Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session C03: Novel Topological Systems |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Haiming Deng, City College of New York Room: BCEC 107B |
Monday, March 4, 2019 2:30PM - 2:42PM |
C03.00001: Observation of topological edge modes via smart patterning David J. Apigo, Kai Qian, Camelia Prodan, Emil Prodan
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Monday, March 4, 2019 2:42PM - 2:54PM |
C03.00002: Electric tuning of topological properties in a symmetric broken gap quantum well Tiago De Campos, Marcelo A. Toloza Sandoval, Leovildo Diago-Cisneros, Guilherme M Sipahi A mainstream within topological insulators, GaSb/InAs quantum wells (QWs) present a broken gap alignment for the energy bands which supports the quantum spin Hall insulator phase [1,2] and forms an important building block in the search of exotic states of matter [3]. Such QWs have electrons and holes are confined in different layers, leading to a wide range of possibilities to tune the topological properties. In this work, we study the inverted band structure of a symmetric GaSb/InAs QWs under the influence of an electric field applied along the growth direction, using the 8-band k.p model and the envelope function approximation. Without external fields, the collapse of the energy gap occurs in ~10 nm for the InAs layer (where the conduction and valence states become degenerate) and the application of an electric field reopens the gap with an inverted band structure. The behavior of the hybridization gap and of the corresponding edge states are investigated considering a gradual increasing of the electric field modulus, which shows that they can be externally tuned in this system. |
Monday, March 4, 2019 2:54PM - 3:06PM |
C03.00003: In-situ Fermi level tuning of semiconductor thin films on graphene during scanning tunneling microscopy studies Yulia Maximenko, Zhenyu Wang, Charles Steiner, Davide Iaia, Vidya Madhavan Doping can often drive the system into interesting quantum phases such as superconductivity or the pseudo-gap phase. Typically, doping is controled during fabrication by adjusting the dopant impurity ratios or tuning the chemical composition of the main elements. For a detailed atomically-resolved study of a material with a complex phase diagram, it is beneficial to implement in-situ Fermi level (EF) tuning that can be done by gating. Gating is a somewhat robust technique in transport measurements, but combining it with epitaxial film growth and scanning tunneling microscopy (STM) presents many technical challenges. Here, we report the design of a robust back-gating device for a versatile thin film growth and subsequent EF tuning inside STM. Graphene serves as the platform for epitaxial growth, while a range of materials can be used as an insulating gate depending on the particular experiment. We demonstrate successful EF tuning in both Bi2Te3 and SnTe films in STM. We study the strength of the effect in terms of Schottky barriers between materials, film thickness, and quality of the gate and report a magnetic field study of back-gated thin films of topological insulators. |
Monday, March 4, 2019 3:06PM - 3:18PM |
C03.00004: Thermodynamic probe of the bulk gap in the quantum spin Hall insulator 1T'-WTe2 Sergio De La Barrera, Qingrui Cao, Jiaqiang Yan, Benjamin Matthew Hunt Single-layer 1T'-phase WTe2 has recently drawn great interest as a candidate material for hosting an intrinsic quantum spin Hall insulator state at low temperature, a highly attractive property for numerous studies and applications. Such a state requires topologically-protected helical edge states coincident with a bulk band gap. Initial calculations of the 1T'-WTe2 electronic structure predicted semimetallic bands, however, several studies have shown simultaneous experimental evidence of edge states and a bulk gap in photoemission, tunneling spectroscopy, and electron transport. The nature of this gap, however, is not fully resolved, with disagreements between predicted gaps from first-principles calculations and spectroscopic measurements in various conditions. Here, we probe the thermodynamic properties of fully-encapsulated single-layer 1T'-WTe2 using low-temperature capacitance to illuminate the discussion of this gap and its single-particle or interaction-driven origin. |
Monday, March 4, 2019 3:18PM - 3:30PM |
C03.00005: Quantifying charge–to–spin conversion efficiency in magnetically–doped topological insulator heterostructures Quanjun Pan, Xiaoyu Che, Qiming Shao, Yabin Fan, Lei Pan, Hao Wu, Peng Zhang, Mohammad Montazeri, Kang L. Wang We deployed a magneto–optical mangetometer and an electrical loop shift method to directly quantify the charge–to–spin conversion efficiency in a magnetically–doped topological insulator heterostructure. While these two approaches are essentially different in their experimental principles, quantitative agreements are found in values obtained by the two approaches. This consistency strongly suggests both methods can accurately estimate the charge–to–spin conversion efficiency without some ambiguity reported previously with other approaches. The charge–to–spin conversion efficiency, which is parameterized by the spin Hall angle tangent, is estimated to be 0.46 and 0.38 at 12K by the magneto–optical mangetometer and the electrical loop shift method, respectively. This value is at least one order larger than those of conventional heavy metals. Our results also reveal that magneto–optical mangetometer and loop shift methods are both reliable and easily accessible for investigation of magnetization dynamics in TI–based magnetic structures. |
Monday, March 4, 2019 3:30PM - 3:42PM |
C03.00006: Investigating the ferromagnetism-induced surface gap formation and effects of optical excitations in Cr-doped topological insulators (TIs) Adrian Llanos, Chien-Chang Chen, Marcus Teague, Peng Zhang, Lei Pan, Koichi Murata, Kang Wang, Nai-Chang Yeh Despite the exciting observation of the quantum anomalous Hall effect (QAHE) in Cr-doped (BixSb1-x)2Te3 compounds, the microscopic origin of ferromagnetism (FM) and the temperature (T) evolution of the anomalous Hall (AH) resistance in these magnetic TIs remain poorly understood. We perform scanning tunneling spectroscopic and electrical transport studies on MBE-grown magnetic TIs, including uniformly 10% Cr-doped (BixSb1-x)2Te3 and bilayer systems consisting of a layer of pure (BixSb1-x)2Te3 on top of a 10% Cr-doped (BixSb1-x)2Te3 layer. In the STS studies on the bilayer system, a surface gap Δ opens up at TC2D ~ 180 K, much higher than the bulk Curie temperature TC3D ~ 35 K determined from the onset of AHE. The spatial distribution of Δ is largely homogeneous, and Δ increases with decreasing T, reaching Δ = (59±7) meV at 147K. Additionally, longitudinal (Rxx) and AH (Rxy) resistance are measured both with and without light. An increase in Rxy and decrease in Rxx is observed in the bilayer system under circularly polarized (CP) light (wavelengths λ =1600 ~ 1700 nm). In contrast, both Rxx and Rxy are suppressed under CP light for uniformly Cr-doped (BixSb1-x)2Te3. The physical implications of these findings will be discussed. |
Monday, March 4, 2019 3:42PM - 3:54PM |
C03.00007: Interfaces and superlattices of the topological insulator Bi2Se3 Niraj Aryal, Efstratios Manousakis Interface states of topological materials are important for both emergent fundamental phenomena and for their potential application in future electronics and computing devices. Here, we propose different ways of manipulating the position of the energy of the surface Dirac states in the topological insulator (TI) Bi2Se3 relative to the bulk Fermi level, their linearity, their location, etc., by making contact with different materials. Our predictions are made by using density functional theory and model Hamiltonians. For example, in the case of an interface of a TI with a band insulator (BI) we find that the location of the interfacial topological state moves out of the topological insulator boundary to the BI side. When a TI forms an interface with a metal we find that the topological surface states can show oscillatory behavior which propagates inside the metal. We also present results for superlattices formed of a topological insulator with a band insulator. |
Monday, March 4, 2019 3:54PM - 4:06PM |
C03.00008: Microwave Impedance Microscopy of WTe2 Joshua Kahn, Yanmeng Shi, Zaiyao Fei, Brian A Francisco, Ben Niu, Xiaodong Xu, Yongtao Cui, David Henry Cobden WTe2 is a van der Waals layered material that behaves as a 2D topological insulator when exfoliated down to a monolayer, supporting edge modes that should be topologically protected. Microwave impedance microscopy (MIM) is a technique that allows real-space mapping of conductivity with a resolution of < 50 nm, and is thus a powerful tool for studying these edge modes that reveals many features that are not accessible to transport measurements alone. Using MIM we observe that the edge is gapless and the edge conduction path conforms to microscopic corners and cracks in the edge and features internal to the monolayer flake. This is consistent with their topological nature and has important consequences for interpreting transport measurements and for making devices employing the 1D helical modes.We also study the behavior of multilayer WTe2, twisted bilayer WTe2, and monolayer WTe2 combined with other materials, such as different capping layers and 2D ferromagnets. |
Monday, March 4, 2019 4:06PM - 4:18PM |
C03.00009: Topological Classification Table Implemented with Classical Passive Meta-Materials Yafis Barlas Topological condensed matter systems from class A and class AII of the classification table have received classical electromagnetic and mechanical analogs. They topological phases exhibit protected wave-guiding which has been demonstrated experimentally. In this talk, we introduce a map which generates classical analogs for all entries of the classification table, using only passive elements. Physical mechanical models can be realized for all strong topological phases in dimension 2, as well as for three classes in dimension 3. As an example, we will show how the Kitaev topological super-conducting chain and it's braiding properties can be realized in classical systems. |
(Author Not Attending)
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C03.00010: Non-Local Spin Transport in Topological Insulator Nanowires Pok Lam Tse, Grace Lu The momentum and spin of charge carriers in Topological Insulators(TI) are constrained to be perpendicular due to spin-orbit coupling. Sb2Te3 is a TI materials with a bulk band gap of 0.28 eV and surface states consisting of a single Dirac cone in the band gap. Single crystalline Sb2Te3 nanowires were synthesised. E-beam lithography were used to pattern two outer Au leads and two magnetic tunnel junction(MTJ) inner leads on individual Sb2Te3 nanowires. The MTJ leads consist of a free Py (Ni80Fe20) layer, whose magnetization determines the magnitude and direction of spin current injected into nanowire. |
Monday, March 4, 2019 4:30PM - 4:42PM |
C03.00011: Numerical signatures of topology in disordered insulators Dominic Reiss, Albert Brown, Fenner Harper, Xu Liu, Rahul Roy We investigate a number of numerical signatures which distinguish the topological and trivial phases of disordered insulators. In particular, we consider three dimensional systems with time-reversal symmetry (class AII) and numerically construct a set of maximally commuting spin-like operators using local unitary circuits. The properties of the resulting spin operators may be used to identify the topology of the underlying phase. Using similar methods, we also construct analogues of Wannier functions for disordered systems with symmetry. Finally, we discuss extensions of this approach to other spatial dimensions and symmetry classes. |
Monday, March 4, 2019 4:42PM - 4:54PM |
C03.00012: Scattering from Monolayer and Bilayer Step Edges on Topological Sb(111) Aditya Mahadevan, Haimei Zhang, Jianfeng Ge, Yang He, Anjan Soumyanarayanan, Shiang Fang, Jennifer Hoffman The topological semimetal antimony (Sb) has a layered structure that cleaves easily along the 111 plane, |
Monday, March 4, 2019 4:54PM - 5:06PM |
C03.00013: Quantized anomalous Hall conductance in a bulk dilute Mn-doped topological insulator Bi2Te3 at high temperatures Haiming Deng, Zhiyi Chen, Shihua Zhao, Simon Turkel, Abhay Pasupathy, Lukasz Plucinski, Marcin Konczykowski, Agnieszka Wolos, Kyungwha Park, Lia Krusin-Elbaum Thus far Quantized anomalous Hall (QAH) has only been realized in the mK range in ultra-thin (5-10nm) MBE films of heavily Cr- or V-doped (Bi,Sb)2Te3, where the Curie Tc could be as high as 60 K but the onset of Anomalous Hall effect (AHE) was at a much lower T. Here we report achieving Hall conductance quantization in the Kelvin range in dilute (2%) Mn-doped thick (50-300 nm) single crystals of Bi2Te3 by taking a different approach to tune the Fermi level EF into the Dirac gap. Our technique employs high energy-electron beams (2.5MeV) to create charged defect states in the bulk and thermal annealing protocols to control both EF and the surface bandstructure. of the initially n-type Mn-doped Bi2Te3. In contrast to previous work, the magnetization M was found to be mean-field-like and carrier independent, with M and AHE onsetting at the same Tc ~ 13 K. We observe conductance quantization Gxy = 1.0098 h/e2 not in the bulk gap but on the bulk conduction band where, remarkably, conduction electrons do not contribute to AHE. We will discuss the correlation of surface and bulk AHE with magnetism in this dilute system. The cross-correlation of transport data with STM and ARPES will be discussed. |
Monday, March 4, 2019 5:06PM - 5:18PM |
C03.00014: In gap states in topological Kondo insulators: Optical properties evidence Roberto Franco Peñaloza, Edwin Ramos, Jereson Silva Valencia, Marcos Sergio Figueira Da Silva We investigate optical properties: ARPES, PES and optical conductivity of topological Kondo insulators; in particular we study the manifestation of In gap states in the ARPES and PES results, taking into account the presence of antiferro-magnetic short range correlations. We consider an additional narrow band to the otherwise completely localized f-electrons, by adding a term to the periodic Anderson model which allows a small hopping of the localized electrons between neighboring sites of the lattice. This new model is adequate to study a novel class of intermetallic 4f and 5f orbitals materials: the Kondo topological insulators. For simplicity, we consider a version of the periodic Anderson model on a two dimensional square lattice. Previously we shows that the anti ferro-magnetic correlations plays an important role on the gap opening of the topological Kondo insulators (1); in this work we discuss the manifestation of the in gap states in the ARPES and PES results, our results for the different symmetry points are similar to the reported for SmB6 (2). |
Monday, March 4, 2019 5:18PM - 5:30PM |
C03.00015: Engineering topological superlattices : Phase diagrams and electronic properties Deepti Jain, Pavel P Shibayev, Elio Koenig, Maryam Salehi, Jisoo Moon
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