphene on thin films of the high-\u03ba dielectric PbZr0. 3Ti0. 7O3 (PZT). Significant modulation (up to\u00b175%) of the plasmon wavelength was achieved with application of ultrasmall voltages (<\u00b11 V) across PZT. Analysis of the observed plasmonic fringes at the graphene edge indicates that carriers in graphene on PZT behave as noninteracting Dirac Fermions approximated by a semiclassical Drude response, which may be attributed to strong dielectric screening at the graphene\/PZT interface. Additionally, significant plasmon scattering occurs at the grain boundaries of PZT from topographic and\/or polarization induced graphene conductivity variation in the interior of graphene, reducing the overall plasmon propagation length. Lastly, through application of 2 V across PZT, we demonstrate the capability to persistently modify the plasmonic \u2026","id":83},{"numerical_label":"085","title":"Hamiltonian optics of hyperbolic polaritons in nanogranules","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"http:\/\/pubs.acs.org\/doi\/abs\/10.1021\/acs.nanolett.5b00814","authors":"Z. Sun, \u00c1. Guti\u00e9rrez-Rubio, D.N. Basov, M.M. Fogler","journal":"Nano Lett.","volume":"15","issue_page":"4455-4460","year":"2015","pdf":"bib\/Sun2015HOO.pdf","view_link":"","abstract":"Semiclassical quantization rules and numerical calculations are applied to study polariton modes of materials whose permittivity tensor has principal values of opposite sign (so-called hyperbolic materials). The spectra of volume-and surface-confined polaritons are computed for spheroidal nanogranules of hexagonal boron nitride, a natural hyperbolic crystal. The field distribution created by polaritons excited by an external dipole source is predicted to exhibit raylike patterns due to classical periodic orbits. Near-field infrared imaging and Purcell-factor measurements are suggested to test these predictions.","id":84},{"numerical_label":"084","title":"Graphene on hexagonal boron nitride as a tunable hyperbolic metamaterial","category":"Graphene and 2D Materials; Near Field Optics","label":"","hyperlink":"https:\/\/www.nature.com\/articles\/nnano.2015.131?cacheBust=1507912270149","authors":"S. Dai, Q. Ma, M.K. Liu, T. Andersen, Z. Fei, M.D. Goldflam, M. Wagner, K. Watanabe, T. Taniguchi, M. Thiemens, F. Keilmann, G.C.A.M. Janssen, S.E. Zhu, P. Jarillo-Herrero, M.M. Fogler, D.N. Basov","journal":"Nat. Nanotechnol","volume":"10","issue_page":"682\u2013686","year":"2015","pdf":"bib\/Dai2015GOH.pdf","view_link":"","abstract":"Hexagonal boron nitride (h-BN) is a natural hyperbolic material 1, in which the dielectric constants are the same in the basal plane (\u03b5 t\u2261 \u03b5 x= \u03b5 y) but have opposite signs (\u03b5 t \u03b5 z< 0) in the normal plane (\u03b5 z) 1, 2, 3, 4. Owing to this property, finite-thickness slabs of h-BN act as multimode waveguides for the propagation of hyperbolic phonon polaritons 1, 2, 5\u2014collective modes that originate from the coupling between photons and electric dipoles 6 in phonons. However, control of these hyperbolic phonon polaritons modes has remained challenging, mostly because their electrodynamic properties are dictated by the crystal lattice of h-BN 1, 2, 7. Here we show, by direct nano-infrared imaging, that these hyperbolic polaritons can be effectively modulated in a van der Waals heterostructure 8 composed of monolayer graphene on h-BN. Tunability originates from the hybridization of surface \u2026","id":85},{"numerical_label":"083","title":"Electronic response of graphene to linelike charge perturbations","category":"Graphene and 2D Materials","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.91.235422","authors":"B.Y. Jiang, M.M. Fogler","journal":"Phys. Rev. B","volume":"91","issue_page":"235422","year":"2015","pdf":"bib\/Jiang2015ERO.pdf","view_link":"","abstract":"The problem of electrostatic screening of a charged line by undoped or weakly doped graphene is treated beyond the linear-response theory. The induced electron density is found to be approximately doping independent, n (x)\u223c x\u2212 2 log 2 x, at intermediate distances x from the charged line. At larger x, twin p\u2212 n junctions may form if the external perturbation is repulsive for graphene charge carriers. The effect of such inhomogeneities on conductance and quantum capacitance of graphene is calculated. The results are relevant for transport properties of graphene grain boundaries and for local electrostatic control of graphene with ultrathin gates.","id":86},{"numerical_label":"082","title":"Subdiffractional focusing and guiding of polaritonic rays in a natural hyperbolic material","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/www.nature.com\/articles\/ncomms7963","authors":"S. Dai, Q Ma, T. Andersen, A.S. Mcleod, Z. Fei, M.K. Liu, M. Wagner, K. Watanabe, T. Taniguchi, M. Thiemens, F. Keilmann, P. Jarillo-Herrero, M.M. Fogler, D.N. Basov","journal":"Nat. Commun.","volume":"6","issue_page":"6963","year":"2015","pdf":"bib\/Dai2015SFA.pdf","view_link":"https:\/\/www.nature.com\/articles\/ncomms7963","abstract":"Uniaxial materials whose axial and tangential permittivities have opposite signs are referred to as indefinite or hyperbolic media. In such materials, light propagation is unusual leading to novel and often non-intuitive optical phenomena. Here we report infrared nano-imaging experiments demonstrating that crystals of hexagonal boron nitride, a natural mid-infrared hyperbolic material, can act as a 'hyper-focusing lens' and as a multi-mode waveguide. The lensing is manifested by subdiffractional focusing of phonon\u2013polaritons launched by metallic disks underneath the hexagonal boron nitride crystal. The waveguiding is revealed through the modal analysis of the periodic patterns observed around such launchers and near the sample edges. Our work opens new opportunities for anisotropic layered insulators in infrared nanophotonics complementing and potentially surpassing concurrent artificial \u2026","id":87},{"numerical_label":"081","title":"Scattering of two-dimensional massless Dirac electrons by a circular potential barrier","category":"Graphene and 2D Materials","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.90.235402","authors":"J.S. Wu, M.M. Fogler","journal":"Phys. Rev. B","volume":"90","issue_page":"235402","year":"2014","pdf":"bib\/Wu2014SOT.pdf","view_link":"","abstract":"We calculate the differential, total, and transport cross-sections for scattering of two-dimensional massless Dirac electrons by a circular barrier. For scatterer of a small radius, the cross-sections are dominated by quantum effects such as resonant scattering that can be computed using the partial-wave series. Scattering by larger size barriers is better described within the classical picture of reflection and refraction of rays, which leads to phenomena of caustics, rainbow, and critical scattering. Refraction can be negative if the potential of the scatterer is repulsive, so that ap\u2212 n junction forms at its boundary. Qualitative differences of this case from the n\u2212 N doping case are examined. Quantum interference effects beyond the classical ray picture are also considered, such as normal and anomalous diffraction, and also whispering-gallery resonances. Implications of these results for transport and \u2026","id":88},{"numerical_label":"080","title":"Colloquium: Graphene spectroscopy","category":"Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1103\/RevModPhys.86.959","authors":"D.\u2009N. Basov, M.\u2009M. Fogler, A. Lanzara, F. Wang, Y. Zhang","journal":"Rev. Mod. Phys.","volume":"86","issue_page":"959","year":"2014","pdf":"bib\/Basov2014CGS.pdf","view_link":"","abstract":"Spectroscopic studies of electronic phenomena in graphene are reviewed. A variety of methods and techniques are surveyed, from quasiparticle spectroscopies (tunneling, photoemission) to methods probing density and current response (infrared optics, Raman) to scanning probe nanoscopy and ultrafast pump-probe experiments. Vast complimentary information derived from these investigations is shown to highlight unusual properties of Dirac quasiparticles and many-body interaction effects in the physics of graphene.","id":89},{"numerical_label":"079","title":"Sub-diffractional volume-confined polaritons in the natural hyperbolic material hexagonal boron nitride","category":"Graphene and 2D Materials","label":"","hyperlink":"https:\/\/www.nature.com\/articles\/ncomms6221","authors":"J.D. Caldwell, A.V. Kretinin, Y. Chen, V. Giannini, M.M. Fogler, Y. Francescato, C.T. Ellis, J.G. Tischler, C.R. Woods, A.J. Giles, M. Hong, K. Watanabe, T. Taniguchi, S.A. Maier, K.S. Novoselov","journal":"Nat. Commun.","volume":"5","issue_page":"5221","year":"2014","pdf":"bib\/Caldwell2014SDV.pdf","view_link":"https:\/\/www.nature.com\/articles\/ncomms6221","abstract":"Strongly anisotropic media, where the principal components of the dielectric tensor have opposite signs, are called hyperbolic. Such materials exhibit unique nanophotonic properties enabled by the highly directional propagation of slow-light modes localized at deeply sub-diffractional length scales. While artificial hyperbolic metamaterials have been demonstrated, they suffer from high plasmonic losses and require complex nanofabrication, which in turn induces size-dependent limitations on optical confinement. The low-loss, mid-infrared, natural hyperbolic material hexagonal boron nitride is an attractive alternative. Here we report on three-dimensionally confined 'hyperbolic polaritons' in boron nitride nanocones that support four series (up to the seventh order) modes in two spectral bands. The resonant modes obey the predicted aspect ratio dependence and exhibit high-quality factors (Q up \u2026","id":90},{"numerical_label":"078","title":"Model for quantitative tip-enhanced spectroscopy and the extraction of nanoscale-resolved optical constants","category":"Near Field Optics","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.90.085136","authors":"A.S. McLeod, P. Kelly, M.D. Goldflam, Z. Gainsforth, A.J. Westphal, G. Dominguez, M.H. Thiemens, M.M. Fogler, D.N. Basov","journal":"Phys. Rev. B","volume":"90","issue_page":"085136","year":"2014","pdf":"bib\/McLeod2014MFQ.pdf","view_link":"","abstract":"Near-field infrared spectroscopy by elastic scattering of light from a probe tip resolves optical contrasts in materials at dramatically subwavelength scales across a broad energy range, with the demonstrated capacity for chemical identification at the nanoscale. However, current models of probe-sample near-field interactions still cannot provide a sufficiently quantitatively interpretation of measured near-field contrasts, especially in the case of materials supporting strong surface phonons. We present a model of near-field spectroscopy derived from basic principles and verified by finite-element simulations, demonstrating superb predictive agreement both with tunable quantum cascade laser near-field spectroscopy of SiO 2 thin films and with newly presented nanoscale Fourier transform infrared (nanoFTIR) spectroscopy of crystalline SiC. We discuss the role of probe \u2026","id":91},{"numerical_label":"077","title":"High-temperature superfluidity with indirect excitons in van der Waals heterostructures","category":"Excitons; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/www.nature.com\/articles\/ncomms5555","authors":"M.M. Fogler, L.V. Butov, K.S. Novoselov","journal":"Nat. Commun.","volume":"54","issue_page":"555","year":"2014","pdf":"bib\/Fogler2014HTS.pdf","view_link":"https:\/\/www.nature.com\/articles\/ncomms5555","abstract":"All known superfluid and superconducting states of condensed matter are enabled by composite bosons (atoms, molecules and Cooper pairs) made of an even number of fermions. Temperatures where such macroscopic quantum phenomena occur are limited by the lesser of the binding energy and the degeneracy temperature of the bosons. High-critical temperature cuprate superconductors set the present record of~ 100 K. Here we propose a design for artificially structured materials to rival this record. The main elements of the structure are two monolayers of a transition metal dichalcogenide separated by an atomically thin spacer. Electrons and holes generated in the system would accumulate in the opposite monolayers and form bosonic bound states\u2014the indirect excitons. The resultant degenerate Bose gas of indirect excitons would exhibit macroscopic occupation of a \u2026","id":92},{"numerical_label":"076","title":"Ultrafast dynamics of surface plasmons in InAs by time-resolved infrared nanospectroscopy","category":"Near Field Optics","label":"","hyperlink":"http:\/\/pubs.acs.org\/doi\/abs\/10.1021\/nl501558t","authors":"Martin Wagner, Alexander S. McLeod, Scott J. Maddox, Zhe Fei, Mengkun Liu, Richard D. Averitt, Michael M. Fogler, Seth R. Bank, Fritz Keilmann, Dmitri N. Basov","journal":"Nano Lett.","volume":"14","issue_page":"4529-4534","year":"2014","pdf":"bib\/Wagner2014UDO.pdf","view_link":"","abstract":"We report on time-resolved mid-infrared (mid-IR) near-field spectroscopy of the narrow bandgap semiconductor InAs. The dominant effect we observed pertains to the dynamics of photoexcited carriers and associated surface plasmons. A novel combination of pump\u2013probe techniques and near-field nanospectroscopy accesses high momentum plasmons and demonstrates efficient, subpicosecond photomodulation of the surface plasmon dispersion with subsequent tens of picoseconds decay under ambient conditions. The photoinduced change of the probe intensity due to plasmons in InAs is found to exceed that of Miscellaneous mid-IR or near-IR media by 1\u20132 orders of magnitude. Remarkably, the required control pulse fluence is as low as 60 \u03bcJ\/cm2, much smaller than fluences of\u223c 1\u201310 mJ\/cm2 previously utilized in ultrafast control of near-IR plasmonics. These low excitation \u2026","id":93},{"numerical_label":"075","title":"Infrared nanospectroscopy and imaging of collective superfluid excitations in anisotropic superconductors","category":"Near Field Optics","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.90.014502","authors":"H.T. Stinson, J.S. Wu, B.Y. Jiang, Z. Fei, A.S. Rodin, B.C. Chapler, A.S. McLeod, A. Castro Neto, Y.S. Lee, M.M. Fogler, D.N. Basov","journal":"Phys. Rev. B","volume":"90","issue_page":"014502","year":"2014","pdf":"bib\/Stinson2014INA.pdf","view_link":"","abstract":"We investigate near-field infrared spectroscopy and superfluid polariton imaging experiments on conventional and unconventional superconductors. Our modeling shows that near-field spectroscopy can measure the magnitude of the superconducting energy gap in Bardeen-Cooper-Schrieffer-like superconductors with nanoscale spatial resolution. We demonstrate how the same technique can measure the c-axis plasma frequency, and thus the c-axis superfluid density, of layered unconventional superconductors with a similar spatial resolution. Our modeling also shows that near-field techniques can image superfluid surface mode interference patterns near physical and electronic boundaries. We describe how these images can be used to extract the collective mode dispersion of anisotropic superconductors with subdiffractional spatial resolution.","id":94},{"numerical_label":"074","title":"Tunable phonon polaritons in atomically thin van der Waals crystals of boron nitride","category":"Graphene and 2D Materials; Near Field Optics","label":"","hyperlink":"http:\/\/science.sciencemag.org\/content\/343\/6175\/1125.short","authors":"S. Dai, Z. Fei, Q. Ma, A.S. Rodin, M. Wagner, A.S. McLeod, M.K. Liu, W. Gannett, W. Regan, K. Watanabe, T. Taniguchi, M. Thiemens, G. Dominguez, A.H. Castro Neto, A. Zettl, F. Keilmann, P. Jarillo-Herrero, M.M. Fogler, D.N. Basov","journal":"Science","volume":"343","issue_page":"1125-1129","year":"2014","pdf":"bib\/Dai2014TPP.pdf","view_link":"","abstract":"van der Waals heterostructures assembled from atomically thin crystalline layers of diverse two-dimensional solids are emerging as a new paradigm in the physics of materials. We used infrared nanoimaging to study the properties of surface phonon polaritons in a representative van der Waals crystal, hexagonal boron nitride. We launched, detected, and imaged the polaritonic waves in real space and altered their wavelength by varying the number of crystal layers in our specimens. The measured dispersion of polaritonic waves was shown to be governed by the crystal thickness according to a scaling law that persists down to a few atomic layers. Our results are likely to hold true in Miscellaneous polar van der Waals crystals and may lead to new functionalities.","id":95},{"numerical_label":"073","title":"Ultrafast and nanoscale plasmonic phenomena in exfoliated graphene revealed by infrared pump\u2013probe nanoscopy","category":"Graphene and 2D Materials; Near Field Optics","label":"","hyperlink":"http:\/\/pubs.acs.org\/doi\/abs\/10.1021\/nl4042577","authors":"M. Wagner, Z. Fei, A.S. McLeod, A.S. Rodin, W. Bao, E.G. Iwinski, Z. Zhao, M. Goldflam, M. Liu, G. Dominguez, M. Thiemens, M.M. Fogler, A.H. Castro Neto, C.N. Lau, S. Amarie, F. Keilmann, D.N. Basov","journal":"Nano Lett.","volume":"14","issue_page":"894-900","year":"2014","pdf":"bib\/Wagner2014UAN.pdf","view_link":"","abstract":"Pump\u2013probe spectroscopy is central for exploring ultrafast dynamics of fundamental excitations, collective modes, and energy transfer processes. Typically carried out using conventional diffraction-limited optics, pump\u2013probe experiments inherently average over local chemical, compositional, and electronic inhomogeneities. Here, we circumvent this deficiency and introduce pump\u2013probe infrared spectroscopy with\u223c 20 nm spatial resolution, far below the diffraction limit, which is accomplished using a scattering scanning near-field optical microscope (s-SNOM). This technique allows us to investigate exfoliated graphene single-layers on SiO2 at technologically significant mid-infrared (MIR) frequencies where the local optical conductivity becomes experimentally accessible through the excitation of surface plasmons via the s-SNOM tip. Optical pumping at near-infrared (NIR) frequencies \u2026","id":96},{"numerical_label":"072","title":"Electronic and plasmonic phenomena at graphene grain boundaries","category":"Graphene and 2D Materials; Near Field Optics","label":"","hyperlink":"https:\/\/www.nature.com\/articles\/nnano.2013.197","authors":"Z. Fei, A.S. Rodin, W. Gannett, S. Dai, W. Regan, M. Wagner, M.K. Liu, A.S. McLeod, G. Dominguez, M. Thiemens, A.H. Castro Neto, F. Keilmann, A. Zettl, R. Hillenbrand, M.M. Fogler, D.N. Basov","journal":"Nat. Nanotechnol.","volume":"8","issue_page":"821","year":"2013","pdf":"bib\/Fei2013EAP.pdf","view_link":"","abstract":"Graphene 1, a two-dimensional honeycomb lattice of carbon atoms of great interest in (opto) electronics 2, 3 and plasmonics 4, 5, 6, 7, 8, 9, 10, 11, can be obtained by means of diverse fabrication techniques, among which chemical vapour deposition (CVD) is one of the most promising for technological applications 12. The electronic and mechanical properties of CVD-grown graphene depend in large part on the characteristics of the grain boundaries 13, 14, 15, 16, 17, 18, 19. However, the physical properties of these grain boundaries remain challenging to characterize directly and conveniently 15, 16, 17, 18, 19, 20, 21, 22, 23. Here we show that it is possible to visualize and investigate the grain boundaries in CVD-grown graphene using an infrared nano-imaging technique. We harness surface plasmons that are reflected and scattered by the graphene grain boundaries, thus causing plasmon \u2026","id":97},{"numerical_label":"071","title":"Plasmonic hot spots in triangular tapered graphene microcrystals","category":"Graphene and 2D Materials; Near Field Optics","label":"","hyperlink":"https:\/\/arxiv.org\/abs\/1309.1909","authors":"A.S. Rodin, Z. Fei, A.S. McLeod, M. Wagner, A.H. Castro Neto, M.M. Fogler, D.N. Basov","journal":"arXiv","volume":"","issue_page":"1309.1909","year":"2013","pdf":"bib\/Robin2013PHS.pdf","view_link":"","abstract":"Recently, plasmons in graphene have been observed experimentally using scattering scanning near-field optical microscopy. In this paper, we develop a simplified analytical approach to describe the behavior in triangular samples. Replacing Coulomb interaction by a short-range one reduces the problem to a Helmholtz equation, amenable to analytical treatment. We demonstrate that even with our simplifications, the system still exhibits the key features seen in the experiment.","id":98},{"numerical_label":"070","title":"Anisotropic electronic state via spontaneous phase separation in strained vanadium dioxide films","category":"Near Field Optics","label":"","hyperlink":"https:\/\/journals.aps.org\/prl\/abstract\/10.1103\/PhysRevLett.111.096602","authors":"M.K. Liu, M. Wagner, E. Abreu, S. Kittiwatanakul, A. McLeod, Z. Fei, M. Goldflam, S. Dai, M.M. Fogler, J. Lu, S.A. Wolf, R.D. Averitt, D.N. Basov","journal":"Phys. Rev. Lett.","volume":"111","issue_page":"096602","year":"2013","pdf":"bib\/Liu2013AES.pdf","view_link":"","abstract":"We resolved the enigma of anisotropic electronic transport in strained vanadium dioxide (VO 2) films by inquiring into the role that strain plays in the nanoscale phase separation in the vicinity of the insulator-to-metal transition. The root source of the anisotropy was visualized as the formation of a peculiar unidirectional stripe state which accompanies the phase transition. Furthermore, nanoscale infrared spectroscopy unveils distinct facets of electron-lattice interplay at three different stages of the phase transition. These stages include the initial formation of sparse nonpercolating metallic domains without noticeable involvement of the lattice followed by an electron-lattice coupled anisotropic stripe state close to percolation which ultimately evolves into a nearly isotropic rutile metallic phase. Our results provide a unique mesoscopic perspective for the tunable macroscopic phenomena in strained \u2026","id":99},{"numerical_label":"069","title":"Universal behavior of repulsive two-dimensional fermions in the vicinity of the quantum freezing point","category":"Miscellaneous","label":"","hyperlink":"http:\/\/iopscience.iop.org\/article\/10.1209\/0295-5075\/103\/16002\/meta","authors":"M. Babadi, B. Skinner, M.M. Fogler, E. Demler","journal":"EPL","volume":"103","issue_page":"16002","year":"2013","pdf":"bib\/Babadi2013UBO.pdf","view_link":"","abstract":"We show by a meta-analysis of the available Quantum Monte Carlo (QMC) results that two-dimensional fermions with repulsive interactions exhibit universal behavior in the strongly correlated regime, and that their freezing transition can be described using a quantum generalization of the classical Hansen-Verlet freezing criterion. We calculate the liquid-state energy and the freezing point of the 2D dipolar Fermi gas (2DDFG) using a variational method by taking ground-state wave functions of 2D electron gas (2DEG) as trial states. A comparison with the recent fixed-node diffusion Monte Carlo analysis of the 2DDFG shows that our simple variational technique captures more than $95\\% $ of the correlation energy, and predicts the freezing transition within the uncertainty bounds of QMC. Finally, we utilize the ground-state wave functions of 2DDFG as trial states and provide a variational \u2026","id":100},{"numerical_label":"068","title":"Interaction corrections to the polarization function of graphene","category":"Graphene and 2D Materials","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.86.115408","authors":"I. Sodemann, M.M. Fogler","journal":"Phys. Rev. B","volume":"86","issue_page":"115408","year":"2012","pdf":"bib\/Sodemann2012ICT.pdf","view_link":"","abstract":"The first-order interaction correction to the irreducible polarization function of pristine graphene is studied at arbitrary relation between momentum and frequency. The results are used to calculate the dielectric function and the dynamical conductivity of graphene beyond the standard random-phase approximation. The computed static dielectric constant compares favorably with recent experiments.","id":101},{"numerical_label":"067","title":"Gate-tuning of graphene plasmons revealed by infrared nano-imaging","category":"Graphene and 2D Materials; Near Field Optics","label":"","hyperlink":"https:\/\/www.nature.com\/articles\/nature11253","authors":"Z. Fei, A.S. Rodin, G.O. Andreev, W. Bao, A.S. McLeod, M. Wagner, L.M. Zhang, Z. Zhao, M. Thiemens, G. Dominguez, M.M. Fogler, A.H. Castro Neto, C.N. Lau, F. Keilmann, D.N. Basov","journal":"Nature","volume":"487","issue_page":"82","year":"2012","pdf":"bib\/Fei2012GTO.pdf","view_link":"","abstract":"Surface plasmons are collective oscillations of electrons in metals or semiconductors that enable confinement and control of electromagnetic energy at subwavelength scales 1, 2, 3, 4, 5. Rapid progress in plasmonics has largely relied on advances in device nano-fabrication 5, 6, 7, whereas less attention has been paid to the tunable properties of plasmonic media. One such medium\u2014graphene\u2014is amenable to convenient tuning of its electronic and optical properties by varying the applied voltage 8, 9, 10, 11. Here, using infrared nano-imaging, we show that common graphene\/SiO 2\/Si back-gated structures support propagating surface plasmons. The wavelength of graphene plasmons is of the order of 200 nanometres at technologically relevant infrared frequencies, and they can propagate several times this distance. We have succeeded in altering both the amplitude \u2026","id":102},{"numerical_label":"066","title":"Quenching of the quantum Hall effect in graphene with scrolled edges","category":"Graphene and 2D Materials","label":"","hyperlink":"https:\/\/journals.aps.org\/prl\/abstract\/10.1103\/PhysRevLett.108.166602","authors":"A. Cresti, M.M. Fogler, F. Guinea, A.H. Castro Neto, S. Roche","journal":"Phys. Rev. Lett.","volume":"108","issue_page":"166602","year":"2012","pdf":"bib\/Cresti2012QOT.pdf","view_link":"","abstract":"Edge nanoscrolls are shown to strongly influence transport properties of suspended graphene in the quantum Hall regime. The relatively long arclength of the scrolls in combination with their compact transverse size results in formation of many nonchiral transport channels in the scrolls. They short circuit the bulk current paths and inhibit the observation of the quantized two-terminal resistance. Unlike competing theoretical proposals, this mechanism of disrupting the Hall quantization in suspended graphene is not caused by ill-chosen placement of the contacts, singular elastic strains, or a small sample size.","id":103},{"numerical_label":"065","title":"Spontaneous coherence in a cold exciton gas","category":"Excitons","label":"","hyperlink":"https:\/\/www.nature.com\/articles\/nature10903","authors":"A.A. High, J.R. Leonard, A.T. Hammack, M.M. Fogler, L.V. Butov, A.V. Kavokin, K.L. Campman, A.C. Gossard","journal":"Nature","volume":"483","issue_page":"584","year":"2012","pdf":"bib\/High2012SCI.pdf","view_link":"","abstract":"If bosonic particles are cooled down below the temperature of quantum degeneracy, they can spontaneously form a coherent state in which individual matter waves synchronize and combine. Spontaneous coherence of matter waves forms the basis of a number of fundamental phenomena in physics, including superconductivity, superfluidity and Bose\u2013Einstein condensation 1, 2. Spontaneous coherence is the key characteristic of condensation in momentum space 3. Excitons\u2014bound pairs of electrons and holes\u2014form a model system to explore the quantum physics of cold bosons in solids 4, 5. Cold exciton gases can be realized in a system of indirect excitons, which can cool down below the temperature of quantum degeneracy owing to their long lifetimes 6. Here we report measurements of spontaneous coherence in a gas of indirect excitons. We found that \u2026","id":104},{"numerical_label":"064","title":"GraXe, graphene and xenon for neutrinoless double beta decay searches","category":"Miscellaneous","label":"","hyperlink":"http:\/\/iopscience.iop.org\/article\/10.1088\/1475-7516\/2012\/02\/037\/meta","authors":"J.J. G\u00f3mez-Cadenas, F. Guinea, M.M. Fogler, M.I. Katsnelson, J. Martin-Albo, F. Monrabal, J. Mu\u00f1oz Vidal","journal":"J. Cosmol. Astropart. Phys.       ","volume":"2012","issue_page":"037","year":"2012","pdf":"bib\/Gomez-Cadenas2012GGA.pdf","view_link":"","abstract":"The International School for Advanced Studies (SISSA) was founded in 1978 and was the first institution in Italy to promote post-graduate courses leading to a Doctor Philosophiae (or PhD) degree. A centre of excellence among Italian and international universities, the school has around 65 teachers, 100 post docs and 245 PhD students, and is located in Trieste, in a campus of more than 10 hectares with wonderful views over the Gulf of Trieste \u2026 SISSA hosts a very high-ranking, large and multidisciplinary scientific research output. The scientific papers produced by its researchers are published in high impact factor, well-known international journals, and in many cases in the world's most prestigious scientific journals such as Nature and Science. Over 900 students have so far started their careers in the field of mathematics, physics and neuroscience research at SISSA \u2026 We propose a new detector concept, GraXe (to \u2026","id":105},{"numerical_label":"063","title":"Two-dimensional electrostatic lattices for indirect excitons","category":"Excitons","label":"","hyperlink":"http:\/\/aip.scitation.org\/doi\/abs\/10.1063\/1.3682302","authors":"M. Remeika, M.M. Fogler, L.V. Butov, M. Hanson, A.C. Gossard","journal":"Appl. Phys. Lett.","volume":"100","issue_page":"061103","year":"2012","pdf":"bib\/Remeika2012TDE.pdf","view_link":"","abstract":"We report on a method for the realization of two-dimensional electrostatic lattices for excitons using patterned interdigitated electrodes. Lattice structure is set by the electrode pattern and depth of the lattice potential is controlled by applied voltages. We demonstrate square, hexagonal, and honeycomb lattices created by this method.","id":106},{"numerical_label":"062","title":"Near-field spectroscopy of silicon dioxide thin films","category":"Near Field Optics","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.85.075419","authors":"L.M. Zhang, G.O. Andreev, Z. Fei, A.S. McLeod, G. Dominguez, M. Thiemens, A.H. Castro Neto, D.N. Basov, M.M. Fogler","journal":"Phys. Rev. B","volume":"85","issue_page":"075419","year":"2011","pdf":"bib\/Zhang2011NFS.pdf","view_link":"","abstract":"We analyze the results of scanning near-field infrared spectroscopy performed on thin films of a-SiO 2 on Si substrate. The measured near-field signal exhibits surface-phonon resonances whose strength has a prominent thickness dependence in the range from 2 to 300 nm. These observations are compared with calculations in which the tip of the near-field infrared spectrometer is modeled either as a point dipole or an elongated spheroid. The latter model accounts for the antenna effect of the tip and gives a better agreement with the experiment. Possible applications of the near-field technique for depth profiling of layered nanostructures are discussed.","id":107},{"numerical_label":"061","title":"Model of large volumetric capacitance in graphene supercapacitors based on ion clustering","category":"Graphene and 2D Materials","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.84.235133","authors":"B. Skinner, M.M. Fogler, B.I. Shklovskii","journal":"Phys. Rev. B","volume":"84","issue_page":"235133","year":"2011","pdf":"bib\/Skinner2011AMO.pdf","view_link":"","abstract":"Electric double-layer supercapacitors (SCs) are promising devices for high-power energy storage based on the reversible absorption of ions into porous conducting electrodes. Graphene is a particularly good candidate for the electrode material in SCs due to its high conductivity and large surface area. In this paper, we consider SC electrodes made from a stack of graphene sheets with randomly inserted spacer molecules. We show that the large volumetric capacitances C\u2273 100 F\/cm 3 observed experimentally can be understood as a result of collective intercalation of ions into the graphene stack and the accompanying nonlinear screening by graphene electrons that renormalizes the charge of the ion clusters.","id":108},{"numerical_label":"060","title":"Infrared nanoscopy of Dirac plasmons at the graphene\u2013SiO_2 interface","category":"Graphene and 2D Materials","label":"","hyperlink":"http:\/\/pubs.acs.org\/doi\/abs\/10.1021\/nl202362d","authors":"Z. Fei, G.O. Andreev, W. Bao, L.M. Zhang, A.S. McLeod, C. Wang, M.K. Stewart, Z. Zhao, G. Dominguez, M. Thiemens, M.M. Fogler, M.J. Tauber, A.H. Castro Neto, C.N. Lau, F. Keilmann, D. N. Basov","journal":"Nano Lett.","volume":"11","issue_page":"4701-4705","year":"2011","pdf":"bib\/Fei2011INO.pdf","view_link":"","abstract":"We report on infrared (IR) nanoscopy of 2D plasmon excitations of Dirac fermions in graphene. This is achieved by confining mid-IR radiation at the apex of a nanoscale tip: an approach yielding 2 orders of magnitude increase in the value of in-plane component of incident wavevector q compared to free space propagation. At these high wavevectors, the Dirac plasmon is found to dramatically enhance the near-field interaction with mid-IR surface phonons of SiO2 substrate. Our data augmented by detailed modeling establish graphene as a new medium supporting plasmonic effects that can be controlled by gate voltage.","id":109},{"numerical_label":"059","title":"Hopping transport in systems of finite thickness or length","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.84.125447","authors":"A.S. Rodin, M.M. Fogler","journal":"Phys. Rev. B","volume":"84","issue_page":"125447","year":"2011","pdf":"bib\/Robin2011HTI.pdf","view_link":"","abstract":"Variable-range hopping transport along short one-dimensional wires and across the shortest dimension of thin three-dimensional films and narrow two-dimensional ribbons is studied theoretically. Geometric and transport characteristics of the hopping resistor network are shown to depend on temperature T and the dimensionality of the system. In two and three dimensions, the usual Mott law applies at high T where the correlation length of the network is smaller than the sample thickness. As T decreases, the network breaks into sparse filamentary paths while the Mott law changes to a different T dependence, which is derived using the percolation theory methods. In one dimension, deviations from the Mott law are known to exist at all temperatures because of rare fluctuations. The evolution of such fluctuations from highly resistive \u201cbreaks\u201d at high T to highly conducting \u201cshorts\u201d at low T is \u2026","id":110},{"numerical_label":"058","title":"Integer quantum Hall effect in trilayer graphene","category":"Graphene and 2D Materials","label":"","hyperlink":"https:\/\/journals.aps.org\/prl\/abstract\/10.1103\/PhysRevLett.107.126806","authors":"A. Kumar, W. Escoffier, J.M. Poumirol, C. Faugeras, D.P. Arovas, M.M. Fogler, F. Guinea, S. Roche, M. Goiran, B. Raquet","journal":"Phys. Rev. Lett.","volume":"107","issue_page":"126806","year":"2011","pdf":"bib\/Kumar2011IQH.pdf","view_link":"","abstract":"By using high-magnetic fields (up to 60 T), we observe compelling evidence of the integer quantum Hall effect in trilayer graphene. The magnetotransport fingerprints are similar to those of the graphene monolayer, except for the absence of a plateau at a filling factor of \u03bd= 2. At a very low filling factor, the Hall resistance vanishes due to the presence of mixed electron and hole carriers induced by disorder. The measured Hall resistivity plateaus are well reproduced theoretically, using a self-consistent Hartree calculations of the Landau levels and assuming an ABC stacking order of the three layers.","id":111},{"numerical_label":"057","title":"Microwaving and stirring the quantum Hall spaghetti","category":"Miscellaneous","label":"","hyperlink":"http:\/\/physics.aps.org\/articles\/v4\/39?referer=rss","authors":"M.M. Fogler","journal":"Physics","volume":"4","issue_page":"39","year":"2011","pdf":"bib\/Fogler2011MAS.pdf","view_link":"http:\/\/physics.aps.org\/articles\/v4\/39?referer=rss","abstract":"Figure 1: Quantum Hall stripes (dark wavy lines) are perturbed by surface acoustic waves propagating with momentum q (bottom arrow). A microwave field of frequency \u03c9 (red arrows) excites magnetophonons. Photoluminescence (vertical wavy lines) signals how much energy is absorbed as a function of \u03c9 and q. Quantum Hall stripes (dark wavy lines) are perturbed by surface acoustic waves propagating with momentum q (bottom arrow). A microwave field of frequency \u03c9 (red arrows) excites magnetophonons. Photoluminescence (vertical wavy lines) signals how much... Show more","id":112},{"numerical_label":"056","title":"Magnetoelectric coupling, Berry phase, and Landau level dispersion in a biased bilayer graphene","category":"Graphene and 2D Materials","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.84.075451","authors":"L.M. Zhang, M.M. Fogler, D.P. Arovas","journal":"Phys. Rev. B","volume":"84","issue_page":"075451","year":"2010","pdf":"bib\/Zhang2010MCB.pdf","view_link":"","abstract":"We study the energy spectrum of a graphene bilayer in the presence of transverse electric and magnetic fields. We find that the resulting Landau levels exhibit a nonmonotonic dependence on the electric field, as well as numerous level crossings. This behavior is explained using quasiclassical quantization rules that properly take into account the pseudospin of the quasiparticles. The pseudospin generates the Berry phase, which leads to a shift in energy quantization and results in a pseudo-Zeeman effect. The latter depends on the electric field, alternates in sign among the two valleys, and also reduces the band gap. Analytic formulas for Miscellaneous pseudospin-related quantities, such as the anomalous Hall conductivity, are derived and compared with prior theoretical work.","id":113},{"numerical_label":"055","title":"Comment on \u201cScreening in gated bilayer graphene\u201d","category":"Graphene and 2D Materials","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.82.197401","authors":"M.M. Fogler, E. McCann","journal":"Phys. Rev. B","volume":"82","issue_page":"197401","year":"2010","pdf":"bib\/Fogler2010COS.pdf","view_link":"","abstract":"We analyze the response of bilayer graphene to an external transverse electric field using a variational method. A previous attempt to do so in a recent paper by Falkovsky [Phys. Rev. B 80, 113413 (2009)] is shown to be flawed. Our calculation reaffirms the original results obtained by one of us [E. McCann, Phys. Rev. B 74, 161403(R) (2006)] by a different method. Finally, we generalize these original results to describe a dual-gated bilayer graphene device.","id":114},{"numerical_label":"054","title":"Simple variational method for calculating energy and quantum capacitance of an electron gas with screened interactions","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.82.201306","authors":"B. Skinner, M.M. Fogler","journal":"Phys. Rev. B","volume":"82","issue_page":"201306","year":"2010","pdf":"bib\/Skinner2010SVM.pdf","view_link":"","abstract":"We describe a variational procedure for calculating the energy of an electron gas in which the long-range Coulomb interaction is truncated, for example by the screening effect of a nearby metallic gate. We use this procedure to compute the quantum capacitance of a two-dimensional electron gas next to a metallic gate as a function of electron density and spin polarization. The accuracy of the method is verified against published Monte Carlo data. The results compare favorably with a recent experiment.","id":115},{"numerical_label":"053","title":"Apparent power-law behavior of conductance in disordered quasi-one-dimensional systems","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/journals.aps.org\/prl\/abstract\/10.1103\/PhysRevLett.105.106801","authors":"A.S. Rodin, M.M. Fogler","journal":"Phys. Rev. Lett.","volume":"105","issue_page":"106801","year":"2010","pdf":"bib\/Robin2010APL.pdf","view_link":"","abstract":"The dependence of hopping conductance on temperature and voltage for an ensemble of modestly long one-dimensional wires is studied numerically using the shortest-path algorithm. In a wide range of parameters this dependence can be approximated by a power law rather than the usual stretched-exponential form. The relation to recent experiments and prior analytical theory is discussed.","id":116},{"numerical_label":"052","title":"Effect of external conditions on the structure of scrolled graphene edges","category":"Graphene and 2D Materials","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.81.161408","authors":"M.M. Fogler, A.H. Castro Neto, F. Guinea","journal":"Phys. Rev. B","volume":"81","issue_page":"161408","year":"2010","pdf":"bib\/Fogler2010EOE.pdf","view_link":"","abstract":"Characteristic dimensions of carbon nanoscrolls\u2014\u201cbuckyrolls\u201d\u2014are calculated by analyzing the competition between elastic, van der Waals, and electrostatic energies for representative models of suspended and substrate-deposited graphene samples. The results are consistent with both atomistic simulations and experimental observations of scrolled graphene edges. Electrostatic control of the wrapping is shown to be practically feasible and its possible device applications are indicated.","id":117},{"numerical_label":"051","title":"Singular elastic strains and magnetoconductance of suspended graphene","category":"Graphene and 2D Materials","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.81.161402","authors":"Elsa Prada, Pablo San-Jose, Gladys Le\u00f3n, Michael M. Fogler, Francisco Guinea","journal":"Phys. Rev. B","volume":"81","issue_page":"161402","year":"2010","pdf":"bib\/Prada2010SES.pdf","view_link":"","abstract":"Graphene membranes suspended off electric contacts or Miscellaneous rigid supports are prone to elastic strain, which is concentrated at the edges and corners of the samples. Such a strain leads to an algebraically varying effective magnetic field that can reach a few Tesla in submicron wide flakes. In the quantum Hall regime the interplay of the effective and the physical magnetic fields causes backscattering of the chiral edge channels, which can destroy the quantized conductance plateaus.","id":118},{"numerical_label":"050","title":"Neutrality point of graphene with coplanar charged impurities","category":"Graphene and 2D Materials","label":"","hyperlink":"https:\/\/journals.aps.org\/prl\/abstract\/10.1103\/PhysRevLett.103.236801","authors":"M.M. Fogler","journal":"Phys. Rev. Lett.","volume":"103","issue_page":"236801","year":"2009","pdf":"bib\/Fogler2009NPO.pdf","view_link":"","abstract":"The ground state and the transport properties of graphene subject to the potential of in-plane charged impurities are studied. The screening of the impurity potential is shown to be nonlinear, producing a fractal structure of electron and hole puddles. Statistical properties of this density distribution as well as the charge compressibility of the system are calculated in the leading-log approximation. The conductivity depends logarithmically on \u03b1, the dimensionless strength of the Coulomb interaction. The theory is asymptotically exact when \u03b1 is small, which is the case for graphene on a substrate with a high dielectric constant.","id":119},{"numerical_label":"049","title":"Numerical studies of variable-range hopping in one-dimensional systems","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.80.155435","authors":"A.S. Rodin, M.M. Fogler","journal":"Phys. Rev. B","volume":"80","issue_page":"155435","year":"2009","pdf":"bib\/Rodin2009NSO.pdf","view_link":"","abstract":"Hopping transport in a one-dimensional system is studied numerically. A fast algorithm is devised to find the lowest-resistance path at arbitrary electric field. Probability distribution functions of individual resistances on the path and the net resistance are calculated and fitted to compact analytic formulas. Qualitative differences between statistics of resistance fluctuations in Ohmic and non-Ohmic regimes are elucidated. The results are compared with prior theoretical and experimental work on the subject.","id":120},{"numerical_label":"048","title":"Trapping indirect excitons in a GaAs quantum-well structure with a diamond-shaped electrostatic trap","category":"Excitons","label":"","hyperlink":"https:\/\/journals.aps.org\/prl\/abstract\/10.1103\/PhysRevLett.103.087403","authors":"A.A. High, A.K. Thomas, G. Grosso, M. Remeika, A.T. Hammack, A.D. Meyertholen, M.M. Fogler, L.V. Butov, M. Hanson, A.C. Gossard","journal":"Phys. Rev. Lett.","volume":"103","issue_page":"087403","year":"2009","pdf":"bib\/High2009TIE.pdf","view_link":"","abstract":"We report on the principle and realization of a new trap for excitons\u2014the diamond electrostatic trap\u2014which uses a single electrode to create a confining potential for excitons. We also create elevated diamond traps which permit evaporative cooling of the exciton gas. We observe the collection of excitons towards the trap center with increasing exciton density. This effect is due to screening of disorder in the trap by the excitons. As a result, the diamond trap behaves as a smooth parabolic potential which realizes a cold and dense exciton gas at the trap center.","id":121},{"numerical_label":"047","title":"Localization-delocalization transition of indirect excitons in lateral electrostatic lattices","category":"Excitons","label":"","hyperlink":"https:\/\/journals.aps.org\/prl\/abstract\/10.1103\/PhysRevLett.102.186803","authors":"M. Remeika, J.C. Graves, A.T. Hammack, A.D. Meyertholen, M.M. Fogler, L.V. Butov, M. Hanson, A.C. Gossard","journal":"Phys. Rev. Lett.","volume":"102","issue_page":"186803","year":"2009","pdf":"bib\/Remeika2009LDT.pdf","view_link":"","abstract":"We study transport of indirect excitons in GaAs\/AlGaAs coupled quantum wells in linear lattices created by laterally modulated gate voltage. The localization-delocalization transition for transport across the lattice was observed with reducing lattice amplitude or increasing exciton density. The exciton interaction energy at the transition is close to the lattice amplitude. These results are consistent with the model, which attributes the localization-delocalization transition to the interaction-induced percolation of the exciton gas through the external potential. We also discuss applications of the lattice potentials for estimating the strength of disorder and exciton interaction.","id":122},{"numerical_label":"046","title":"Biexcitons in two-dimensional systems with spatially separated electrons and holes","category":"Excitons","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.78.235307","authors":"A.D. Meyertholen, M.M. Fogler","journal":"Phys. Rev. B","volume":"78","issue_page":"235307","year":"2008","pdf":"bib\/Meyertholen2008BIT.pdf","view_link":"","abstract":"The binding energy and wave functions of two-dimensional indirect biexcitons are studied analytically and numerically. It is proven that stable biexcitons exist only when the distance between electron and hole layers is smaller than a certain critical threshold. Numerical results for the biexciton binding energies are obtained using the stochastic variational method and compared with the analytical asymptotics. The threshold interlayer separation and its uncertainty are estimated. The results are compared with those obtained by Miscellaneous techniques, in particular, the diffusion Monte Carlo method and the Born-Oppenheimer approximation.","id":123},{"numerical_label":"045","title":"Determination of the electronic structure of bilayer graphene from infrared spectroscopy","category":"Graphene and 2D Materials","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.78.235408","authors":"L.M. Zhang, Z.Q. Li, D.N. Basov, M.M. Fogler, Z. Hao, M.C. Martin","journal":"Phys. Rev. B","volume":"78","issue_page":"235408","year":"2008","pdf":"bib\/Zhang2008DOT.pdf","view_link":"","abstract":"We present an experimental study of the infrared conductivity, transmission, and reflection of a gated bilayer graphene and their theoretical analysis within the Slonczewski-Weiss-McClure (SWMc) model. The infrared response is shown to be governed by the interplay of the interband and the intraband transitions among the four bands of the bilayer. The position of the main conductivity peak at the charge-neutrality point is determined by the interlayer tunneling frequency. The shift of this peak as a function of the gate voltage gives information about less known parameters of the SWMc model such as those responsible for the electron-hole and sublattice asymmetries. These parameter values are shown to be consistent with recent electronic structure calculations for the bilayer graphene and the SWMc parameters commonly used for the bulk graphite.","id":124},{"numerical_label":"044","title":"Pseudomagnetic fields and ballistic transport in a suspended graphene sheet","category":"Graphene and 2D Materials","label":"","hyperlink":"https:\/\/journals.aps.org\/prl\/abstract\/10.1103\/PhysRevLett.101.226804","authors":"M.M. Fogler, F. Guinea, M.I. Katsnelson","journal":"Phys. Rev. Lett.","volume":"101","issue_page":"226804","year":"2008","pdf":"bib\/Fogler2008PFA.pdf","view_link":"","abstract":"We study a suspended graphene sheet subject to the electric field of a gate underneath. We compute the elastic deformation of the sheet and the corresponding effective gauge field, which modifies the electronic transport. In a clean system the two-terminal conductance of the sample is reduced below the ballistic limit and is almost totally suppressed at low carrier concentrations in samples under tension. Residual disorder restores a small finite conductivity.","id":125},{"numerical_label":"043","title":"Effect of spatial resolution on the estimates of the coherence length of excitons in quantum wells","category":"Excitons","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.78.035411","authors":"M.M. Fogler, S. Yang, A.T. Hammack, L.V. Butov, A.C. Gossard","journal":"Phys. Rev. B","volume":"78","issue_page":"035411","year":"2008","pdf":"bib\/Fogler2008EOS.pdf","view_link":"","abstract":"We evaluate the effect of diffraction-limited resolution of the optical system on the estimates of the coherence length of two-dimensional excitons deduced from the interferometric study of the exciton emission. The results are applied for refining our earlier estimates of the coherence length of a cold gas of indirect excitons in coupled quantum wells [S. Yang et al., Phys. Rev. Lett. 97, 187402 (2006)]. We show that the apparent coherence length is well approximated by the quadratic sum of the actual exciton coherence length and the diffraction correction given by the conventional Abbe limit divided by \u03c0. In practice, accounting for diffraction is necessary only when the coherence length is smaller than about one wavelength. The earlier conclusions regarding the strong enhancement of the exciton coherence length at low temperatures remain intact.","id":126},{"numerical_label":"042","title":"Effect of disorder on a graphene p-n junction","category":"Graphene and 2D Materials","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.77.075420","authors":"M.M. Fogler, D.S. Novikov, L. Glazman, B.I. Shklovskii","journal":"Phys. Rev. B","volume":"77","issue_page":"075420","year":"2008","pdf":"bib\/Fogler2008EOD.pdf","view_link":"","abstract":"We propose the theory of transport in a gate-tunable graphene p\u2212 n junction, in which the gradient of the carrier density is controlled by the gate voltage. Depending on this gradient and on the density of charged impurities, the junction resistance is dominated by either diffusive or ballistic contribution. We find the conditions for observing ballistic transport and show that in existing devices they are satisfied only marginally. We also simulate numerically the trajectories of charge carriers and illustrate challenges in realizing more delicate ballistic effects, such as Veselago lensing.","id":127},{"numerical_label":"041","title":"Nonlinear screening and ballistic transport in a graphene p-n junction","category":"Graphene and 2D Materials","label":"","hyperlink":"https:\/\/journals.aps.org\/prl\/abstract\/10.1103\/PhysRevLett.100.116804","authors":"L. M. Zhang, M.M. Fogler","journal":"Phys. Rev. Lett.","volume":"100","issue_page":"116804","year":"2007","pdf":"bib\/Zhang2007NSA.pdf","view_link":"","abstract":"We study the charge density distribution, the electric field profile, and the resistance of an electrostatically created lateral p\u2212 n junction in graphene. We show that the electric field at the interface of the electron and hole regions is strongly enhanced due to limited screening capacity of Dirac quasiparticles. Accordingly, the junction resistance is lower than estimated in previous literature.","id":128},{"numerical_label":"040","title":"Screening of a hypercritical charge in graphene","category":"Graphene and 2D Materials","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.76.233402","authors":"M.M. Fogler, D.S. Novikov, B.I. Shklovskii","journal":"Phys. Rev. B","volume":"76","issue_page":"233402","year":"2007","pdf":"bib\/Fogler2007SOA.pdf","view_link":"","abstract":"Screening of a large external charge in graphene is studied. The charge is assumed to be displaced away or smeared over a finite region of the graphene plane. The initial decay of the screened potential with distance is shown to follow the 3\u2215 2 power. It gradually changes to the Coulomb law outside of a hypercritical core whose radius is proportional to the external charge.","id":129},{"numerical_label":"039","title":"Concentration-dependent mobility in organic field-effect transistors probed by infrared spectromicroscopy of the charge density profile","category":"Miscellaneous","label":"","hyperlink":"http:\/\/aip.scitation.org\/doi\/abs\/10.1063\/1.2745223","authors":"A.D. Meyertholen, Z.Q. Li, D.N. Basov, M.M. Fogler, M.C. Martin, G.M. Wang, A.S. Dhoot, D. Moses, A.J. Heeger.","journal":"Appl. Phys. Lett.","volume":"90","issue_page":"222108","year":"2007","pdf":"bib\/Meyertholen2007CDM.pdf","view_link":"","abstract":"The authors show that infrared imaging of the charge density profile in organic field-effect transistors (FETs) can probe transport characteristics which are difficult to access by conventional contact-based measurements. Specifically, they carry out experiments and modeling of infrared spectromicroscopy of poly (3-hexylthiophene)(P3HT) FETs in which charge injection is affected by a relatively low resistance of the gate insulators. They conclude that the mobility of P3HT has a power-law density dependence, which is consistent with the activated transport in disorder-induced tails of the density of states.","id":130},{"numerical_label":"038","title":"Coherence length of cold exciton gases in coupled quantum wells","category":"Excitons","label":"","hyperlink":"https:\/\/journals.aps.org\/prl\/abstract\/10.1103\/PhysRevLett.97.187402","authors":"S. Yang, A.T. Hammack, M.M. Fogler, L.V. Butov, A.C. Gossard","journal":"Phys. Rev. Lett.","volume":"97","issue_page":"187402","year":"2006","pdf":"bib\/Yang2006CLO.pdf","view_link":"","abstract":"A Mach-Zehnder interferometer with spatial and spectral resolution was used to probe spontaneous coherence in cold exciton gases, which are implemented experimentally in the ring of indirect excitons in coupled quantum wells. A strong enhancement of the exciton coherence length is observed at temperatures below a few Kelvin. The increase of the coherence length is correlated with the macroscopic spatial ordering of excitons. The coherence length at the lowest temperature corresponds to a very narrow spread of the exciton momentum distribution, much smaller than that for a classical exciton gas.","id":131},{"numerical_label":"037","title":"Scanned gate microscopy of a one-dimensional quantum dot","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"http:\/\/pubs.acs.org\/doi\/abs\/10.1021\/nl061445+","authors":"L.M. Zhang, M.M. Fogler","journal":"Nano Lett.","volume":"6","issue_page":"2206-2210","year":"2006","pdf":"bib\/Zhang2006SGM.pdf","view_link":"","abstract":"We analyze electrostatic interaction between a sharp conducting tip and a thin one-dimensional wire, eg, a carbon nanotube, in a scanned gate microscopy (SGM) experiment. The problem is analytically tractable if the wire resides on a thin dielectric substrate above a metallic backgate. The characteristic spatial scale of the electrostatic coupling to the tip is equal to its height above the substrate. Numerical simulations indicate that imaging of individual electrons by SGM is possible once the mean electron separation exceeds this scale (typically, a few tens of nm). Differences between weakly and strongly invasive SGM regimes are pointed out.","id":132},{"numerical_label":"036","title":"Coulomb blockade and transport in a chain of one-dimensional quantum dots","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/journals.aps.org\/prl\/abstract\/10.1103\/PhysRevLett.97.096601","authors":"M.M. Fogler, S.V. Malinin, T. Nattermann","journal":"Phys. Rev. Lett.","volume":"97","issue_page":"096601","year":"2006","pdf":"bib\/Fogler2006CBA.pdf","view_link":"","abstract":"A long one-dimensional wire with a finite density of strong random impurities is modeled as a chain of weakly coupled quantum dots. At low temperature T and applied voltage V its resistance is limited by breaks: randomly occurring clusters of quantum dots with a special length distribution pattern that inhibit the transport. Because of the interplay of interaction and disorder effects the resistance can exhibit T and V dependences that can be approximated by power laws. The corresponding two exponents differ greatly from each Miscellaneous and depend not only on the intrinsic electronic parameters but also on the impurity distribution statistics.","id":133},{"numerical_label":"035","title":"Effects of interactions and disorder on the compressibility of two-dimensional electron and hole systems","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/www.sciencedirect.com\/science\/article\/pii\/S1386947706002098","authors":"E.A. Galaktionov, G.D. Allison, M.M. Fogler, A.K. Savchenko, S.S. Safonov, M.Y. Simmons, D.A. Ritchie","journal":"Physica E.","volume":"34","issue_page":"240-243","year":"2006","pdf":"bib\/Galaktionov2006EOI.pdf","view_link":"","abstract":"The compressibility \u03c7 of dilute two-dimensional electron and hole gases in GaAs semiconductor structures has been studied in the ranges of the interaction parameter rs= 1\u20132.5 and rs= 10\u201330 for the electron and hole system, respectively. Nonmonotonic dependence of \u03c7-1 with an upturn at low carrier densities is observed. Despite the large difference in rs the behavior of \u03c7-1 in both systems can be accurately described by the theory of nonlinear screening of disorder by the carriers.","id":134},{"numerical_label":"034","title":"Thermodynamic density of states of two-dimensional GaAs systems near the apparent metal-insulator transition","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/journals.aps.org\/prl\/abstract\/10.1103\/PhysRevLett.96.216407","authors":"G. Allison, E.A. Galaktionov, A.K. Savchenko, S.S. Safonov, M.M. Fogler, M.Y. Simmons, D.A. Ritchie","journal":"Phys. Rev. Lett.","volume":"96","issue_page":"216407","year":"2006","pdf":"bib\/Allison2006TDO.pdf","view_link":"","abstract":"We perform combined resistivity and compressibility studies of two-dimensional hole and electron systems which show the apparent metal-insulator transition\u2014a crossover in the sign of\u2202 R\/\u2202 T with changing density. No thermodynamic anomalies have been detected in the crossover region. Instead, despite a tenfold difference in rs, the compressibility of both electrons and holes is well described by the theory of nonlinear screening of the random potential. We show that the resistivity exhibits a scaling behavior near the percolation threshold found from analysis of the compressibility. Notably, the percolation transition occurs at a much lower density than the crossover.","id":135},{"numerical_label":"033","title":"Effects of interactions and disorder on the compressibility of two\u2010dimensional electron and hole systems","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"http:\/\/onlinelibrary.wiley.com\/doi\/10.1002\/pssc.200562760\/full","authors":"A.K. Savchenko, E.A. Galaktionov, G.D. Allison, M.M. Fogler, S.S. Safonov, M.Y. Simmons, D.A. Ritchie","journal":"Phys. Stat. Solidi (C)","volume":"3","issue_page":"343-346","year":"2006","pdf":"bib\/Savchenko2006EQI.pdf","view_link":"","abstract":"We have studied the thermodynamic compressibility of 2D electron and 2D hole gases near the apparent metal-insulator transition, in the ranges of the interaction parameter rs= 1\u20132.5 and rs= 10\u201330, respectively. Nonmonotonic dependence of the inverse compressibility on the carrier density is observed with an upturn at low densities. Despite the large difference in rs, this behaviour of inverse compressibility in both electron and hole systems can be quantitatively accounted for by a nonlinear screening of disorder by the carriers, in particular, by the disorder-driven proliferation of depletion regions in the 2D channel.(\u00a9 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)","id":136},{"numerical_label":"032","title":"Spin exchange in quantum rings and wires in the Wigner-crystal limit","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"http:\/\/iopscience.iop.org\/article\/10.1088\/0953-8984\/18\/1\/L02\/meta","authors":"M.M. Fogler, E. Pivovarov","journal":"J. Phys. Condens. Matter","volume":"18","issue_page":"L7","year":"2006","pdf":"bib\/Fogler2006SEI.pdf","view_link":"","abstract":"We present a controlled method for computing the exchange coupling in strongly correlated one-dimensional electron systems. It is based on the asymptotically exact relation between the exchange constant and the pair-correlation function of spinless electrons. Explicit results are obtained for thin quantum rings with realistic Coulomb interactions, by calculating this function via a many-body instanton approach.","id":137},{"numerical_label":"031","title":"Exchange interaction in quantum rings and wires in the Wigner-crystal limit","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.72.195344","authors":"M.M. Fogler, E. Pivovarov","journal":"Phys. Rev. B","volume":"72","issue_page":"195344","year":"2005","pdf":"bib\/Fogler2005EII.pdf","view_link":"","abstract":"We present a controlled method for computing the exchange coupling in correlated one-dimensional electron systems based on the relation between the exchange constant and the pair-correlation function of spinless electrons. This relation is valid in several independent asymptotic regimes, including the low-electron-density case, under the general condition of a strong spin-charge separation. Explicit formulas for the exchange constant are obtained for thin quantum rings and wires with realistic Coulomb interactions by calculating the pair-correlation function via a many-body instanton approach. A remarkably smooth interpolation between high-and low-electron-density results is shown to be possible. These results are applicable to the case of one-dimensional wires of intermediate width as well. Our method can be easily generalized to Miscellaneous interaction laws, such as the inverse distance squared \u2026","id":138},{"numerical_label":"030","title":"Non-ohmic variable-range hopping transport in one-dimensional conductors","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/journals.aps.org\/prl\/abstract\/10.1103\/PhysRevLett.95.166604","authors":"M.M. Fogler, R.S. Kelley","journal":"Phys. Rev. Lett.","volume":"95","issue_page":"166604","year":"2005","pdf":"bib\/Fogler2005NOV.pdf","view_link":"","abstract":"We investigate theoretically the effect of a finite electric field on the resistivity of a disordered one-dimensional system in the variable-range hopping regime. We find that at low fields the transport is inhibited by rare fluctuations in the random distribution of localized states that create high-resistance breaks in the hopping network. As the field increases, the breaks become less resistive. In strong fields the breaks are overrun and the electron distribution function is driven far from equilibrium. The logarithm of the resistance initially shows a simple exponential drop with the field, followed by a logarithmic dependence, and finally, by an inverse square-root law.","id":139},{"numerical_label":"029","title":"Short-range correlations and spin-mode velocities in ultrathin one-dimensional conductors","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.71.161304","authors":"M.M. Fogler","journal":"Phys. Rev. B","volume":"71","issue_page":"161304","year":"2005","pdf":"bib\/Fogler2005SRC.pdf","view_link":"","abstract":"In ultrathin wires positioned on high-\u03ba dielectric substrates or nearby metallic gates, electrons can form strongly correlated one-dimensional fluids already at rather high electron densities. The density-density correlation function, charge compressibility, spin susceptibility, and electron specific heat of such fluids are calculated analytically. The results are relevant for transport and thermodynamics of carbon nanotube field-effect transistors and semiconductor quantum wires.","id":140},{"numerical_label":"028","title":"Ground-state energy of the electron liquid in ultrathin wires","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/journals.aps.org\/prl\/abstract\/10.1103\/PhysRevLett.94.056405","authors":"Michael M. Fogler","journal":"Phys. Rev. Lett.","volume":"94","issue_page":"056405","year":"2005","pdf":"bib\/Fogler2005GSE.pdf","view_link":"","abstract":"The ground-state energy and the density correlation function of the electron liquid in a thin one-dimensional wire are computed. The calculation is based on an approximate mapping of the problem with a realistic Coulomb interaction law onto exactly solvable models of mathematical physics. This approach becomes asymptotically exact in the limit of a small wire radius but remains numerically accurate even for modestly thin wires.","id":141},{"numerical_label":"027","title":"Electrostatics of two-dimensional structures: Exact solutions and approximate methods","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.69.245321","authors":"M.M. Fogler","journal":"Phys. Rev. B","volume":"69","issue_page":"245321","year":"2004","pdf":"bib\/Fogler2004EOT.pdf","view_link":"","abstract":"We consider a set of electrostatic problems relevant for determining the real-space structure and the ground-state energy of a two-dimensional electron liquid subject to smooth external potentials. Three fundamental geometries are investigated: an elongated metallic island, an antidot, and a constriction. In the first two cases complete closed-form analytical solutions are obtained, despite the absence of rotational or translational symmetries. These solutions govern the shape and size of large quantum dots, and also the size of the depletion regions and the density profiles around isolated antidots. For the constriction, an exact asymptotical formula for boundary shape is derived and arguments are given in favor of its universality. For the cases where the full analytical solution cannot be obtained, an approximate method is proposed as an alternative. Its accuracy is verified against numerical simulations in a \u2026","id":142},{"numerical_label":"026","title":"Effective theory of incompressible quantum Hall liquid crystals","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"http:\/\/iopscience.iop.org\/article\/10.1209\/epl\/i2003-10234-8\/meta","authors":"M.M. Fogler","journal":"EPL","volume":"66","issue_page":"572","year":"2004","pdf":"bib\/Fogler2004ETO.pdf","view_link":"","abstract":"I propose an effective theory of zero-temperature phases of the quantum Hall stripes: a smectic phase where the stripes are static and a novel quantum nematic phase where the positional order is destroyed by quantum fluctuations. The nematic is viewed as a Bose condensate of dislocations whose interactions are mediated by a U (1) gauge field. Collective mode spectrum and the dynamical structure factor in the two phases are calculated.","id":143},{"numerical_label":"025","title":"Variable-range hopping in quasi-one-dimensional electron crystals","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.69.035413","authors":"M.M. Fogler, S. Teber, B.I. Shklovskii","journal":"Phys. Rev. B","volume":"69","issue_page":"035413","year":"2004","pdf":"bib\/Fogler2003VRH.pdf","view_link":"","abstract":"We study the effect of impurities on the ground state and the low-temperature Ohmic dc transport in a one-dimensional chain and quasi-one-dimensional systems of many parallel chains. We assume that strong interactions impose a short-range periodicity of the electron positions. The long-range order of such an electron crystal (or equivalently, a 4 k F charge-density wave) is destroyed by impurities, which act as strong pinning centers. We show that a three-dimensional array of chains behaves differently at large and at small impurity concentrations N. At large N, impurities divide the chains into metallic rods. Additions or removal of electrons from such rods correspond to charge excitations whose density of states exhibits a quadratic Coulomb gap. At low temperatures the conductivity is due to the variable-range hopping of electrons between the rods. It obeys the Efros-Shklovskii (ES) \u2026","id":144},{"numerical_label":"024","title":"Nonlinear screening and percolative transition in a two-dimensional electron liquid","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.69.121409","authors":"M.M. Fogler","journal":"Phys. Rev. B","volume":"69","issue_page":"121409","year":"2003","pdf":"bib\/Fogler2003NSA.pdf","view_link":"","abstract":"A variational method is proposed for calculating the percolation threshold, the real-space structure, and the ground-state energy of a disordered two-dimensional electron liquid. Its high accuracy is verified against exact asymptotics and prior numerical results. The inverse thermodynamical density of states is shown to have a strongly asymmetric minimum at a density that is approximately the triple of the percolation threshold. This implies that the experimentally observed metal-insulator transition takes place well before the percolation point is reached.","id":145},{"numerical_label":"023","title":"Dynamics of disordered quantum Hall crystals","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/www.sciencedirect.com\/science\/article\/pii\/S1386947703007987","authors":"M.M. Fogler","journal":"Physica E.","volume":"22","issue_page":"98-103","year":"2003","pdf":"bib\/Fogler2003DOD.pdf","view_link":"","abstract":"Charge density waves are thought to be common in two-dimensional electron systems in quantizing magnetic fields. Such phases are formed by the quasiparticles of the topmost occupied Landau level when it is partially filled. One class of charge density wave phases can be described as electron solids. In weak magnetic fields (at high Landau levels) solids with many particles per unit cell\u2014bubble phases\u2014predominate. In strong magnetic fields (at the lowest Landau level) only crystals with one particle per unit cell\u2014Wigner crystals\u2014can form. Experimental identification of these phases is facilitated by the fact that even a weak disorder influences their DC and AC magnetotransport in a very specific way. In the AC domain, a range of frequencies appears where the electromagnetic response is dominated by magnetophonon collective modes. The effect of disorder is to localize the collective \u2026","id":146},{"numerical_label":"022","title":"Low frequency dynamics of disordered XY spin chains and pinned density waves: From localized spin waves to soliton tunneling","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/journals.aps.org\/prl\/abstract\/10.1103\/PhysRevLett.88.186402","authors":"M.M. Fogler","journal":"Phys. Rev. Lett.","volume":"88","issue_page":"186402","year":"2002","pdf":"bib\/Fogler2002LFD.pdf","view_link":"","abstract":"A long-standing problem of the low-energy dynamics of a disordered XY spin chain is reexamined. The case of a rigid chain is studied, where the quantum effects can be treated quasiclassically. It is shown that, as the frequency decreases, the relevant excitations change from localized spin waves to two-level systems to soliton-antisoliton pairs. The linear-response correlation functions are calculated. The results apply to Miscellaneous periodic glassy systems such as pinned density waves, planar vortex lattices, stripes, and disordered Luttinger liquids.","id":147},{"numerical_label":"021","title":"Stripe and Bubble Phases in Quantum Hall Systems","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/doi.org\/10.1007\/3-540-45649-X_4","authors":"M.M. Fogler","journal":"High Magnetic Fields","volume":"","issue_page":"98-138","year":"2002","pdf":"bib\/Fogler2002SAB1.pdf","view_link":"","abstract":"We present a brief survey of the charge density wave phases of a two-dimensional electron liquid in moderate to weak magnetic fields where several higher Landau levels are occupied. The review follows the chronological development of this new and emerging field: from the ideas that led to the original theoretical prediction of the novel ground states, to their dramatic experimental discovery, to the currently pursued directions and open questions.","id":148},{"numerical_label":"020","title":"Comment on \u201cAnalytic Structure of One-Dimensional Localization Theory: Reexamining Mott's Law\u201d","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/doi.org\/10.1103\/PhysRevLett.86.4715","authors":"M.M. Fogler, Z. Q. Wang","journal":"Phys. Rev. Lett.","volume":"86","issue_page":"4715","year":"2001","pdf":"bib\/Fogler2001COA.pdf","view_link":"","abstract":"A Comment on the Letter by Alexander O. Gogolin, Phys. Rev. Lett. 84, 1760 (2000).","id":149},{"numerical_label":"019","title":"Josephson effect without superconductivity: realization in quantum Hall bilayers","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/journals.aps.org\/prl\/abstract\/10.1103\/PhysRevLett.86.1833","authors":"M.M. Fogler, F. Wilczek","journal":"Phys. Rev. Lett.","volume":"86","issue_page":"1833","year":"2001","pdf":"bib\/Fogler2001JEW.pdf","view_link":"","abstract":"We show that a quantum Hall bilayer with the total filling \u03bd= 1 should exhibit a dynamical regime similar to the flux flow in large Josephson junctions. This analogy may explain a conspicuous peak in the interlayer tunneling conductance [Phys. Rev. Lett. 84, 5808 (2000)]. The flux flow is likely to be spatiotemporally chaotic at low-bias voltage, which will manifest itself through broadband noise. The peak position can be controlled by an in-plane magnetic field.","id":150},{"numerical_label":"018","title":"Dynamical response of a pinned two-dimensional Wigner crystal","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.62.7553","authors":"M.M. Fogler, D.A. Huse","journal":"Phys. Rev. B","volume":"62","issue_page":"7553","year":"2000","pdf":"bib\/Fogler2000DRO.pdf","view_link":"","abstract":"We reexamine a long-standing problem of the finite-frequency conductivity of a weakly pinned two-dimensional classical Wigner crystal. In this system an inhomogeneously broadened absorption line (pinning mode) centered at disorder-and magnetic-field-dependent frequency \u03c9 p is known to appear. We show that the relative linewidth \u0394 \u03c9 p\/\u03c9 p of the pinning mode is of the order of 1 in weak magnetic fields, exhibits a power-law decrease in intermediate fields, and eventually saturates at a small value in strong magnetic fields. The linewidth narrowing is due to a peculiar mechanism of mixing between the stiffer longitudinal and the softer transverse components of the collective excitations. The width of the high-field resonance proves to be related to the density of states in the low-frequency tail of the zero-field phonon spectrum. We find a qualitative agreement with recent \u2026","id":151},{"numerical_label":"017","title":"Hydrodynamics of the quantum Hall smectics","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/journals.aps.org\/prl\/abstract\/10.1103\/PhysRevLett.84.5828","authors":"M.M. Fogler, V.M. Vinokur","journal":"Phys. Rev. Lett.","volume":"84","issue_page":"5828","year":"2000","pdf":"bib\/Fogler2000HOT.pdf","view_link":"","abstract":"We propose a dynamical theory of the stripe phase arising in a two-dimensional electron liquid near half-integral fillings of high Landau levels. The system is modeled as a novel type of a smectic liquid crystal with Lorentz force dominated dynamics. We calculate the structure factor, the dispersion relation of the collective modes, and their intrinsic attenuation rate. We show that thermal fluctuations cause a strong power-law renormalization of the elastic and dissipative parameters familiar from the conventional smectics but with different dynamical scaling exponents.","id":152},{"numerical_label":"016","title":"Unconventionally sharp dynamic resonances from a disordered Wigner crystal","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/jp4.journaldephysique.org\/articles\/jp4\/abs\/1999\/10\/jp4199909PR1055\/jp4199909PR1055.html","authors":"M.M. Fogler, D.A. Huse","journal":"J. Phys. IV France","volume":"9","issue_page":"219-222","year":"1999","pdf":"bib\/Fogler1999USD.pdf","view_link":"","abstract":"Motivated by puzzling results of recent experiments, we re-examine the response of a weakly pinned two-dimensional Wigner crystal to a uniform AC electric field. We confirm that at some disorder and magnetic field dependent frequency \u03c9 p, an inhomogeneously broadened absorption line emerges. Although the line is conventionally broad in zero magnetic field, in strong fields it appears as a sharp resonance whose width is related to the density of states in the low-frequency tail of the zero-field phonon spectrum. This behavior originates due to the long-range Coulomb interactions.","id":153},{"numerical_label":"015","title":"Cyclotron resonance in a two-dimensional electron gas with long-range randomness","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/journals.aps.org\/prl\/abstract\/10.1103\/PhysRevLett.80.4749","authors":"M.M. Fogler, B.I. Shklovskii","journal":"Phys. Rev. Lett.","volume":"80","issue_page":"4749","year":"1998","pdf":"bib\/Fogler1998CRI.pdf","view_link":"","abstract":"We show that the cyclotron resonance in a two-dimensional electron gas has nontrivial properties if the correlation length of the disorder is larger than the Fermi wavelength:(a) The line shape assumes three different forms in strong, intermediate, and weak magnetic fields.(b) The linewidth collapses at the transition from the intermediate to the weak fields via the motional narrowing mechanism brought about by a dramatic enhancement of the localization length.","id":154},{"numerical_label":"014","title":"Quasiclassical approach to the weak levitation of extended states in the quantum Hall effect","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.57.11947","authors":"M.M. Fogler","journal":"Phys. Rev. B","volume":"57","issue_page":"11947","year":"1998","pdf":"bib\/Fogler1998QAT.pdf","view_link":"","abstract":"The two-dimensional motion of a charged particle in a random potential and a transverse magnetic field is believed to be delocalized only at discrete energies E N. In strong fields there is a small positive deviation of EN from the center of the N th Landau level, which is referred to as the \u201cweak levitation\u201d of the extended state. I calculate the size of the weak levitation effect for the case of a smooth random potential rederiving earlier results of Haldane and Yang [Phys. Rev. Lett. 78, 298 (1997)] and extending their approach to lower magnetic fields. I find that as the magnetic field decreases, this effect remains weak down to the lowest field B min where such a quasiclassical approach is still justified. Moreover, in the immediate vicinity of B min the weak levitation becomes additionally suppressed. This indicates that the \u201cstrong levitation\u201d expected at yet even lower magnetic fields must be of \u2026","id":155},{"numerical_label":"013","title":"Localization length at the conductivity minima of the quantum Hall effect","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/www.sciencedirect.com\/science\/article\/pii\/S1386947797000301","authors":"M.M. Fogler, A. Yu Dobin, B.I. Shklovskii","journal":"Physica E","volume":"1","issue_page":"132-134","year":"1997","pdf":"bib\/Fogler1997LLAT.pdf","view_link":"","abstract":"The quantum localization is known to be responsible for the deep conductivity minima of the quantum Hall effect. In this paper we calculate the localization length \u03be as a function of magnetic field B at such minima for several models of disorder (\u201cwhite-noise\u201d, short-range, and long-range random potentials). We find that \u03be\u221d B\u2212 \u03b1 with the exponent \u03b1 between one and 10 3, depending on the model. In particular, for the \u201cwhite-noise\u201d random potential \u03be roughly coincides with the classical cyclotron radius. Our results are in agreement with available experimental data.","id":156},{"numerical_label":"012","title":"Localization length at the resistivity minima of the quantum Hall effect","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.57.4614","authors":"M.M. Fogler, A. Yu Dobin, B.I. Shklovskii","journal":"Phys. Rev. B","volume":"57","issue_page":"4614","year":"1997","pdf":"bib\/Fogler1997LLA.pdf","view_link":"","abstract":"The resistivity minima of the quantum Hall effect arise due to the localization of the electron states at the Fermi energy, when it is positioned between adjacent Landau levels. In this paper we calculate the localization length \u03be of such states at even filling factors \u03bd= 2 N. The calculation is done for several models of disorder (\u201cwhite-noise,\u201d short-range, and long-range random potentials). We find that the localization length has a power-law dependence on the Landau level index, \u03be\u221d N \u03b1 with the exponent \u03b1 between one and 10 3, depending on the model. In particular, for a \u201cwhite-noise\u201d random potential \u03be roughly coincides with the classical cyclotron radius. Our results are in reasonable agreement with experimental data on low and moderate mobility samples.","id":157},{"numerical_label":"011","title":"Suppression of chaotic dynamics and localization of two-dimensional electrons by a weak magnetic field","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.56.6823","authors":"M.M. Fogler, A. Yu Dobin, V.I. Perel, B.I. Shklovskii","journal":"Phys. Rev. B","volume":"56","issue_page":"6823","year":"1997","pdf":"bib\/Fogler1997SOC.pdf","view_link":"","abstract":"We study a two-dimensional motion of a charged particle in a weak random potential and a perpendicular magnetic field. The correlation length of the potential is assumed to be much larger than the de Broglie wavelength. Under such conditions, the motion on not too large length scales is described by classical equations of motion. We show that the phase-space averaged diffusion coefficient is given by the Drude-Lorentz formula only at magnetic fields B smaller than certain value B c. At larger fields, the chaotic motion is suppressed and the diffusion coefficient becomes exponentially small. In addition, we calculate the quantum-mechanical localization length as a function of B at the minima of \u03c3 xx. At B< B c it is exponentially large but decreases with increasing B. At B> B c, this decrease becomes very rapid and the localization length ceases to be exponentially large at a field B*, which is \u2026","id":158},{"numerical_label":"010","title":"Laughlin liquid to charge-density-wave transition at high Landau levels","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.55.9326","authors":"M.M. Fogler, A.A. Koulakov","journal":"Phys. Rev. B","volume":"55","issue_page":"9326","year":"1997","pdf":"bib\/Fogler1997LLT.pdf","view_link":"","abstract":"We compare the energies of the Laughlin liquid and a charge density wave in a weak magnetic field for the upper Landau-level filling factors \u03bd N= and. The charge-density-wave period has been optimized and was found to be\u2243 3. 3 R c, where R c is the cyclotron radius. We conclude that the optimal charge density wave is lower in energy than the Laughlin liquid for the Landau-level numbers N\u2a7e 2 at \u03bd N= and for N\u2a7e 3 at \u03bd N=. This implies that the quantum Hall plateaus cannot be observed for N\u2a7e 2, in agreement with the experiment.","id":159},{"numerical_label":"009","title":"Quantum Hall effect in weak magnetic fields","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"http:\/\/adsabs.harvard.edu\/abs\/1997PhDT.......137F","authors":"M.M. Fogler","journal":"PQDT","volume":"58","issue_page":"4287","year":"1997","pdf":"","view_link":"","abstract":"The quantum Hall effect (QHE) is one of the most remarkable phenomena of nature. Since its discovery in 1980 it has motivated an extensive theoretical and experimental work, which led to the development of many fundamental concepts of the condensed matter physics and gave rise to important practical applications. Nowadays, cleaner samples made it possible to observe the QHE in magnetic fields as low as several tenths of a Tesla. Several new interesting findings have been reported. I investigate this new range of magnetic fields in order to explain these findings and establish the connection with previously studied case of strong magnetic fields. Specifically, I study the effects of localization and electron-electron interactions on thermodynamical and transport properties of the system. The thesis covers the following topics: the theory of the collapse of the spin-splitting, the study of the ground \u2026","id":160},{"numerical_label":"008","title":"Activated conductivity in the quantum Hall effect","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/www.sciencedirect.com\/science\/article\/pii\/0039602896003974","authors":"M.M. Fogler, D.G. Polyakov, B.I. Shklovskii","journal":"Surf. Sci.","volume":"361","issue_page":"255-260","year":"1996","pdf":"bib\/Fogler1996ACI.pdf","view_link":"","abstract":"Activated dissipative conductivity \u03c3 xx= \u03c3* xx exp (\u2212 \u0394\/T) and the activated deviation of the Hall conductivity from the precise quantization \u03b4\u03c3 xy= \u03c3 xy\u2212 ie 2\/h= \u03c3* xy exp (\u2212 \u0394\/T) are studied in a plateau range of the quantum Hall effect. The prefactors \u03c3* xx and \u03c3* xy are calculated for the case of a long-range random potential in the framework of a classical theory. There is a range of temperatures T 1\u226a T\u226a T 2 where \u03c3* xy= e 2\/h. In this range \u03c3* xy\u2248(e 2\/h)(T\/T 2) 80\/21\u226a \u03c3* xx. At large T\u226b T 2, on the Miscellaneous hand, \u03c3* xy= e 2\/h and \u03c3* xx=(e 2 h)(T 2 T) 10\/13\u226a \u03c3* xy. Similar results are valid for a fractional plateau near the filling factor p\/q if charge e is replaced by e\/q.","id":161},{"numerical_label":"007","title":"The ground state of a two-dimensional electron liquid in a weak magnetic field","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.54.1853","authors":"M.M. Fogler, A.A. Koulakov, B.I. Shklovskii","journal":"Phys. Rev. B","volume":"54","issue_page":"1853","year":"1996","pdf":"bib\/Fogler1996TGS.pdf","view_link":"","abstract":"We consider a clean two-dimensional electron liquid in a weak magnetic field where N\u226b 1 lower Landau levels are completely filled, while the upper level is only partially filled. Due to a screening by the lower Landau levels, the repulsive interaction between any two electrons at the upper level as a function of the separation between the guiding centers of their cyclotron orbits abruptly drops at the distance of two cyclotron radii. Such a ''box-like''component in the interaction potential makes the uniform distribution of the electron density at the upper Landau level unstable, and domains with filling factor equal to one and zero are formed. The shape of domains is studied both analytically and numerically. We show that when the filling factor of the upper Landau level is close to one-half, the domains have the form of parallel stripes alternating with a spatial period close to three cyclotron \u2026","id":162},{"numerical_label":"006","title":"Charge density wave in two-dimensional electron liquid in weak magnetic field","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/journals.aps.org\/prl\/abstract\/10.1103\/PhysRevLett.76.499","authors":"A.A. Koulakov, M.M. Fogler, B.I. Shklovskii","journal":"Phys. Rev. Lett.","volume":"76","issue_page":"499","year":"1995","pdf":"bib\/Koulakov1995CDW.pdf","view_link":"","abstract":"We study the ground state of a clean two-dimensional electron liquid in a weak magnetic field where N\u226b 1 lower Landau levels are completely filled and the upper level is partially filled. It is shown that the electrons at the upper Landau level form domains with filling factors equal to 1 and zero. The domains alternate with a spatial period of order of the cyclotron radius, which is much larger than the interparticle distance at the upper Landau level. The one-particle density of states, which can be probed by tunneling experiments, is shown to have a gap linearly dependent on the magnetic field in the limit of large N.","id":163},{"numerical_label":"005","title":"Collapse of spin splitting in the quantum Hall effect","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.52.17366","authors":"M.M. Fogler, B.I. Shklovskii","journal":"Phys. Rev. B","volume":"52","issue_page":"17366","year":"1995","pdf":"bib\/Fogler1995COS.pdf","view_link":"","abstract":"It is known experimentally that at not very large filling factors \u03bd the quantum Hall conductivity peaks corresponding to the same Landau level number N and two different spin orientations are well separated. These peaks occur at half-integer filling factors \u03bd= 2N+ 1\/2 and \u03bd= 2N+ 3\/2 so that the distance \u03b4\u03bd between them is unity. As \u03bd increases \u03b4\u03bd shrinks. Near certain N= N c two peaks merge into a single peak at \u03bd= 2N+ 1. We argue that this collapse of the spin splitting at low magnetic fields is attributed to the disorder-induced destruction of the exchange enhancement of the electron g factor. We use the mean-field approach to show that in the limit of zero Zeeman energy \u03b4\u03bd experiences a second-order phase transition as a function of the magnetic field. We give explicit expressions for N c in terms of a sample's parameters. For example, we predict that for high-mobility heterostructures N c= 0.9 dn 5 \u2026","id":164},{"numerical_label":"004","title":"Probability of an eigenvalue number fluctuation in an interval of a random matrix spectrum","category":"Miscellaneous","label":"","hyperlink":"https:\/\/journals.aps.org\/prl\/abstract\/10.1103\/PhysRevLett.74.3312","authors":"M.M. Fogler, B.I. Shklovskii","journal":"Phys. Rev. Lett.","volume":"74","issue_page":"3312","year":"1995","pdf":"bib\/Fogler1995TPO.pdf","view_link":"","abstract":"We calculate the probability to find exactly n eigenvalues in a spectral interval of a large random N\u00d7 N matrix when this interval contains s\u226a N eigenvalues on average. The calculations exploit an analogy to the problem of finding a two-dimensional charge distribution on the interface of a semiconductor heterostructure under the influence of a split gate.","id":165},{"numerical_label":"003","title":"Thermally Activated Deviations from Quantum Hall Plateaus","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/www.sciencedirect.com\/science\/article\/pii\/0038109895001190?via%3Dihub","authors":"M.M. Fogler, B.I. Shklovskii","journal":"Solid State Comm.","volume":"94","issue_page":"503","year":"1995","pdf":"bib\/Fogler1995TAD.pdf","view_link":"","abstract":"The Hall conductivity $\\sigma_ {\\rm xy} $ of a two-dimensional electron system is quantized in units of $ e^ 2\/h $ when the Fermi level is located in the mobility gap between two Landau levels. We consider the deviation of $\\sigma_ {\\rm xy} $ from a quantized value caused by the thermal activation of electrons to the extended states for the case of a long range random potential. This deviation is of the form $\\sigma_ {\\rm xy}^*\\exp (-\\Delta\/T) $. The prefactor $\\sigma_ {\\rm xy}^* $ is equal to $ e^ 2\/h $ at temperatures above a characteristic temperature $ T_2 $. With the temperature decreasing below $ T_2 $, $\\sigma_ {\\rm xy}^* $ decays according to a power law: $\\sigma_ {\\rm xy}^*=\\frac {e^ 2}{h}(T\/T_2)^\\gamma $. Similar results are valid for a fractional Hall plateau near filling factor $ p\/q $ if $ e $ is replaced by the fractional charge $ e\/q $.","id":166},{"numerical_label":"002","title":"Resistance of a long wire in the quantum Hall regime","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/doi.org\/10.1103\/PhysRevB.50.1656","authors":"M.M. Fogler, B.I. Shklovskii","journal":"Phys. Rev. B","volume":"50","issue_page":"1656","year":"1994","pdf":"bib\/Fogler1994ROA.pdf","view_link":"","abstract":"We study the two-probe transport in a long narrow channel of two-dimensional electron liquid (a quantum wire) in a strong magnetic field normal to the plane. The wire is split into alternating parallel strips of compressible and incompressible liquids. When the temperature is not too low, in every transverse cross section of the wire the electrochemical potential is different in different compressible strips but is uniform within each strip. The equilibration rate between two neighboring compressible strips falls off exponentially with the distance between them (the width of the separating incompressible strip). Assuming a parabolic confinement potential and small disorder potential, we find the magnetic-field dependence of the strip widths. Using only this dependence, we calculate the shape of the magnetoresistance peaks. We show that the low-field sides of these peaks are much steeper than the high-field sides, in agreement with recent experimental data.","id":167},{"numerical_label":"001","title":"Chemical potential and magnetization of a Coulomb island","category":"Other Work on Low-Dim Systems","label":"","hyperlink":"https:\/\/journals.aps.org\/prb\/abstract\/10.1103\/PhysRevB.49.13767","authors":"M.M. Fogler, E.I. Levin, B.I. Shklovskii","journal":"Phys. Rev. B","volume":"49","issue_page":"13767","year":"1994","pdf":"bib\/Fogler1994CPA.pdf","view_link":"","abstract":"We consider variations of the low temperature chemical potential and magnetization of laterally confined two-dimensional interacting electrons in a strong magnetic field. Cases of axially symmetrical (a quantum dot) and strongly elongated (a quantum wire) spatial distributions of electron density are studied. We commonly refer to both types of systems as ''islands.''The calculations are performed in the mean-field approximation. In this approximation the system consists of alternating regions of compressible and incompressible liquids. We are focusing on islands with typical widths b, which are much larger than the Bohr radius in a semiconductor a B, and argue that the density profile is formed mainly by electrostatic forces, has a domelike shape, and changes only slightly in the whole range of magnetic fields. We calculated numerically the chemical potential and the \u2026","id":168},{"numerical_label":"122","title":"Photonic crystals for nano-light in moir\u00e9 graphene superlattices","category":"Near Field Optics; Graphene and 2D Materials","label":"","hyperlink":"https:\/\/doi.org\/10.1126\/science.aau5144","authors":"S.S. Sunku, G.X. Ni, B.Y. Jiang, H. Yoo, A. Sternbach, A.S. McLeod, T. Stauber, L. Xiong, T. Taniguchi, K. Watanabe, P. Kim, M.M. Fogler, D.N. Basov","journal":"Science","volume":"362","issue_page":"1153-1156","year":"2018","pdf":"bib\/Sunku2018PCF.pdf","view_link":"","abstract":"Graphene is an atomically thin plasmonic medium that supports highly confined plasmon polaritons, or nano-light, with very low loss. Electronic properties of graphene can be drastically altered when it is laid upon another graphene layer, resulting in a moir\u00e9 superlattice.The relative twist angle between the two layers is a key tuning parameter of the interlayer coupling in thus-obtained twisted bilayer graphene (TBG). We studied the propagation of plasmon polaritons in TBG by infrared nano-imaging. We discovered that the atomic reconstruction occurring at small twist angles transforms the TBG into a natural plasmon photonic crystal for propagating nano-light. This discovery points to a pathway for controlling nano-light by exploiting quantum properties of graphene and other atomically layered van der Waals materials, eliminating the need for arduous top-down nanofabrication.","id":169},{"numerical_label":"121","title":"Theory of plasmon reflection by a 1D junction","category":"Near Field Optics","label":"","hyperlink":"https:\/\/www.osapublishing.org\/abstract.cfm?uri=oe-26-13-17209","authors":"B.Y. Jiang, E.J. Mele, M.M. Fogler","journal":"Opt. Express","volume":"26","issue_page":"17209-17226","year":"2018","pdf":"bib\/Jiang2018TOP.pdf","view_link":"","abstract":"We report on infrared (IR) nanoscopy of 2D plasmon excitations of Dirac fermions in graphene. This is achieved by confining mid-IR radiation at the apex of a nanoscale tip: an approach yielding 2 orders of magnitude increase in the value of in-plane component of incident wavevector qcompared to free space propagation. At these high wavevectors, the Dirac plasmon is found to dramatically enhance the near-field interaction with mid-IR surface phonons of SiO2 substrate. Our data augmented by detailed modeling establish graphene as a new medium supporting plasmonic effects that can be controlled by gate voltage.","id":170},{"numerical_label":"120","title":"Fundamental limits to graphene plasmonics","category":"Graphene and 2D Materials; Near Field Optics","label":"","hyperlink":"https:\/\/www.nature.com\/articles\/s41586-018-0136-9","authors":"G.X. Ni, A.S. McLeod, Z. Sun, L. Wang, L. Xiong, K.W. Post, S.S. Sunku, B.Y. Jiang, J. Hone, C.R. Dean, M.M. Fogler, D.N. Basov ","journal":"Nature","volume":"557","issue_page":"530-533","year":"2018","pdf":"bib\/Ni2018FLT.pdf","view_link":"","abstract":"Plasmon polaritons are hybrid excitations of light and mobile electrons that can confine the energy of long-wavelength radiation at the nanoscale. Plasmon polaritons may enable many enigmatic quantum effects, including lasing, topological protection and dipole-forbidden absorption. A necessary condition for realizing such phenomena is a long plasmonic lifetime, which is notoriously difficult to achieve for highly confined modes. Plasmon polaritons in graphene\u2014hybrids of Dirac quasiparticles and infrared photons\u2014provide a platform for exploring light\u2013matter interaction at the nanoscale. However, plasmonic dissipation in graphene is substantial and its fundamental limits remain undetermined. Here we use nanometre-scale infrared imaging to investigate propagating plasmon polaritons in high-mobility encapsulated graphene at cryogenic temperatures. In this regime, the propagation of plasmon polaritons is primarily restricted by the dielectric losses of the encapsulated layers, with a minor contribution from electron\u2013phonon interactions. At liquid-nitrogen temperatures, the intrinsic plasmonic propagation length can exceed 10\u2009micrometres, or 50 plasmonic wavelengths, thus setting a record for highly confined and tunable polariton modes. Our nanoscale imaging results reveal the physics of plasmonic dissipation and will be instrumental in mitigating such losses in heterostructure engineering applications.","id":171}]