Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session B08: Topological Insulator Experiment and Computation |
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Sponsoring Units: DCMP Chair: Ching-Kai Chiu, University of Maryland, College Park Room: LACC 153C |
Monday, March 5, 2018 11:15AM - 11:27AM |
B08.00001: THz spectroscopy of electric field modulated Dirac surface states of topological insulator thin films Mintu Mondal, Dipanjan Chaudhuri, Maryam Salehi, Cheng Wan, Nicholas Laurita, Bing Cheng, Michael Quintero, Deepti Jain, Jisoo Moon, Pavel Shibayev, Tyrel McQueen, Seongshik Oh, Peter Armitage Topological insulators (TIs) exhibit symmetry protected conducting surface states with Dirac like dispersion and novel spin textures. TIs can also be described as bulk magnetoelectric materials with quantized magneto-optical response functions. However, observing such phenomena in real materials can be challenging because of the finite bulk contributions which tend to screen the surface state effects. Here, we present a novel ionic gel gating technique that can be used to tune the chemical potential while simultaneously measuring the optical spectra. We demonstrate that the Fermi level for naturally n-doped Bi_{2}Se_{3} films can be lowered down to ~10 meV above the Dirac point, resulting in a reduction in the carrier concentration by an order of magnitude. At high magnetic fields, quantization of Faraday angle is observed which provides evidence for the topological magnetoelectric effects. Furthermore, we also demonstrate that the filling factor in the surface states can be altered by changing the applied bias voltage. |
Monday, March 5, 2018 11:27AM - 11:39AM |
B08.00002: Insulator-semimetal transitions from annihilating and restoring topological surface Dirac points Yang Xu, I. Miotkowski, Yong Chen Pairwise merging and annihilation of Dirac points is known to give a topological transition from a semimetal to an insulator. Here, we report transport studies on thin films of BiSbTeSe_{2} (BSTS), which in its bulk form is a three-dimensional topological insulator (3D TI) that hosts gapless (semi-metallic) Dirac fermion surface states (SS). When the sample is reduced to ~10 nm thick, the hybridization between opposite surfaces opens a gap thus annihilates the Dirac points, giving rise to an insulating behavior. We observe that an in-plane magnetic field can drive the system again towards a metallic behavior, with a prominent negative magnetoresistance and a temperature-insensitive resistivity close to h/2e^{2} at the charge neutral point. This is in general agreement with a predicted in-plane magnetic field induced restoration of Dirac points, split in the momentum space and giving rise to a tunable graphene-like band structure. Such topological transitions of Dirac cones as controlled by the TI thickness and magnetic fields can be exploited for topological electronics and spintronics applications. |
Monday, March 5, 2018 11:39AM - 11:51AM |
B08.00003: Non-magnetic disorder threshold of the topological state in Sb_{2}Te_{3} thin films Shihua Zhao, Inna Korzhovska, Zhiyi Chen, Haiming Deng, Lukas Zhao, Marcin Konczykowski, Simone Raoux, Lia Krusin-Elbaum We establish a disorder threshold for the topological state in thin films of Sb_{2}Te_{3} by following the disorder evolution of transport across large range of structural nonmagnetic disorder. Starting at the high disorder end, where 3D transport is found to be governed by dynamic spin correlations, we recover the low disorder state by thermal annealing. Weak antilocalization interference correction (WAL), characteristic of 2D topological transport related to the Berry phase, is recovered at a relatively low disorder level corresponding to W_{th} = 0.05E_{g}, where E_{g} is the bulk gap. The 3D-2D transition coincides with the disappearance of disorder-induced glassy spin correlations, directly measured using micro-Hall sensors. The sharp onset of magnetic response appears independent of disorder, consistent with the presence of FM clusters with T_{C} ~ 200 K that shrink in size as disorder is reduced. At W_{th}, transport is dominated by crystalline regions with minimal spin scattering, thereby supplying a percolative path for the carriers to travel along, with conductance of about G_{0}=e^{2}/h _{. }We discuss the role of spin relaxation in inducing the dimensionality transition into the topological state. |
Monday, March 5, 2018 11:51AM - 12:03PM |
B08.00004: Hydrodynamic flow of topological electrons Valentin Mueller, Yuan Yan, Oleksiy Kashuba, Raimund Schlereth, Björn Trauzettel, Hartmut Buhmann, Laurens Molenkamp The electron transport in metals and semiconductors is conventionally determined by the momentum-relaxing scattering, e.g., electron-phonon and electron-impurity scattering. However, an exotic hydrodynamic flow of topological electrons is experimentally observed in three-dimensional topological insulator HgTe, where the transport seems to be dominated by the momentum conserved electron-electron scattering. In these experiments, HgTe narrow channels with different length-to-width ratios (1:3, 1:5, and 1:10) were fabricated via a refined wet etching process, which defines the physical boundary and maintains high electron mobility. In particular, to modulate the strength of the electron-electron scattering, a DC bias current was applied to heat up the channel by Joule heating. As the DC heating current increases, the differential resistances of the HgTe narrow channels initially increases and then decreases, which is reminiscent of the Gurzhi effect reported by M.J.M. de Jong and L.W. Molenkamp. |
Monday, March 5, 2018 12:03PM - 12:15PM |
B08.00005: Transport in topological insulator thin films with bulk-surface coupling: interference corrections and conductance fluctuations Hristo Velkov, Tobias Micklitz, Georg Schwiete Motivated by the experimental difficulty to produce topological insulators of the Bi2Se3 family with pure surface-state conduction, we study the effect that the bulk can have on the low-temperature transport properties of gated thin TI films. In this talk, I will mainly discuss conductance fluctuations in various parameter regimes. I will argue that combined measurements of conductance fluctuations and interference corrections, weak localization or weak anti-localization, can provide an improved understanding of the physics underlying the low-temperature transport processes in thin TI films with bulk-surface coupling. |
Monday, March 5, 2018 12:15PM - 12:27PM |
B08.00006: Electronic Properties of thin films of strained HgTe. Jihang Zhu, Chao Lei, Allan MacDonald Strained HgTe is a small band gap 3D topological insulator. Bulk HgTe therefore has 2D Dirac-like surface states that are protected by the time-reversal symmetry. Because of the small gap, the surface state localization length is however fairly large and the separation between bulk and surface effects in the thin film samples that are available experimentally is not always clear. With this motivation we have examined the multi-band envelope description of strained HgTe thin films with thicknesses between 10 and 100 nm, accounting carefully for both electrostatic and exchange-correlation interaction effects, to provide a basis for interpreting measurements of surface and bulk state electronic properties that are tuned by varying gate voltages. We find that the films respond like semiconductors to gates that induce electron gases, but like metals to gates that induce hole gases. We are able to provide a simple explanation for recent experiments that characterize HgTe thin films using capacitance and thermopower measurements. In particular, we find that it is not necessary to invoke phonon drag to explain the large difference in thermopower between electron and hole cases. |
Monday, March 5, 2018 12:27PM - 12:39PM |
B08.00007: Weyl Points of the Entanglement Spectrum for Topological Insulators from First Principles Hiromu Araki, Takahiro Fukui, Yasuhiro Hatsugai As for the three dimensional topological insulators, the parity of the Weyl points of an entanglement spectrum in the half of the Brillouin zone characterizes the strong topological insulator (STI) phase from the weak one (WTI phase)[1]. The STI/WTI phase corresponds to the odd/even parity. Without using any symmetry such as spatial inversion, one can easily specify the Weyl points by the section Chern number of the entanglement Hamiltonians. We apply this characterization method to realistic topological insulator materials using the first principle calculations. |
Monday, March 5, 2018 12:39PM - 12:51PM |
B08.00008: Strain Tuning of the Optical Absorbance in Topological Insulator Material Bismuth Selenide Mathias Jensen, Jens Paaske, Anders Lunde, Morten Willatzen Bismuth selenide is among the first materials discovered to be a three-dimensional topological insulator (TI) [Zhang et al., Nat. Phys. 5, 438 - 442 (2009)]. The topology of the bandstructure engenders electronic surface states located at the boundary of the material with linear dispersion within the bulk band gap. Similar to graphene TIs show great potential for optoelectronic applications [Zhang et al., PRB 82, 245107 (2010)]. We present calculations of the optical absorbance using a k.p model and Fermi's golden rule including all symmetry allowed strain terms to first order. The absorbance edge for a thin film can be effectively tuned by applying uniaxial strain, and for large strains a Van Hove singularity leads to a diverging band edge absorbance. Shear strain breaks the isotropy of the model leading to a polarization dependent absorbance. Remarkably the directional average is always found to increase, independent of the material parameters |
Monday, March 5, 2018 12:51PM - 1:03PM |
B08.00009: Band Structure Engineering in 3D Topological Insulators Lukasz Plucinski, Gregor Mussler, Gustav Bihlmayer, Ewa Mlynczak, Stefan Bluegel, Detlev Gruetzmacher, Claus Schneider We will present our recent combined experimental and theoretical results on band structure engineering in 3D topological insulator (3D TI) bilayers and superlattices. These results show how new topologies emerge in complex structures, as compared to the routine Fermi level control by alloying, and provide a starting point in search for novel topological phases. In topological pn-junction we have demonstrated Fermi level control by the relative thicknesses of the layers [1], while in superlattices which combine Bi_{2}Te_{3} quintuple layers and Bi bilayers we have predicted dual topological properties, and experimentally demonstrated the existence of non-trivial topological crystalline insulator (TCI) crossings away from the surface Brillouin zone center [2]. |
Monday, March 5, 2018 1:03PM - 1:15PM |
B08.00010: Hinge states in SnTe Zhijun Wang, Frank Schindler, Ashley Cook, Maia Vergniory, Stuart Parkin, Andrei Bernevig, Titus Neupert Three-dimensional topological insulators are materials with an insulating bulk, but conducting surface states topologically protected by time-reversal symmetry. Here, we extend the notion of three-dimensional topological insulators to systems that host no gapless surface states, but exhibit topologically protected gapless hinge states. Their topological character is protected by spatio-temporal symmetries, where we distinguish two cases: (1) Chiral higher-order topological insulators protected by the combination of time-reversal and a four-fold rotation symmetry. Their hinge states are chiral modes and the topology is Z2 classified. (2) Helical higher-order topological insulators protected by time-reversal and mirror symmetries. Their hinge states come in Kramers pairs and the topology is Z classified. Furthermore we show that uniaxially strained SnTe is a helical higher-order topological insulator with one Kramers pair of gapless modes on each hinge. |
Monday, March 5, 2018 1:15PM - 1:27PM |
B08.00011: Topological nodal-line semimetal in nonsymmorphic Cmce-phase Ag_{2}S Huaqing Huang, Kyung-Hwan Jin, Feng Liu Based on first-principles calculations and symmetry analysis, we discovery that the Ag_{2}S with Cmce symmetry is a topological nodal-line semimetal in the absence of spin-orbit coupling. A single nodal loop as protected by the glide symmetry exists around the center of the Brillouin zone, dispersing slightly in the momentum space to form both electron and hole pockets around the loop. Moreover, a nearly flat drumheadlike surface state appears on the (001) surface of this material. The nodal-line semimetal phase and its drumheadlike surface states are expected to be experimentally detectable in Cmce-phase Ag_{2}S, because spin-orbit coupling will only open a negligible gap comparing to the energy dispersion of the nodal loop. Our finding provides a new member to the growing family of nodal-line semimetals with a single nodal loop structure. |
Monday, March 5, 2018 1:27PM - 1:39PM |
B08.00012: Weakly Protected 1D Topological Step Edge States at the Surface of a 3D Topological Insulator Yishuai Xu, Janet Chiu, Lin Miao, Erica Kotta, Rudro Biswas, Lewis Wray Topological insulators in the Bi_{2}Se_{3} family manifest helical Dirac surface states that span the topologically ordered bulk band gap. Recent scanning tunneling microscopy measurements have discovered in-gap states at step edges in the bulk band gap of Bi_{2}Se_{3} and Bi_{2}Te_{3}, possibly caused by a band bending-like phenomenon. We use numerical simulations of a topological insulator surface to explore the phenomenology of edge state formation at the single-quintuple-layer step defects found ubiquitously on these materials. The modeled one-dimensional edge states are found to exhibit a protected topological connection to the two-dimensional surface state Dirac point. I will discuss the emergence and physical significance of this topologically connected 1D edge state, as well as how it evolves under broken symmetries and disorder. |
Monday, March 5, 2018 1:39PM - 1:51PM |
B08.00013: Coexisting Surface States in the Weak and Crystalline Topological Insulator Bi_{2}TeI Nurit Avraham, andrew norris, Yan Sun, yanpeng qi, Anna Isaeva, Claudia Felser, Binghai Yan, Haim Beidenkopf The established diversity of electronic topology classes lends the opportunity to pair them into dual topological complexes. We show that the layered compound Bi_{2}TeI realizes a dual topological insulator. It exhibits band inversions at two time reversal symmetry points of the bulk band which classify it as a weak topological insulator. Additional mirror symmetry of the crystal structure concurrently classifies it as a topological crystalline insulator. Bi_{2}TeI is therefore predicted to host a pair of Dirac cones protected by time reversal symmetry on its ’side’ surfaces and three pairs of Dirac cones protected by mirror symmetry on its top and bottom surfaces. We spectroscopically map the top cleaved surface of Bi_{2}TeI, and crystallographic step edges therein. We show the existence of both two dimensional surface states which are susceptible to mirror symmetry breaking, as well as one dimensional channels that reside along the step edges. Their mutual coexistence on the step edge where both facets join is facilitated by momentum and energy segregation. |
Monday, March 5, 2018 1:51PM - 2:03PM |
B08.00014: Discovery of a weak topological phase in a transition-metal pnictide Niels Schröter, Wujun Shi, Juan Jiang, Fernando De Juan, Yanfeng Guo, Lexian Yang, Cheng Chen, Han Peng, Sandy Ekahana, Yiwei Li, Claudia Felser, Binghai Yan, Yulin Chen Topological insulators (TIs) can host a variety of exotic phenomena, such as spin-polarized topological surface states (TSSs), topological magnetoelectric effects, and topological superconductivity. Early on, it was realized that TIs can be grouped into two families: strong TIs that host TSSs on all crystal surfaces, and weak TIs, which host TSSs only on some surfaces. Initially, it was believed that unlike for strong TIs, TSSs in weak TIs would be susceptible to localization by disorder. However, subsequent theoretical studies found that weak TIs should be more robust against disorder than originally assumed and that they can harbour unusual physical properties, such as 1D conduction channels at step edges and in screw dislocations, which are protected from localization by topology. Yet, despite multiple theoretical proposals and experimental reports, “smoking-gun” evidence for the existence of a weak topological phase - such as momentum resolved measurement of a TSS dispersion - is still elusive. Here, by performing angle-resolved photoemission measurements and ab-initio calculations for the surface states of two crystal surfaces, we can conclusively confirm the existence of a weak topological phase in a transition-metal pnictide. |
Monday, March 5, 2018 2:03PM - 2:15PM |
B08.00015: The Study of Plasmons in Topological Insulators and Semimetals Using High Resolution Electron Energy Loss Spectroscopy Xun Jia, Xuetao Zhu, Jiandong Guo Plasmon is a fundamental collective excitation in physics related to numerous technological applications. However plasmons usually have very short lifetime in large momentum range due to strong damping, independent of their origin from normal massive electrons or from Dirac massless electrons. Then the properties of plasmons in topologically nontrivial systems that have massless electrons with longer lifetime are extremely important. Here, we report the study of the properties of plasmons in three-dimensional (3D) Topological insulators (TIs), Dirac Semimetals (DSMs) and Weyl Semimetals (WSMs). In a typical 3D TI, Bi_{2}Se_{3}, we observed an anomalous acoustic plasmon on the surface with weak damping in a large momentum range, which likely relates to the spin-momentum locking feature of the TI surface states. However, in DSM and WSM, it's more complicated because both topologically trivial and nontrivial 3D bands contribute to Fermi Surface (FS) and thus they are both involved in the origination of plasmons. In PdTe_{2} (a type-II DSM) and MoTe_{2} (a type-II WSM),_{ }we both observed possible surface and bulk plasmon modes. By carefully examining the dependence of the plasmon energy on the momentum, electron density and Fermi velocity, we provide possible explanations to these modes. |
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