Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session D10: Optical and THz Properties of Topological Insulators |
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Sponsoring Units: DCMP Chair: Dennis Drew, University of Maryland Room: 007A |
Monday, March 2, 2015 2:30PM - 2:42PM |
D10.00001: Transient reflectance of photoexcited electrons and holes in cadmium arsenide Chris Weber, Bryan Berggren, Ernest Arushanov, Tahereh Hosseini, Nikolai Kouklin We report ultrafast transient-grating measurements of crystals of the three-dimensional Dirac semimetal cadmium arsenide, Cd$_3$As$_2$, at both room temperature and 80 K. After photoexcitation with 1.5-eV photons, charge-carriers relax by two processes, one of sub-picosecond duration and the other of duration 3 ps. By measuring the complex phase of the change in reflectance, we determine that the faster signal corresponds to an increase in phase velocity, and the slower signal to a decrease in absorption, at the probe energy. We assign the fast signal to free-carrier absorption from photoexcited electron and hole populations, which relax by recombination, and the slower signal to phase-space filling by thermally excited electrons. The proposed processes closely mirror the response of graphene to photoexcitation. We also present evidence that both the electrons and the lattice are strongly heated. [Preview Abstract] |
Monday, March 2, 2015 2:42PM - 2:54PM |
D10.00002: Gated Terahertz Magneto-optical Measurements of 3D Dirac Semimetals Don C. Schmadel, Gregory S. Jenkins, Andrei B. Sushkov, Remington L. Carey, H. Dennis Drew, Jason W. Krizan, Satya Kushwaha, Quinn Gibson, Robert J. Cava We report gated terahertz magneto-optical measurements of the bulk and surface states of Na$_3$Bi and Cd$_3$As$_2$. The onset of interband transitions within the bulk 3D Dirac cone is observed. A gate is used to modulate surface carriers and bulk carriers near the surface resulting in differential optical signals. Gate-modulated cyclotron resonance and Fourier transform infrared spectroscopy characterize the bulk Dirac carriers along all three crystal axes as well as the Fermi-arc carriers with Fermi Energy. The transport scattering and Fermi velocity anisotropy as well as potential fluctuations characterized near the Dirac point will be discussed. [Preview Abstract] |
Monday, March 2, 2015 2:54PM - 3:06PM |
D10.00003: Theory of Kerr and Faraday rotation in Topological Weyl Semimetals Mehdi Kargarian, Mohit Randeria, Nandini Trivedi Topological Weyl semimetals are characterized by bulk Dirac nodes separated in momentum space by a distance $2b$ and lead to Fermi arcs in the surfaces electronic structure. We calculate the Faraday $\theta_F$ and Kerr $\theta_K$ angles for electromagnetic waves scattered from such a Weyl semimetal using the Kubo formalism. (1) For thin films with electromagnetic radiation incident on a surface without arcs, we show that $\theta_K=bd/\alpha\pi$ and $\theta_F=\alpha\pi/bd$ where $\alpha$ is the fine structure constant, and the film thickness $d \ll \lambda$, the wavelength. We further show multiple reflections give rise to giant Kerr rotation, under certain conditions, for a film on a substrate. (2) In the case when the electromagnetic radiation is incident on the surface with arcs, the wave propagating inside the material acquires a longitudinal component of the electric field proportional to $b$. We discuss the implications of our results for thin films of pyrochlore iridates, and also for the recently discovered Dirac semimetals in a magnetic field. [Preview Abstract] |
Monday, March 2, 2015 3:06PM - 3:18PM |
D10.00004: Temperature-driven band inversion in Pb$_{0.77}$Sn$_{0.23}$Se: Optical and Hall-effect studies Naween Anand, Zhiguo Chen, Sanal Buvaev, C. Martin, Kamal Choudhary, Genda Gu, S. Sinnott, Zhiqiang Li, A. Hebard, D. Tanner Optical and Hall-effect measurements have been performed on single crystals of Pb$_{0.77}$Sn$_{0.23}$Se, a IV-VI mixed chalcogenide. The temperature dependent (10-300 K) reflectance was measured over 40-7000 cm$^{-1}$ (5-870 meV) with an extension to 15,500 cm$^{-1}$ (1.92 eV) at room temperature. The reflectance was fit to the Drude-Lorentz model using a single Drude component and several Lorentz oscillators. The optical properties at the measured temperatures were estimated via Kramers-Kronig analysis as well as by the Drude-Lorentz fit. The carriers were p-type with the carrier density determined by Hall measurements. A signature of valence intraband transition is found in the low-energy optical spectra. It is found that the valence-conduction band transition energy as well as the free carrier effective mass reach minimum values at 100 K, suggesting temperature-driven band inversion in the material. Some of the results from optical studies were compared with density function theory calculations. [Preview Abstract] |
Monday, March 2, 2015 3:18PM - 3:30PM |
D10.00005: Single Photon Transport through an Atomic Chain Coupled to a One-dimensional Photonic Waveguide Zeyang Liao, Xiaodong Zeng, M. Suhail Zubairy We study the dynamics of a single photon pulse travels through a linear atomic chain coupled to a one-dimensional (1D) single mode photonic waveguide. We derive a time-dependent dynamical theory for this collective many-body system which allows us to study the real time evolution of the photon transport and the atomic excitations. Our result is consistent with previous calculations when there is only one atom. For an atomic chain, the collective interaction between the atoms mediated by the waveguide mode can significantly change the dynamics of the system. The reflectivity can be tuned by changing the ratio of coupling strength and the photon linewidth or by changing the number of atoms in the chain. The reflectivity of a single photon pulse with finite bandwidth can even approach 100{\%}. The spectrum of the reflected and transmitted photon can also be significantly different from the single atom case. Many interesting physics can occur in this system such as the photonic bandgap effects, quantum entanglement generation, Fano-type interference, superradiant effects and nonlinear frequency conversion. For engineering, this system may be used as a single photon frequency filter, single photon modulation and photon storage. [Preview Abstract] |
Monday, March 2, 2015 3:30PM - 3:42PM |
D10.00006: Electrostatic tuning of the surface states of irradiated topological insulators Inna Korzhovska, Lukas Zhao, Yury Deshko, Ghidewon Arefe, Marcin Konczykowsky, Lia Krusin-Elbaum One of the main obstacles to accessing charge transport through Dirac surface states of topological insulators (TIs) is a significant conduction in the bulk. We have developed a new approach of reaching a \textit{stable} charge neutrality point (CNP) using irradiation with 2.5 MeV energy electrons. By controlling the beam fluence and annealing protocol we can convert bulk conductivity from \textit{p}- (hole-like) to \textit{n}-type (electron-like) and back, crossing the Dirac point while preserving the robust topological signatures of surface channels. Electron beams act to compensate charged bulk defects and pull the Fermi level into the bulk gap -- a process that decreases bulk conductivity by orders of magnitude to a minimum, $\sigma_{min}$, at CNP. We study the origins of minimum conductivity in electron-irradiated TIs in a transistor-like gated structures fabricated in inert environment by mechanical exfoliation with Bi$_2$Te$_3$ as a prototypical TI and h-BN as a gate dielectric. The effects of electrostatic tuning by the gate bias voltage on surface conductivity near CNP will be presented. [Preview Abstract] |
Monday, March 2, 2015 3:42PM - 3:54PM |
D10.00007: Effects of the topological phase transition and band inversion on ultrafast dynamics in topological crystalline insulators Y.M. Dai, J. Bowlan, A.J. Taylor, D.A. Yarotski, R.P. Prasankumar, R.D. Zhong, G.D. Gu, T. Valla, C.C. Homes, T. Yilmaz, B. Sinkovic Topological crystalline insulators, characterized by a gapless metallic state on their high-symmetry surfaces that is protected by crystalline symmetry, are realized both theoretically and experimentally in the Pb$_{1-x}$Sn$_{x}$Te and Pb$_{1-x}$Sn$_{x}$Se compounds. In these materials, a topological phase transition and band inversion can be induced by doping, pressure or temperature. We use femtosecond optical pump-probe spectroscopy to study the evolution of the ultrafast dynamics as a function of both temperature and doping in the Pb$_{1-x}$Sn$_{x}$Te system. The influence of these parameters on the topological phase transition and band inversion, as well as on quasiparticle dynamics, will be discussed. [Preview Abstract] |
Monday, March 2, 2015 3:54PM - 4:06PM |
D10.00008: Cyclotron resonance of surface states in the bulk-insulating topological insulator Bi$_2$Se$_3$ by THz spectroscopy Liang Wu, Wang-Kong Tse, Christopher Morris, Matthew Brahlek, Nikesh Koirala, Seongshik Oh, Peter Armitage We have utilized magneto-optical time-domain terahertz spectroscopy to investigate the low frequency optical response of topological insulator films of Cu$_{x}$Bi$_2$Se$_3$ and Bi$_2$Se$_3$. Such experiments give sufficient information to measure the mobility and density of multiple conduction channels simultaneously. Sharp cyclotron resonances (CRs) were observed in both samples by Faraday rotation experiments. We find that the Cu$_{x}$Bi$_2$Se$_3$ films with certain Cu concentration are bulk insulators with only surface conduction channels. This is consistent with pure topological surface states conduction and an E$_{F}$ that is $\sim$150 meV above Dirac point (around 70meV below conduction band minimum). Hence, a true topological insulator with insulating bulk is realized. In contrast, we find that Bi$_2$Se$_3$ with E$_{F}$ $\sim$350 meV above Dirac point has two channels; a dominant one that exhibits a CR in the Faraday rotation comes from surface states and a second channel which does not show a CR comes from bulk and/or 2DEG. Orbital effect on the electrodynamics of surface states and electron-phonon interaction are also discussed. [Preview Abstract] |
Monday, March 2, 2015 4:06PM - 4:18PM |
D10.00009: Probing topological transitions in HgTe/CdTe quantum wells by magneto-optical measurements Benedikt Scharf, Alex Matos-Abiague, Jaroslav Fabian, Igor Zutic In two-dimensional topological insulators, helical Quantum Spin Hall (QSH) states persist even at finite magnetic fields below a critical magnetic field $B_{\mathrm{c}}$, above which only Quantum Hall (QH) states can be found [1]. Using linear response theory, we theoretically investigate the magneto-optical properties of inverted HgTe/CdTe quantum wells, both for infinite two-dimensional and finite-strip geometries, and possible signatures of the transition between the QSH and QH regimes. In the absorption spectrum, several peaks arise due to non-equidistant Landau levels in both regimes. However, in the QSH regime, we find an additional absorption peak at low energies in the finite-strip geometry. This peak arises due to the presence of edge states in this geometry and persists for any Fermi level in the QSH regime, while in the QH regime the peak vanishes if the Fermi level is situated in the bulk gap. Thus, by sweeping the gate voltage, it is potentially possible to distinguish between the QSH and QH regimes. Moreover, we investigate the effect of spin-orbit coupling and finite temperature on this measurement scheme.\\[4pt] [1] B. Scharf, et al., Phys. Rev. B 86, 075418 (2012). [Preview Abstract] |
Monday, March 2, 2015 4:18PM - 4:30PM |
D10.00010: Coupling two lasers on a dielectric surface Cristian Bahrim, Md Khairuzzaman, Md Mozammal Raju, Wei-Tai Hsu We can modify the radiation perceived by a dielectric surface using a capacitor voltage set up across. The associated uniform electric field allows us to shift toward shorter wavelengths the optical response of the dielectric surface for a given monochromatic laser radiation incident on it. We use this capacitor configuration for coupling two laser beams incident simultaneously on the surface. The stronger coupling laser couples with the electric dipoles and impedes a probe laser to oscillate the same dipoles. The interaction between the two laser beams creates a destructive interference pattern in the Brewster angle region of the probe. Clear evidence of several minima of diffraction shows along the direction of reflection of the probe laser. This diffraction pattern indicates the `lock in' of the probe laser on the surface. This new physics is related to an electromagnetic induced transparency (EIT)-type phenomenon with the major difference that in our case the coupling between two lasers is produced at the dielectric's surface rather than inside the bulk, as is typically the case in classical EIT. Our measurements indicate that the light is reflected by the first layer of dipoles on the surface, within one chemical bond. [Preview Abstract] |
Monday, March 2, 2015 4:30PM - 4:42PM |
D10.00011: Measurement of a topological edge invariant in a microwave network Jason C. Pillay, Wenchao Hu, Kan Wu, Michael Pasek, Perry Ping Shum, Yidong Chong We report on the experimental measurement of topological edge invariants in an electromagnetic analog of a topological insulator, realized by a classical microwave network. This experiment serves as a classical electromagnetic realization of Laughlin's ``topological pumping'' thought experiment for the quantum Hall effect. The experiment consists of determining the electromagnetic scattering matrix, based on the input and output wave amplitudes measured at the edges of the network via a network analyzer. The winding on the scattering matrix eigenvalues, resulting from a tunable phase shift built into the network, forms a topological invariant. We demonstrate the existence of a topologically trivial phase, where the winding is zero (no edge states), as well as a topologically nontrivial phase, where the winding is non-zero (topological edge states present). Unlike most other systems used to study topological insulator physics, the full complex scattering parameters can be measured in this setup. As in most microwave experiments, however, our system is susceptible to losses. We show that topological behavior can be meaningfully defined in the experiment despite the effects of loss. [Preview Abstract] |
Monday, March 2, 2015 4:42PM - 4:54PM |
D10.00012: Experimental realization of microwave photonic topological insulators Jianwen Dong Topological properties play a fundamental role in many physical phenomena. While topology focus on electronic systems, there has been a recent emergence of interest in exploring topological orders with photons. Recent experiments have demonstrated substantial progress towards the implementation of Hamiltonians with topological robustness, from microwave to visible frequency domains. Here, we will show the demonstration on nontrivial photonic bandgaps, as well as the topologically protected edge states. We designed and fabricated a metacrystal comprising non-resonant meta-atoms sandwiched between two metallic plates. Spin Chern number of photonic crystals is calculated based on group theory and accurately predicts topological characters of edge states in different gaps. Topologically nontrivial gaps are achieved by mode exchange at high symmetric k-points. Nontrivial bandgap was confirmed by experimentally measured transmission spectra and calculated nonzero spin Chern number. Gapless spin-filtered edge states were demonstrated experimentally by measuring Ez fields and Hz fields, as well as their phase differences. Robustness of the edge states were also observed when an obstacle is introduced near the edge. [Preview Abstract] |
Monday, March 2, 2015 4:54PM - 5:06PM |
D10.00013: Persistent Optical Gating of a Topological Insulator Heterostructure Andrew L. Yeats, Yu Pan, Anthony Richardella, Peter J. Mintun, Nitin Samarth, David D. Awschalom We demonstrate persistent, bidirectional control of the chemical potential in a (Bi,Sb)$_{2}$Te$_{3}$/SrTiO$_{3}$ heterostructure through a two-color all-optical technique. By manipulating the space-charge distribution in a SrTiO$_{3}$ substrate, we locally tune the field effect in a (Bi,Sb)$_{2}$Te$_{3}$ channel comparably to electrostatic gating techniques but without additional materials or processing. The effect persists for thousands of seconds and functions from cryogenic to ambient temperatures. This enables us to write and re-write arbitrarily shaped $\it{p}$- and $\it{n}$-type regions, which we characterize electrically and image with scanning photocurrent microscopy. The ability to rapidly prototype mesoscopic electronic structures in a topological insulator may aid in the investigation of the spin-polarized surface and edge states unique to the topological insulating phase. The optical patterning technique may be adaptable to other material systems, which could form a basis for reconfigurable electronics. [Preview Abstract] |
Monday, March 2, 2015 5:06PM - 5:18PM |
D10.00014: Circular Dichroism Observed by Photoemission from Ultrathin Bi$_{2}$Te$_{3}$ Films Cai-Zhi Xu, Yang Liu, Ryu Yukawa, Long-Xiang Zhang, Tom Miller, Tai-Chang Chiang Circular dichroism (CD) observed by photoemission from the surface states of topological insulators has drawn much interest. It was initially attributed to the spin polarization or chiral orbital momentum of the initial states, but later proven to also involve the final states. The detailed mechanism remains controversial. To address this question, we have performed measurements of ultrathin films of the prototypical topological insulator Bi$_{2}$Te$_{3}$ over a wide range of film thickness and photon energy. The results show that the CD depends not only on the photon energy, but also on the film thickness in a nontrivial manner. A theoretical model has been developed that involves dipole transition, surface photoemission, and spin-orbit coupling. The computed results are in good agreement with the general trends of the data including sign reversals as a function of photon energy and film thickness. The complex behavior of the measured CD function is partially caused by modifications of both the initial and final states in the thin film geometry. [Preview Abstract] |
Monday, March 2, 2015 5:18PM - 5:30PM |
D10.00015: Rashba effect and beating patterns in the THz magneto-photoresponse of a HgTe-based two-dimensional electron gas Mehdi Pakmehr, Christoph Bruene, Hartmut Buhmann, Laurens Molenkamp, Bruce McCombe HgTe quantum wells with a gapped single Dirac cone electronic dispersion relation have been investigated by THz magneto-photoresponse (PR) and magneto-transport measurements. The HgTe has the conventional band alignment at well thickness of 6.1 nm, slightly smaller than the critical thickness for the topological phase transition. The effective gap is roughly 10 meV, and the large sheet density of electrons (n$_{\mathrm{S}} \approx 1.5 \times$ 10$^{12}$ cm$^{-2}$) results in a very large Fermi energy (E$_{\mathrm{F}}$ $\sim$ 160 meV). We have found several interesting effects at these high densities. We focus here on an observed beating of quantum oscillations in the PR signal (at 1.83 THz) and compare it with direct measurements of oscillations in the longitudinal magneto-resistance (R$_{\mathrm{xx}}$). The mechanism for the PR is cyclotron resonance absorption heating of the electrons (an electron bolometric effect). We attribute the beating to Rashba splitting of the spin states, which is barely observable in direct R$_{\mathrm{xx}}$ measurements even under strong gate-induced electric fields. We will discuss the origin of the enhanced visibility of the Rashba effect in the PR and the magnitude of the Rashba coefficient ($\alpha_{R}$) from these data. [Preview Abstract] |
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