Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session Y03: Transport in Topological Systems: Theory and Experiment |
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Sponsoring Units: DCMP Room: BCEC 107B |
Friday, March 8, 2019 11:15AM - 11:27AM |
Y03.00001: Configuration-sensitive transport at the domain walls of a magnetic topological insulator Yanfeng Zhou, Zhe Hou, Qing-Feng Sun We report on the transport of a two-terminal device containing a domain wall (DW) in a magnetic topological insulator (TI). In the low-energy case, the transport behaviors of the magnetic TI are dominated by chiral edge states (CESs) at the device edges as well as at the DW. We calculate the band structures of magnetic TIs with both a Bloch wall and a Néel wall. For a Bloch wall, two copropagating CESs at the DW are doubly degenerate, while for a Néel wall a split is present. Consequently, the transport is strongly dependent on the DW configuration. In the Bloch wall case, the incoming electron with zero energy is totally reflected regardless of the system parameters. However, in the Néel case, the device functions as a chirality-based Mach-Zehnder interferometry, so that the transmission coefficient oscillates between zero and unity with changes in system parameters. By constructing the scattering matrix of the device from the effective Hamiltonian, these transport behaviors can be well understood. |
Friday, March 8, 2019 11:27AM - 11:39AM |
Y03.00002: Conditions for fully gapped topological superconductivity in topological insulator nanowires Roni Ilan, Fernando De Juan, Jens Bardarson Among the different platforms to engineer Majorana fermions in one-dimensional topological superconductors, topological insulator nanowires remain a promising option. Threading an odd number of flux quanta through these wires induces an odd number of surface channels, which can then be gapped with proximity induced pairing. Because of the flux and depending on energetics, the phase of this surface pairing may or may not wind around the wire in the form of a vortex. We show that for wires with discrete rotational symmetry, a vortex is necessary to produce a fully gapped topological superconductor with localized Majorana end states. Without a vortex the proximitized wire remains gapless, and it is only if the symmetry is broken by disorder that a gap develops, which is much smaller than the one obtained with a vortex. These results are explained with the help of a continuum model and validated numerically with a tight binding model, and highlight the benefit of a vortex for reliable use of Majorana fermions in this platform. |
Friday, March 8, 2019 11:39AM - 11:51AM |
Y03.00003: Topological Properties of Gapped Graphene Nanoribbons with Spatial Symmetries Kuan-Sen Lin, Mei-Yin Chou To date, almost all of the discussions on topological insulators (TIs) have focused on two- and three-dimensional systems. One-dimensional (1D) TIs manifested in real materials, in which localized spin states may exist at the end or near the junctions, have largely been unexplored. Previous studies have considered the system of gapped graphene nanoribbons (GNRs) possessing spatial symmetries with terminations commensurate with inversion- or mirror-symmetric unit cells. In this work, we prove that a symmetry-protected Z2 topological classification exists for any type of termination. Instead of the Berry phase, only the origin-independent part of it gives the correct bulk-boundary correspondence by the π-quantized values. The resulting Z2 invariant depends on the 1D unit cell and is connected to the symmetry eigenvalues at the center and boundary of the Brillouin zone. Using cove-edged GNRs, we demonstrate the existence of localized states at the end of GNR segments and at the junction between two GNRs based on a topological analysis. The current results are expected to shed light on the design of electronic devices based on GNRs as well as the understanding of the topological features in 1D systems. |
Friday, March 8, 2019 11:51AM - 12:03PM |
Y03.00004: Quasi-one-dimensional first- and second-order topological insulators: Bi4X4 (X = Br, I) Chiho Yoon, Cheng-Cheng Liu, Hongki Min, Fan Zhang Recently, quasi-one-dimensional materials Bi4X4 (X = Br, I) in both α and β crystalline phases have been proposed as a promising platform for the realization of various topological insulator (TI) phases [e.g., PRL 116, 066801 (2016)]. This list includes the strong TI, the weak TI, and the second-order TI with helical hinge states. We perform first-principles calculations and construct an effective model for Bi4X4. These results allow us to derive all the aforementioned TI phases and guide future experimental studies. |
Friday, March 8, 2019 12:03PM - 12:15PM |
Y03.00005: PbSnSe: a model system to study topological phases Gauthier Krizman, Badih Assaf, Milan Orlita, Thanyanan Phuphachong, Günther Bauer, Gunther Springholz, Louis-Anne de Vaulchier, Yves Guldner Pb1-xSnxSe hosts three-dimensional (3D) massive Dirac fermions across the entire composition range for which the crystal structure is cubic. In this talk, we will present a comprehensive experimental mapping of the 3D band structure of Pb1-xSnxSe as a function of chemical composition (0<x<0.3), temperature (1.6<T<200 K), and magnetic field (0<B<34 T). We use magneto-infrared spectroscopy to determine all the Dirac parameters in the trivial and topologically non-trivial regimes. |
Friday, March 8, 2019 12:15PM - 12:27PM |
Y03.00006: Higher-order topology in two-dimensional crystals Frank Schindler, Wladimir Benalcazar, Marta Brzezinska, Mikel Iraola, Adrien Bouhon, Stepan Tsirkin, Maia Vergniory, Titus Neupert We study two-dimensional spinful topological phases of matter protected by time-reversal and crystalline symmetries. To define the topology we employ the concept of corner charge fractionalization: We show that corners in a higher-order topological phase can carry charges that are fractions of even multiples of the electric charge. These charges are quantized and topologically stable as long as all symmetries are preserved. We classify the topologies corresponding to different corner charge configurations for all 80 layer groups, and present their bulk topological indices. These can be calculated from the symmetry data and Brillouin zone Wilson loops. To corroborate our findings, we present tight-binding models and density functional theory calculations for various material realizations. |
Friday, March 8, 2019 12:27PM - 12:39PM |
Y03.00007: Exploring electronic properties of topological insulator Bi2Se3 using nuclear magnetic resonance Robin Guehne, Grant V. M. Williams, Shen V Chong, Vojtěch Chlan, Juergen Haase The investigations of 3-dimensional topological insulators such as Bi2Se3 focus chiefly on the gapless surface states that emerge as a consequence of the special energy band inversion near the Fermi level induced by spin-orbit coupling. Not as much studied are the real-space effects in the bulk. The band inversion, for example, changes the wave function of the free carriers, compared to the topologically trivial counterpart. We will show that nuclear magnetic resonance (NMR) as a local, bulk probe can detect this band inversion through the electric quadrupole interaction that, in addition, measures the concentration of free carriers, e.g. originating from self-doping effects. NMR data in external fields up to 17 T, i.e., shifts, linewidths and quadrupole splittings, in doped single crystalline samples of Bi2Se3 are discussed. |
Friday, March 8, 2019 12:39PM - 12:51PM |
Y03.00008: Thermal Hall Effect Measurements on Topological Materials Luke Pritchard-Cairns, Jean Philippe Reid, Robin S. Perry, Dharmalingam Prabhakaran, Andrew Huxley In the past few decades topology has become a central theme in condensed matter physics, allowing a more complete understanding of concepts such as the quantum Hall effect and of the class of materials termed topological insulators. Topological insulators were first predicted in 1987 and have subsequently been experimentally realised in a host of materials. A similar effect has been predicted to occur for SrCu2(BO3)2 (SCBO) but with one crucial difference; unlike those materials previously found, it is the spin waves that have a topological character rather than the conduction bands [3]. SCBO is therefore a bosonic topological material. |
Friday, March 8, 2019 12:51PM - 1:03PM |
Y03.00009: Selective Contact and Patterning of Topological Surface States by Interfacing with Trivial and Ferromagnetic Insulators Ankita Anirban, Abdur Rehman Jalil, Yunbo Ou, Mirko Rocci, Gregor Mussler, Jagadeesh Moodera, Crispin Barnes, Christopher J Ford Topological insulators (TIs) are a novel class of material which host helical surface states with exciting and unique electronic properties. They can provide insights into new physics and also pave the way for potential spintronic applications. Much work has been done to identify and characterise these surface states, but in order to utilise them towards multifunctional devices, we need a way to control the exchange gap opening in a tailored way and pattern them accordingly. We present results from devices which allow selective contact and patterning of topological surface states. We interface a TI with a ferromagnetic insulator to open up a gap in the Dirac cone, in an effort to have independent contact to the top and bottom surface states of a single TI film, enabling the characterisation and utilisation of the two surface states separately We also study heterostructures with two TI layers separated by a trivially insulating tunnel barrier, allowing for a way of probing the dispersion relation of TIs using spin and momentum resolved tunnelling. |
Friday, March 8, 2019 1:03PM - 1:15PM |
Y03.00010: The effect of lattice termination on the Zeeman gap of topological edge states in the quantum spin Hall insulator WTe2 Alexander Lau, Rajyavardhan Ray, Daniel Varjas, Anton Akhmerov We investigate the effect of the sample termination on the electronic properties of the recently discovered quantum spin Hall insulator WTe2. For this purpose, we have derived a minimal 4-orbital tight-binding model by means of a combination of symmetry considerations, density-funcitonal theory calculations and experimental data. Here, we are going to present the results of our theoretical study including spectra of topological edge states and conductance calculations for various sample geometries with and without magnetic field. |
Friday, March 8, 2019 1:15PM - 1:27PM |
Y03.00011: Bulk-boundary correspondence of non-Hermitian systems Zhong Wang, Shunyu Yao, Fei Song An intriguing issue of non-Hermitian systems is the fate of bulk-boundary correspondence. We show that a previously overlooked ``non-Hermitian skin effect'' necessitates a non-Bloch bulk-boundary correspondence, which is based on a complex-valued Brillouin zone. This dramatic generalization of bulk-boundary correspondence is illustrated in the non-Hermitian Su-Schrieffer-Heeger models and Chern insulator models. We analytically obtain their topological phase diagrams, which are remarkably different from those of the Bloch Hamiltonians. As a natural formulation of the non-Bloch bulk-boundary correspondence, we introduce the non-Bloch topological invariants in the complex Brillouin zone (instead of the standard Brillouin zone with real-valued momentum), which faithfully predict the number of topological edge modes. Specifically, the chiral edge modes of two-dimensional non-Hermitian Chern bands are determined by the non-Bloch Chern numbers. |
Friday, March 8, 2019 1:27PM - 1:39PM |
Y03.00012: Epitaxial Nanoribbon Transfer Process for Quantum Spin Hall Devices Siqi Yao, Xiaoxue Liu, Zhongdong Han, Bingbing Tong, Rui-Rui Du It has been shown that nanoribbon transistors consisting of of InAs or GaSb channels can be processed by transferring epitaxial semiconductor layers on to Si substrate, and further integrated into compound semiconductor CMOS circuits. In our research, we adept this technique and study transferring epitaxial InAs/InGaSb topological insulator to Si/SiO2 or FM-insulator substrate. We will describe the processing for the QSH devices and present their low temperature transport data. |
Friday, March 8, 2019 1:39PM - 1:51PM |
Y03.00013: Revealing Proximity Effects at Magnetic Topological Insulator / Antiferromaget Interfaces Alexander Grutter, Qing Lin He, Gen Yin, Lei Pan, Chao-Yao Yang, Xiaoyu Che, Dustin Gilbert, Steven Disseler, Brian Kirby, Julie Borchers, William Ratcliff, Padraic Shafer, Elke Arenholz, Luyan Yu, Guoqiang Yu, Qiming Shao, Yingying Wu, Bin Zhang, Xiaodong Han, Yizhou Liu, Roger Lake, Kam Tuen Law, Kang Wang Magnetic topological insulators are an exciting new platform for the investigation of exciting time-reversal symmetry-breaking physics such as the quantum anomalous hall effect, inverse spin galvanic effect, axion electrodynamics, and spin orbit torque. These materials systems also have great potential for spintronic device applications, but practical development has so far been inhibited by the extremely low (around 30 K) Curie temperature of these materials. Interfacial proximity coupling to other magnetically ordered compounds has been suggested as a mechanism by which this challenge may be addressed. However, separating interfacial exchange coupling from less-desirable effects such as intermixing presents significant challenges. In this work, we use neutron and magnetic X-ray spectroscopy to assemble an depth and element-resolved picture of magnetism within topological insulator/antiferromagnet heterostructures. Our results support true proximity-induced magnetization in (Bi,Sb)2Te3 films interfaced with CrSb and MnTe, with enhanced ordering temperature and chiral magnetic structures. We show that the topology and magnetism of the (Bi,Sb)2Te3 may be readily controlled through exchange coupling with the adjacent antiferromagnet. |
Friday, March 8, 2019 1:51PM - 2:03PM |
Y03.00014: Observation of propagating Dirac plasmons in topological insulators Yong Wang, Stephanie Ann Law Topological insulators (TI) have been widely studied due to their unique band structure. The Dirac cone band dispersion as well as spin-momentum locking at the surface enables the existence of Dirac plasmons with resonance frequencies in the THz range [1]. People have observed localized Dirac plasmons by etching TIs into stripe arrays [2]. There is limited experimental evidence for propagating surface plasmon polaritons in TIs. Due to the nature of TI surface states, the propagating surface plasmon will be both charge density wave and spin density wave. Here we present our work on exciting propagating Dirac plasmons in TIs by lifting off gold gratings on top of the TI surface. We demonstrate the tunability of the plasmon resonance frequency by varying the grating coupler period. Future work may also include other degrees of tunablilty, and the simulation of plasmon dispersion relation. The research on propagating Dirac plasmons give rise to new dimensions of plasmonic and spintronic applications with TI. |
Friday, March 8, 2019 2:03PM - 2:15PM |
Y03.00015: First-Principles Prediction of Room Temperature Quantum Anomalous Hall Effect in 2D Oxalate-Bridged Metal Organic Complexes Lizhi Zhang, Mina Yoon Quantum anomalous Hall (QAH) insulators are a highly promising class of materials for spintronic devices and quantum computations because of their precise quantization nature, robust properties against defects, and relatively low energy consumption for operation. To realize the QAH effect, quantum spin Hall (QSH) insulators must be utilized, which requires transition metal doping or surface functionality control. Here, we propose a new family of high temperature organic QAH insulators of 2D oxalate-bridged metal (M) organic complexes, M2(C2O4)3 (M=Re, Pt, Hg). First-principles calculations show the spin–orbit coupling gaps ~160 meV, with Curie temperature, calculated by non-linear spin wave theory based on the XYZ Heisenberg model, of greater than ~380 K, which indicates a room-temperature QAH effect. The first room-temperature organic QAH insulators with computationally proved high thermal stability can be realized experimentally. |
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