Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session M55: Transport and Thermodynamic Properties of Topological Systems: II |
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Sponsoring Units: DCMP Chair: Yaroslaw Bazaliy, Univ of South Carolina Room: Mile High Ballroom 2B |
Wednesday, March 4, 2020 11:15AM - 11:27AM |
M55.00001: Bulk insulation and surface electrons' properties of Pb(Bi,Sb)2Te4 Topological Insulator Yuya Hattori, Yuki Tokumoto, Keiichi Edagawa In the field of Topological Insulators (TIs), almost all the transport studies are limited to (Bi,Sb)2(Te,Se,S)3 (BSTS)TIs. For now, BSTS-TIs has the largest bulk resistivity and transport properties of surface states have been revealed in detail. However, the utilization of surface states for device application is still a bit difficult for now. |
Wednesday, March 4, 2020 11:27AM - 11:39AM |
M55.00002: Robustness of Helical Edge States Under Proximate Band Reconstruction Niels John, Bernd Rosenow, Adrian Del Maestro We analyze the edge structure of two-dimensional topological insulators described by the Bernevig-Hughes-Zhang model with the help of a self-consistent electrostatic modeling of sample edges, combined with a time-dependent perturbation theory approach. We find that for sufficiently smooth confinement, strongly interacting additional edge states arise under fairly general conditions. While their ground state is spin unpolarized, a spin-exchange coupling between helical and reconstructed edge states can lead to a dynamical spin polarization of the latter. However, we show that spatially random spin-orbit coupling inhibits such dynamical spin polarization and protects the helical edge states against backscattering. Further, we argue that at low electron density, the reconstructed edge states are well described by a Luttinger liquid. |
Wednesday, March 4, 2020 11:39AM - 11:51AM |
M55.00003: Transport properties of a topological system in non-integer dimensions Sonja Fischer, Lars Fritz, Mikael Fremling In recent years, the subject of topological materials has inspired a very active field of research. A considerable amount of effort was put into investigating the stability of topological edge modes under various kinds of disturbances, such as disorder. |
Wednesday, March 4, 2020 11:51AM - 12:03PM |
M55.00004: Probing spin-orbit interactions in monolayer WTe2 edge states Elliott Runburg, Wenjin Zhao, Zaiyao Fei, Joshua Mutch, Paul Malinowski, Bosong Sun, Xiong Huang, Yongtao Cui, Xiaodong Xu, Jiun-Haw Chu, David Cobden Monolayer WTe2 displays edge conduction that is consistent with the material being a two-dimensional topological insulator. The expected helical nature of the edge conduction should limit the conductance between adjacent contacts on an edge to e2/h, and this is consistent with experiments to date. However, the helical nature has not been supported by experiments that probe the form of the Hamiltonian of the edge states. To address this, we investigate the angular dependence of the magnetic field on the conduction of edges and cracks in monolayer WTe2 flakes as a function of gate voltage, temperature, and edge orientation relative to the crystal axes as determined by scanning microwave impedance microscopy. |
Wednesday, March 4, 2020 12:03PM - 12:15PM |
M55.00005: Coulomb Drag in 2D bilayer Topological Insulator systems Kai-Jie Yang, Chaoxing Liu InAs/GaSb quantum wells are 2D bilayer topological insulator systems possessing the inverted band structure with the conduction band bottom of the InAs layer below the valence band top of GaSb layer. Recent transport experiment in InAs/GaSb quantum wells has revealed an anomalous high frequency quantum oscillation coexisting with a large resistance when the chemical potential is tuned into the charge neutrality point. The high oscillation frequency (large carrier density) and large resistance (insulating behaviors) are in contradiction with each other and this fact cannot be explained in a single-particle picture, thus motivating us to study the interaction effect of this system. Based on the linear response theory and diagrammatic methods, we carefully investigated the Coulomb drag effect in this 2D topological insulator systems based on the simplified models. Our results shed the light in understanding the anomalous quantum oscillation in 2D bilayer topological insulator systems. |
Wednesday, March 4, 2020 12:15PM - 12:27PM |
M55.00006: Electrical study of Bi2Te3, Sb2Te3 and (Bi1-xSbx)2Te3 films Soorya Suresh Babu, Yang Bai, James Eckstein Bi2Te3 and Sb2Te3 are semiconductors that also behave as topological insulators when they are thin films. The topological properties mean that the surface conducting states have carriers that have spins locked normal to their momentum states. We have studied the growth of thin films grown by molecular beam epitaxy containing Bi2Te3, Sb2Te3 and alloys of (Bi1-xSbx)2Te3. As was discovered earlier1, this allows the chemical potential to be tuned from p-type to n-type by varying the relative composition. We have also grown short period superlattices and thin bilayers of the tellurides to study how this occurs when the binary phases are kept in separate layers. The growth on c-plane sapphire crystallizes into a flatter film when Bi2Te3 is the interfacial layer. Like in the case of alloys, the carrier type in the bilayers can be tuned by modifying layer thickness. In bilayers with different thicknesses we have studied how the conductivity depends on layer thickness. |
Wednesday, March 4, 2020 12:27PM - 12:39PM |
M55.00007: Wide band gap topological insulator via band engineering Ido Levy, Cody Youmans, Thor A Garcia, Haiming Deng, Steven Alsheimer, Christophe Testelin, Lia Krusin-Elbaum, Pouyan Ghaemi, Maria C Tamargo Molecular beam epitaxy (MBE) growth of Bi2Se3 and Sb2Te3 is hindered by a high bulk background doping due to selenium vacancies (n-type) and tellurium anti-sites defects (p-type). In order to explore the unique properties of these topological insulators (TIs), the bulk carriers must be suppressed. We previously presented the growth of TI/TI superlattices (SLs). By growing SLs of Bi2Se3 and Sb2Te3, which have a type III band alignment, a reduction of bulk background doping by more than one order of magnitude, from 1.2x1020 cm-3 to 8.5x1018 cm-3 was observed as the period was decreased from 12 nm to 5 nm. We attributed this to the formation of SL bands that create an enhancement of the bulk bandgap. Here we present tight binding calculations, which predict a SL gap of up to 400% larger than the bandgaps of the constituent layers when the SL is grown with the appropriate thicknesses. The calculations also predict the preservation of the Dirac cone, along with these large bandgaps. Measurements of magnetoconductance confirm preservation of the surface states for a SL with period thickness of 5nm. Direct measurement of the bandgaps by FTIR spectroscopy will also be reported. |
Wednesday, March 4, 2020 12:39PM - 12:51PM |
M55.00008: Pb-doped p-type Bi2Se3 thin films via interfacial engineering Jisoo Moon, Zengle Huang, Weida Wu, Seongshik Oh Due to high density of native defects, the prototypical topological insulator, Bi2Se3, is naturally n-type. Although Bi2Se3 can be converted into p-type by substituting 2+ ions for Bi, only light elements such as Ca have been so far effective as the compensation dopant. Considering that strong spin-orbit coupling (SOC) is essential for the topological surface states, a light element is undesirable as a dopant, because it weakens the strength of SOC. In this sense, Pb, which is the heaviest 2+ ion, located right next to Bi in the periodic table, is the most ideal p-type dopant for Bi2Se3. However, Pb-doping has so far failed to achieve p-type Bi2Se3 not only in thin films but also in bulk crystals. Here, by utilizing an interface engineering scheme, we have achieved the first Pb-doped p-type Bi2Se3 thin films. Furthermore, at heavy Pb-doping, the mobility turns out to be substantially higher than that of Ca-doped samples, indicating that Pb is a less disruptive dopant than Ca. |
Wednesday, March 4, 2020 12:51PM - 1:03PM |
M55.00009: Thermoelectric transport in helical edge states via chiral hydrodynamics Xinghai Zhang, Matthew Foster We study thermoelectric transport in a 1D helical liquid, as appears at the edge of a 2D topological insulator. We employ semiclassical “chiral hydrodynamics”, which directly incorporates the axial anomaly, and we consider the combination of Rashba-mediated umklapp backscattering and quenched disorder. The conductivity is computed from the balance between umklapp scattering and the anomaly; the results agree with previous bosonization calculations. We also compute the thermoelectric power (TEP) and thermal conductivity. In the clean limit, chiral hydrodynamics gives a TEP equal to the thermodynamic entropy per charge, while the Wiedemann-Franz law is violated for the thermal conductivity by umklapp scattering. In the dirty limit, the electric and thermal conductivities scale the same way with temperature, while the TEP vanishes. We will also discuss results in the nonlinear response regime. |
Wednesday, March 4, 2020 1:03PM - 1:15PM |
M55.00010: Proximity-induced quantum anomalous Hall effect in (Zn,Cr)Te/(Bi,Sb)2Te3/(Zn,Cr)Te heterostructure Ryutaro Yoshimi, Ryota Watanabe, Minoru Kawamura, Masataka Mogi, Atsushi Tsukazaki, Xiuzhen Yu, Kiyomi Nakajima, Kei Takahashi, Masashi Kawasaki, Yoshinori Tokura The quantum anomalous Hall effect (QAHE) is an exotic quantum phenomenon originating from dissipation-less chiral channels at the sample edge. While the QAHE has been observed in magnetically doped topological insulators (TIs), exploiting magnetic proximity effect on the TI surface from adjacent ferromagnet layers may provide an alternative approach to the QAHE by opening an exchange gap with less disorder than that in the doped system. Nevertheless, the engineering of a favorable heterointerface that realizes the QAHE based on the magnetic proximity effect remains to be achieved. Here, we report on the observation of the QAHE in a proximity coupled system of non-magnetic TI and ferromagnetic insulator (FMI). We have designed sandwich heterostructures of (Zn,Cr)Te/(Bi,Sb)2Te3/(Zn,Cr)Te that fulfills two prerequisites for the emergence of the QAHE; the formation of a sizable exchange gap at the TI surface state and the tuning of the Fermi energy into the exchange gap. The efficient proximity coupling in the all-telluride based heterostructure as demonstrated here will enable a realistic design of versatile tailor-made topological materials coupled with ferromagnetism, ferroelectricity, superconductivity, and so on. |
Wednesday, March 4, 2020 1:15PM - 1:27PM |
M55.00011: Franckeite: A new naturally occurring topological insulator Juan Palacios, Wendel Silva Paz, Marcos Menezes, Rodrigo Capaz Franckeite is a layered material formed by alternating tin disulfide-based Sn(Sb)S2 - pseudohexagonal (H) - and lead sulfide-based Pb(Sb)S - pseudotetragonal (Q) - layers. These layers exhibit different symmetry and periodicity, leading to an incommensurate crystal structure bound by not-so-weak van der Waals interactions between interleaved layers. The electronic properties at large concentration of Sb on the H layer are those of an indirect gap heterostructure (holes and electrons belonging to different layers) [1]. Here we present DFT results on the opposite limit of concentration. For 100% Sn on the H layer, franckeite becomes a topological insulator. This is corroborated by Wilson loop and Z2 number evaluations (the latter when forced into a centro-symmetric structure). We have also evaluated the structural composition of the energetically favorable thinnest possible layer, corresponding this to 0.5Q/H/0.5Q. This layer behaves as a 2D topological insulator, very much like InAs/GaSb quantum wells. [1] Molina-Mendoza et al., Nature Communications 8, 14409 (2017). |
Wednesday, March 4, 2020 1:27PM - 1:39PM |
M55.00012: Possible topological protection and superconducting proximity effects in surface states of HfNiSn single crystals Lucia Steinke, William Baker, Michael Babb, Mason Klemm, Harlan R Harris, Meigan Aronson Surface states of HfNiSn single crystals were shown to exhibit unconventional properties like nonlocal transport, time-reversal symmetry breaking in the absence of external fields or magnetism, and nonlinear I(V) characteristics indicating electronic correlations [1], possible characteristics of a correlated topological state similar to a quantum Hall system. Oscillations in the magnetoresistance suggest quantum interference with coherence lengths up to 1 μm. The combination of quantum Hall edges or similar chiral one-dimensional states and superconductors is particularly attractive, as such junctions are expected to host the elusive Majorana fermions that could provide a possible platform for topological quantum computing, where the chiral nature of quantum Hall edge states could enable braiding operations. Our first tests of metal deposition on HfNiSn single crystals show promising results, where proximity to superconducting tin or niobium leads to conductance steps of approximately 0.5 e2/h. We further observe a clear disruption of quantum interference patterns at the superconducting transition, and magnetoresistance features associated with the critical field that can be traced up to 80 K. |
Wednesday, March 4, 2020 1:39PM - 1:51PM |
M55.00013: Influence of electronic band topology on sound waves Sanghita Sengupta, Mohamed Nabil Yacine Lhachemi, Ion Garate We present a semiclassical theory of acoustic waves in topologically nontrivial materials. Combining the Boltzmann equation with the dynamical theory of elasticity, we calculate the influence of the electronic Berry curvature on sound velocity and attenuation. We predict signatures of nontrivial electronic band topology in the magnetic-field-dependence of sound propagation and discuss their possible experimental observation. |
Wednesday, March 4, 2020 1:51PM - 2:03PM |
M55.00014: Thermoelectric Transport Coefficients of a Dirac Electron Gas in High Magnetic Fields Viktor Könye, Masao Ogata We study the massive Dirac Hamiltonian in high magnetic fields focusing on the effects of the mass term and we show the main differences that arise compared to massless Dirac fermions. We calculate the thermoelectric transport coefficients based on the formalism developed by Luttinger, and study the transverse components of the electric conductivity, Seebeck, and heat conductivity tensors in high magnetic fields. We prove that the Mott formula and the Wiedemann-Franz law are valid at low temperatures. |
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M55.00015: Epitaxial growth and electronic properties of few-layer stanene on InSb (111) Jianfeng Zhang, Xiaohu Zheng, Rui-Rui Du Stanene has been predicted to be a 2D topological insulator with a large band gap, potentially hosting a room-temperature quantum spin Hall effect [1]. Here, stanene with controllable layers has been epitaxially grown on InSb (111) and its electronic properties have been investigated by scanning tunneling microscopy/spectroscopy (STM/STS) and low-temperature magnetotransport experiments [2]. STS results reveal a large band gap on both the wetting layer (~ 0.35 eV) and the subsequently grown single-layer (~ 0.2 eV) stanene. Spectroscopy evidences of edge state with energy inside the band gap, are also observed on the step of stanene film. Furthermore, the magnetotransport results show clear Shubnikov–de Haas oscillations in the bulk state of single-layer stanene. A brief discussion along with the data will be presented. |
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