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Search Results: 1 - 10 of 148113 matches for " Andrei B. Sushkov "
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Optical evidence for a Weyl semimetal state in pyrochlore Eu2Ir2O7
Andrei B. Sushkov,Johannes B. Hofmann,Gregory S. Jenkins,Jun Ishikawa,Satoru Nakatsuji,Sankar Das Sarma,H. Dennis Drew
Physics , 2015, DOI: 10.1103/PhysRevB.92.241108
Abstract: A Weyl semimetallic state with pairs of nondegenerate Dirac cones in three dimensions was recently predicted to occur in the antiferromagnetic state of the pyrochlore iridates. Here, we show that the THz optical conductivity and temperature dependence of free carriers in the pyrochlore Eu2Ir2O7 match the predictions for a Weyl semimetal and suggest novel Dirac liquid behavior. The interband optical conductivity vanishes continuously at low frequencies signifying a semimetal. The metal-insulator transition at T_N = 110 K is manifested in the Drude spectral weight, which is independent of temperature in the metallic phase, and which decreases smoothly in the ordered phase. The temperature dependence of the free carrier weight below T_N is in good agreement with theoretical predictions for a Dirac material. The data yield a Fermi velocity v_F=4x10^7 cm/s, a logarithmic renormalization scale Lambda_L=600 K, and require a Fermi temperature of T_F=100 K associated with residual unintentional doping to account for the low temperature optical response and dc resistivity.
Strong surface scattering in ultrahigh mobility Bi2Se3 topological insulator crystals
Nicholas P. Butch,Kevin Kirshenbaum,Paul Syers,Andrei B. Sushkov,Gregory S. Jenkins,H. Dennis Drew,Johnpierre Paglione
Physics , 2010, DOI: 10.1103/PhysRevB.81.241301
Abstract: While evidence of a topologically nontrivial surface state has been identified in surface-sensitive measurements of Bi2Se3, a significant experimental concern is that no signatures have been observed in bulk transport. In a search for such states, nominally undoped single crystals of Bi2Se3 with carrier densities approaching 10^16 cm^-3 and very high mobilities exceeding 2 m^2 V^-1 s^-1 have been studied. A comprehensive analysis of Shubnikov de Haas oscillations, Hall effect, and optical reflectivity indicates that the measured electrical transport can be attributed solely to bulk states, even at 50 mK at low Landau level filling factor, and in the quantum limit. The absence of a significant surface contribution to bulk conduction demonstrates that even in very clean samples, the surface mobility is lower than that of the bulk, despite its topological protection.
Spectral origin of the colossal magnetodielectric effect in multiferroic DyMn2O5
A. B. Sushkov,Ch. Kant,M. Schiebl,A. M. Shuvaev,Anna Pimenov,Andrei Pimenov,B. Lorenz,S. Park,S-W. Cheong,Maxim Mostovoy,H. D. Drew
Physics , 2014, DOI: 10.1103/PhysRevB.90.054417
Abstract: The origin of the colossal magnetodielectric effect in DyMn2O5 [1] has been an outstanding question in multiferroics. Here, we report the activation of the electric dipole mode at 4-5 cm-1 in an applied magnetic field which fully accounts for the CMD effect. We examine two alternative explanations of this mode: an electromagnon and transitions between f-electron levels of Dy3+ ions. The experimental and theoretical evidence supports the electromagnon origin of the CMD effect.
Terahertz Nonlinearity in Graphene Plasmons
Mohammad M. Jadidi,Jacob C. K?nig-Otto,Stephan Winnerl,Andrei B. Sushkov,H. Dennis Drew,Thomas E. Murphy,Martin Mittendorff
Physics , 2015,
Abstract: Sub-wavelength graphene structures support localized plasmonic resonances in the terahertz and mid-infrared spectral regimes. The strong field confinement at the resonant frequency is predicted to significantly enhance the light-graphene interaction, which could enable nonlinear optics at low intensity in atomically thin, sub-wavelength devices. To date, the nonlinear response of graphene plasmons and their energy loss dynamics have not been experimentally studied. We measure and theoretically model the terahertz nonlinear response and energy relaxation dynamics of plasmons in graphene nanoribbons. We employ a THz pump-THz probe technique at the plasmon frequency and observe a strong saturation of plasmon absorption followed by a 10 ps relaxation time. The observed nonlinearity is enhanced by two orders of magnitude compared to unpatterned graphene with no plasmon resonance. We further present a thermal model for the nonlinear plasmonic absorption that supports the experimental results.
Giant plateau in the THz Faraday angle in gated Bi2Se3
Gregory S. Jenkins,Andrei B. Sushkov,Don C. Schmadel,M. -H. Kim,Matthew Brahlek,Namrata Bansal,Seongshik Oh,H. Dennis Drew
Physics , 2012, DOI: 10.1103/PhysRevB.86.235133
Abstract: We report gated terahertz Faraday angle measurements on epitaxial Bi2Se3 thin films capped with In2Se3. A plateau is observed in the real part of the Faraday angle at an onset gate voltage corresponding to no band bending at the surface which persists into accumulation. The plateau is two orders of magnitude flatter than the step size expected from a single Landau Level in the low frequency limit, quantized in units of the fine structure constant. At 8 T, the plateau extends over a range of gate voltage that spans an electron density greater than 14 times the quantum flux density. Both the imaginary part of the Faraday angle and transmission measurements indicate dissipative off-axis and longitudinal conductivity channels associated with the plateau.
Dirac cone shift of a passivated topological Bi2Se3 interface state
Gregory S. Jenkins,Andrei B. Sushkov,Don C. Schmadel,Max Bichler,Gregor Koblmueller,Matthew Brahlek,Namrata Bansal,Seongshik Oh,H. Dennis Drew
Physics , 2012, DOI: 10.1103/PhysRevB.87.155126
Abstract: Gated terahertz cyclotron resonance measurements on epitaxial Bi2Se3 thin films capped with In2Se3 enable the first spectroscopic characterization of a single topological interface state from the vicinity of the Dirac point to above the conduction band edge. A precipitous drop in the scattering rate with Fermi energy is observed that is interpreted as the surface state decoupling from bulk states and evidence of a shift of the Dirac point towards mid-gap. Near the Dirac point, potential fluctuations of 50 meV are deduced from an observed loss of differential optical spectral weight near the Dirac point. Potential fluctuations are reduced by a factor of two at higher surface Fermi levels in the vicinity of the conduction band edge inferred from the width of the scattering rate step. The passivated topological interface state attains a high mobility of 3500 cm2/Vsec near the Dirac point.
Sensitive Room-Temperature Terahertz Detection via Photothermoelectric Effect in Graphene
Xinghan Cai,Andrei B. Sushkov,Ryan J. Suess,Mohammad M. Jadidi,Greg S. Jenkins,Luke O. Nyakiti,Rachael L. Myers-Ward,Shanshan Li,Jun Yan,D. Kurt Gaskill,Thomas E. Murphy,H. Dennis Drew,Michael S. Fuhrer
Physics , 2013, DOI: 10.1038/nnano.2014.182
Abstract: Terahertz (THz) radiation has uses from security to medicine; however, sensitive room-temperature detection of THz is notoriously difficult. The hot-electron photothermoelectric effect in graphene is a promising detection mechanism: photoexcited carriers rapidly thermalize due to strong electron-electron interactions, but lose energy to the lattice more slowly. The electron temperature gradient drives electron diffusion, and asymmetry due to local gating or dissimilar contact metals produces a net current via the thermoelectric effect. Here we demonstrate a graphene thermoelectric THz photodetector with sensitivity exceeding 10 V/W (700 V/W) at room temperature and noise equivalent power less than 1100 pW/Hz^1/2 (20 pW/Hz^1/2), referenced to the incident (absorbed) power. This implies a performance which is competitive with the best room-temperature THz detectors for an optimally coupled device, while time-resolved measurements indicate that our graphene detector is eight to nine orders of magnitude faster than those. A simple model of the response, including contact asymmetries (resistance, work function and Fermi-energy pinning) reproduces the qualitative features of the data, and indicates that orders-of-magnitude sensitivity improvements are possible.
Hybrid Metal-Graphene Plasmons for Tunable Terahertz Technology
Mohammad M. Jadidi,Andrei B. Sushkov,Rachael L. Myers-Ward,Anthony K. Boyd,Kevin M. Daniels,D. Kurt Gaskill,Michael S. Fuhrer,H. Dennis Drew,Thomas E. Murphy
Physics , 2015, DOI: 10.1021/acs.nanolett.5b03191
Abstract: Among its many outstanding properties, graphene supports terahertz surface plasma waves -- sub-wavelength charge density oscillations connected with electromagnetic fields that are tightly localized near the surface[1,2]. When these waves are confined to finite-sized graphene, plasmon resonances emerge that are characterized by alternating charge accumulation at the opposing edges of the graphene. The resonant frequency of such a structure depends on both the size and the surface charge density, and can be electrically tuned throughout the terahertz range by applying a gate voltage[3,4]. The promise of tunable graphene THz plasmonics has yet to be fulfilled, however, because most proposed optoelectronic devices including detectors, filters, and modulators[5-10] desire near total modulation of the absorption or transmission, and require electrical contacts to the graphene -- constraints that are difficult to meet using existing plasmonic structures. We report here a new class of plasmon resonance that occurs in a hybrid graphene-metal structure. The sub-wavelength metal contacts form a capacitive grid for accumulating charge, while the narrow interleaved graphene channels, to first order, serves as a tunable inductive medium, thereby forming a structure that is resonantly-matched to an incident terahertz wave. We experimentally demonstrate resonant absorption near the theoretical maximum in readily-available, large-area graphene, ideal for THz detectors and tunable absorbers. We further predict that the use of high mobility graphene will allow resonant THz transmission near 100%, realizing a tunable THz filter or modulator. The structure is strongly coupled to incident THz radiation, and solves a fundamental problem of how to incorporate a tunable plasmonic channel into a device with electrical contacts.
Horizon closeness bounds for static black hole mimickers
Sergey V. Sushkov,Oleg B. Zaslavskii
Physics , 2009, DOI: 10.1103/PhysRevD.79.067502
Abstract: We consider the question whether a wormhole can be converted into a non-extremal quasi-black black hole by continuous change of parameters. In other words, we ask whether "black" wormholes can exist as end points of families of static wormhole geometries. The answer is negative since the corresponding limit is shown to be singular. Similar conclusions are valid also for other types of black hole mimickers such as gravastars and quasi-black holes without wormhole behavior. Our treatment is model-independent and applies to any static geometries without requirement of special symmetries. We also find an asymptotic expression for the Kretschmann scalar for wormholes on the threshold of horizon formation that can be used as an the bound on proximity of the configuration to the would-be horizon. The derived bound is very weak for astrophysical black holes but becomes relevant for microscopic ones. We point out complementarity between ability of wormholes to mimic black holes and their ability to be traversable "in practice".
Low-temperature far-infrared ellipsometry of convergent beam
A. B. Sushkov,E. A. Tishchenko
Physics , 1998, DOI: 10.1007/BF02086222
Abstract: Development of an ellipsometry to the case of a coherent far infrared irradiation, low temperatures and small samples is described, including a decision of the direct and inverse problems of the convergent beam ellipsometry for an arbitrary wavelength, measurement technique and a compensating orientation of cryostat windows. Experimental results are presented: for a gold film and UBe13 single crystal at room temperature (lambda=119 um), temperature dependencies of the complex dielectric function of SrTiO3 (lambda=119, 84 and 28 um) and of YBa2Cu3O7-delta ceramic (lambda=119 um).
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