Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session A42: Focus Session: Non-equilibrium Effects in Topological Insulators |
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Sponsoring Units: DMP Chair: Tom Devereaux, SLAC National Accelerator Laboratory Room: Mile High Ballroom 4A |
Monday, March 3, 2014 8:00AM - 8:12AM |
A42.00001: Time-resolved terahertz dynamics in thin films of the topological insulator Bi$_{2}$Se$_3$ Rolando Vald\'es Aguilar, J. Qi, A.J. Taylor, D.A. Yarotski, R.P. Prasankumar, M. Brahlek, N. Bansal, S. Oh Experiments at terahertz frequencies (1 THz $\sim$ 4 meV) in thin films of Bi$_2$Se$_3$ have provided evidence of the surface response, and give a picture of relatively mobile surface carriers with a bulk response that makes a small contribution to the THz conductivity. In this report we use optically pumped time-resolved THz spectroscopy at low temperature to distinguish the bulk and surface contribution on thin films of Bi$_2$Se$_3$ of several thicknesses. We find that for very thin films, where pure 2D behavior is expected, the optical pump induces a change in the 2D transport scattering rate which decays in a time-scale of 20 picoseconds. For thicker films, we see an additional contribution that increases the conductivity and scales with the increase in both the film thickness and the fluence of the pump beam. This contribution has much faster rise and decay times of approximately 5 ps, as well as a much larger scattering rate than the previously identified surface term. The different dynamics between surface and bulk electrons close to the Fermi energy evidenced in this study indicate a decoupling of surface and bulk carriers at low temperature, and present the possibility of accessing long-lived surface photo-carriers for optoelectronic applications. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A42.00002: Observation of Floquet-Bloch states on the surface of a topological insulator Yihua Wang, Hadar Steinberg, Pablo Jarillo-Herrero, Nuh Gedik The unique electronic properties of the surface electrons in a topological insulator are protected by time-reversal symmetry. Circularly polarized light naturally breaks time-reversal symmetry, which may lead to an exotic surface quantum Hall state. Using time- and angle-resolved photoemission spectroscopy, we show that an intense ultrashort mid-infrared pulse with energy below the bulk band gap hybridizes with the surface Dirac fermions of a topological insulator to form Floquet-Bloch bands. These photon dressed surface bands exhibit polarization-dependent band gaps at avoided crossings. Circularly polarized photons induce an additional gap at the Dirac point, which is a signature of broken time-reversal symmetry on the surface. These observations establish the Floquet-Bloch bands in solids and pave the way for optical manipulation of topological quantum states of matter. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A42.00003: Tunable Floquet Majorana Modes in Coupled Quantum Dots Yantao Li, Arijit Kundu, Fan Zhong, Babak Seradjeh We study theoretically the appearance of Floquet Majorana fermions in a double quantum dot system coupled by a superconducting lead and driven by separate AC potentials. We argue that the system could be fine tuned controllably in the expanded parameter space of the drive frequency, amplitude, and phase difference across the two dots. While these Majorana fermions are not topologically protected, the all-electric, highly tunable setup could provide a realistic system for observing the exotic physics associated with Majorana fermions. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A42.00004: Ultrafast transient decoupling and multi-phonon effects in driven electron-phonon systems Alexander Kemper, Michael Sentef, Brian Moritz, James Freericks, Thomas Devereaux Pump-probe experiments have become an increasingly important tool in studying the interaction between electrons and bosons in condensed matter. Here we discuss some of the transient effects that occur during the pumping process using the non-equilibrium Keldysh technique to numerically solve the equations of motion for a strongly coupled electron-phonon system. The scattering of spectral weight while the pump is on decouples the electrons from the phonons, leading to a transient weakening of the electron-phonon spectral features known as kinks. We further note that higher order kinks in the spectra are more readily visible in the time domain by observing changes in the time-resolved ARPES spectra. Finally, we revisit the question of time-resolved relaxation dynamics and highlight the effects of the transients spectral weight re-arrangement there. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A42.00005: Photo-induced topological phase transition in graphene studied by exact simulation of pump-probe photoemission spectroscopy Michael Sentef, Alexander Kemper, Brian Moritz, James Freericks, Thomas Devereaux The idea of inducing a nontrivial topological band structure using circularly polarized light was triggered by the observation that in a steady ``Floquet'' state, periodically driven Dirac fermions can be mapped [1] to the Haldane model for a quantum Hall state without Landau levels [2]. A recent observation of Floquet-Bloch states on the surface of a spin-orbit driven topological insulator and a surface state energy gap opened by time-reversal symmetry breaking [3] poses the question how a topological phase transition occurs in real time on ultrashort time scales. We use a well developed Keldysh Green function technique [4] to compute the exact time evolution of tight-binding electrons on the honeycomb lattice coupled to realistic short laser pulses. The time- and angle-resolved photoemission response reveals a photo-induced topological phase transition with energy gaps $>$ 100 meV at the Dirac point that should be observable experimentally. [1] T.~Oka and H.~Aoki, Phys. Rev. B 79, 081406 (2009); T.~Kitagawa et.~al., Phys. Rev. B 82, 235114 (2010); N.~H.~Lindner et.~al., Nature Physics 7, 490-495 (2011). [2] F.~D.~M.~Haldane, Phys. Rev. Lett. 61, 2015-2018 (1988). [3] Y.~H.~Wang et.~al., Science 342, 453 (2013). [4] M.~Sentef et.~al., arXiv:1212.4841 (Phys. Rev. X 2013). [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A42.00006: Polarization-controlled picosecond currents in topological insulators Alexander Holleitner, Christoph Karnetzky, Helmut Karl, Christoph Kastl Controlling spin currents in topological insulators may lead to applications in future spintronic devices [1]. Here, we show that surface currents in Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ can be controlled by circularly polarized light on a time-scale of a picosecond with a fidelity near unity even at room temperature. We reveal the temporal interplay of such ultrafast spin currents with photo-induced thermoelectric and drift currents in optoelectronic circuits [2]. [1] C. Kastl, et al, APL 101, 251110 (2012). [2] C. Kastl et al. under review (2013). [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A42.00007: Nonequilibrium Spectroscopy of Topological Edge Liquids Alex Levchenko, Stanislav Apostolov We develop theory for the energy and spatially resolved tunneling spectroscopy of the topological quantum spin Hall helical states driven out of equilibrium. When helical liquid is constrained between two superconducting reservoirs transport at the edge is governed by the multiple Andreev reflections. The resulting distribution functions of the edge channels exhibit multiple discontinuities at the subgap energies with the periodicity of an applied voltage. The combined effect of interactions and disorder leads to the inelastic backscattering processes mixing different helicity modes thus causing smearing of these singularities. If equilibration is strong then distribution functions of the edge channels collapse into a single Fermi-like function with an effective temperature determined by the superconducting gap, applied voltage and interaction parameter. We conclude that mapping out nonequilibrium distribution function in the experiments may provide valuable information about the relevant perturbations that spoil ballistic edge transport. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A42.00008: Time evolution of wave-packets in topological insulators Poliana H. Penteado, Sebastian Duque Mesa, Gerson J. Ferreira, J. Carlos Egues Topological insulators (TIs) are a fantastic new class of materials that have gapless helical surface (3D TIs) or edge (2D TIs) states embedded within the bulk gap of its host material. This unique property rises from an interface between materials with topologically inequivalent sets of bands structures, i.e. gaps with different signs. Here we investigate the time-evolution of wave-packets in TIs. Within the Dirac equation, the interference between eigenstates from positive and negative energy bands leads to the relativistic oscillatory behavior well known as Zitterbewegung. It was recently discussed the time evolution of the guiding center of a wave-packet in TIs converging towards the edge states. Here we show a more detailed discussion of the evolution of the full wave-packet and its behavior regarding the collision with the edges of the system. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A42.00009: Steady States of Floquet Topological Insulators in the presence of Electron-Phonon Interaction Karthik Seetharam, Netanel Lindner, Gil Refael Floquet topological insulators (FTI) employ a well chosen periodic drive to induce a non-equilibrium topological state in an otherwise trivial semiconductor system. By using a periodic drive, time translation symmetry is partially broken and the topological features are captured by the resulting quasienergy bands defined modulo the drive frequency. When considering a solid state system with electron-phonon interaction, the inevitable contribution of relaxation processes involving multi-photon transitions (``Umklapp''-type processes in quasienergy) leads to unique steady states, which differ from a Fermi-Dirac distribution expected in the absence of such processes. Understanding these steady states is crucial for calculating transport properties of the driven system. Using kinetic equations we study the evolution of the quantum state of the FTI in the presence of electron-phonon interaction, obtain a description of the steady state of the driven system, and study the transport properties of the FTI. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A42.00010: Ultrafast Dynamics of Bi1.5Sb0.5Te1.8Se1.2 Topological Insulator Liang Cheng, Chi-Sin Tang, Saritha Krishnankutty Nair, Bin Xia, Lan Wang, Jian-Xin Zhu, Ee Min Elbert Chia Bi1.5Sb0.5Te1.8Se1.2~(BSTS) is a type of topological insulator, which is an insulator in bulk but surface states are gapless. In this work, we took optical pump-probe data on BSTS crystal to analyze the dynamics of phonons and charge carriers. The ultrafast dynamics were obtained as a function of temperature ranging from 10K to 300K, as well as fluence ranging from 1 $\mu $J/cm2 to 10 $\mu $J/cm2. In additional to the coherent optical phonon mode found in other topological insulators, acoustic phonon mode was observed in our experiment. We also observed phonon softening and the temperature dependence of carrier lifetime in BSTS. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A42.00011: Towards Phononic Topological Insulators Pai Wang, Katia Bertoldi Recent studies in optics have shown that the concept of topological insulators can be extended to band theories of classical waves and bosonic systems. Here, we present some design considerations in realization and observation of topological edge states for phonons. The goal is to achieve topologically protected one-way propagation of surface acoustic / elastic waves against back-scattering and localization due to defects and disorders by utilizing phononic crystals, which have micro-structures with periodicity comparable to the wavelength of the propagating elastic waves. Both theoretical and practical challenges in creating non-reciprocal elastic media will be discussed. Possible candidates include temporal modulation of phononic crystals, coupled wave guides, chiral local resonators, artificial magneto-acoustic effects and asymmetric body forces induced by external fields. These symmetry breaking mechanisms can potentially lead to the phononic analogue of electronic quantum hall effect. The robustness of reflection-immune unidirectional elastic wave has promising applications in surface acoustic wave (SAW) devices that are widely used in modern telecommunication, geophysics as well as micro-fluidics. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A42.00012: Dielectric Screening of Surface States in a Topological Insulator J.P.F. LeBlanc, J.P. Carbotte Hexagonal warping provides an anisotropy to the dispersion curves of the helical Dirac fermions that exist at the surface of a topological insulator. We show how modifications to the Dirac spectrum by inclusion of hexagonal warping, as well as a Schr\"odinger and gap term modify the polarization function of the surface states. We derive in the long wavelength limit the plasmon dispersion and show that it obtains a weak dependence on the direction of scattering momentum, q. Further, we show numerically the plasmon dispersions at large q and find considerable directional anisotropy of the plasmon bands in comparison to the pure Dirac plasmons. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A42.00013: Observation of resonance in second harmonic generation measurements of topologically insulating Bi2Se3 thin films Mikel Holcomb, Yuri Glinka, Sercan Babakiray, Trent Johnson, Alan Bristow, David Lederman Second harmonic generation is an ideal probe of topological insulator surface states due to its sensitivity to space inversion symmetry breaking, which naturally occurs at a material's surface. We measured the angular dependence of second harmonic intensity for s-s, p-p, s-p and p-s polarization configurations of the incoming and outgoing light in films ranging in thickness from 6 to 40 nm of the topological insulator Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$. We assign each of these angular arrangements to the symmetry of specific Se and Bi atomic layers near the surface of the material. Exploiting this information, we separate the bulk and surface crystal structure contributions. Modelling the response requires use of a conventional second-order nonlinear term and a third-order electric-field induced SHG term. The latter dominates the thickness dependence showing a strong peak at about 10 nm. We apply appropriate models to explain this behavior and will discuss the resonance-like feature observed within a small thickness range and its implications. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A42.00014: Topological polaritons from non-topological quantum-wells in an optical cavity Torsten Karzig, Netanel Lindner, Gil Refael We study the formation of topological polaritons from semiconductor excitons coupled to cavity photons. Oscillating classical electromagnetic fields can turn a trivial band structure into a Floquet topological insulator. In a similar spirit cavity photons can induce topology when coupling to otherwise trivial excitons. We discuss the necessary ingredients to lend the polaritons a non-trivial topology through a ``winding'' coupling of the excitons to the photons. One hallmark signature of the topological polaritons are chiral edge modes which allow for unidirectional photon propagation as a part of the exciton-photon edge mode. [Preview Abstract] |
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