Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session F01: Exotic Electronic States in Topological Systems |
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Sponsoring Units: DCMP Chair: Predrag Nikolic, George Mason University Room: BCEC 106 |
Tuesday, March 5, 2019 11:15AM - 11:27AM |
F01.00001: Exactly Solvable Majorana-Anderson Impurity Models Shankar Ganesh, Joseph Maciejko Majorana fermions emerge in topological superconductors as end zero modes in one dimension, and vortex-trapped zero modes or chiral edge modes in two dimensions. A question of much recent interest is the effect of electron-electron interactions on such Majorana fermions. We introduce exactly solvable Majorana-Anderson impurity models that describe an interacting Anderson impurity (i) immersed in tight-binding lattices of Majorana modes in one and two dimensions, (ii) coupled to the Majorana end mode of a Kitaev chain, and (iii) coupled to the edge mode of a chiral p-wave superconductor. The Ising slave-spin representation is used to map our interacting models to models of free auxiliary Majorana fermions. In each case, we calculate exact spectral functions for the impurity fermions and observe a nontrivial temperature dependence as a signature of interactions. Spectral functions for the host fermions are also calculated to study the feedback effect of the impurity on the superconducting host. We compare our exact results to those obtained from strong-coupling perturbation theory. |
Tuesday, March 5, 2019 11:27AM - 11:39AM |
F01.00002: Tuning Many-Body Interactions in a Thin Topological Insulator Nicholas Dale, Alexander Nguyen, Drew W Latzke, Nikesh Koirala, Pavel P Shibayev, Jisoo Moon, Seongshik Oh, Alessandra Lanzara The conical dispersion of spinless Dirac fermion systems can become renormalized upon introduction to electron-electron interactions. It is an open question whether or not this behavior extends to the spin-momentum locked surface states of Topological Insulators. We investigate in Photoemission Spectroscopy the surface states of a thin Topological Insulator Bi2Se3 on SrTiO3 using temperature as a knob to tune electron-electron interactions. |
Tuesday, March 5, 2019 11:39AM - 11:51AM |
F01.00003: Surface band bending on cleaved strongly correlated topological insulators Christian Matt, Harris Pirie, Wendel S. Paz, Juan Jose Palacios, Daniel Larson, Mohammad H Hamidian, Jennifer Hoffman Strongly correlated topological surface states are promising platforms for next-generation quantum applications, but they remain elusive in real materials. Although angle-resolved photoemission (ARPES) experiments on the correlated insulator SmB6 appear to show spin-textured surface states, the Dirac point – the hallmark of any topological system – has not been resolved by ARPES. A key challenge is that SmB6 lacks a natural cleaving plane, thus limiting the ordered surface domains to tens of nanometers, with local energy band features shifted by tens of meV as observed by our scanning tunneling microscopy experiments. Here we simulate the full spectral function as an average over multiple domains with different surface potentials and band-bending. We thus explain the discrepancy between large-area measurements that average over multiple band-shifted domains and atomically-resolved measurements. |
Tuesday, March 5, 2019 11:51AM - 12:03PM |
F01.00004: Band topology in twisted bilayer graphene Hoi Chun Po, Liujun Zou, Senthil Todadri, Ashvin Vishwanath Superconductivity and correlated insulators have been observed in “magic-angle” twisted bilayer graphene when the nearly flat bands close to charge neutrality are partially filled. The observed phenomenology resembles that of high-temperature superconductors like cuprates. Yet, the building blocks of the two systems are vastly different, since the states in TBG descend from graphene’s Dirac dispersion. We argue that the Dirac character of the relevant states endows the nearly flat bands with a nontrivial band topology, which forbids any fully symmetric tight-binding description for the nearly flat bands alone. Extended models incorporating the higher energy bands, however, are possible. We constructing a family of such tight-binding models, and from them establish that the band topology is “fragile” in nature, in that it can be dissolved simply by adding additional atomic bands. Our models pave the way to developing a theoretical understanding of twisted bilayer graphene from a strong-coupling perspective. |
Tuesday, March 5, 2019 12:03PM - 12:15PM |
F01.00005: Magnetic structure of the antiferromagnetic Weyl semimetal candidates Mn3(Ge/Sn) Youzhe Chen, Jonathan Gaudet, Guy G Marcus, Naoki kiyohara, Agustinus A. Nugroho, Yang Zhao, Satoru Nakatsuji, Collin Broholm The recent discovery of Anomalous Hall Effect in Mn3X (X=Sn,Ge) suggests the existence of Weyl nodes in the electronic band structure of these non-colinear antiferromagnets. The magnetic structure of Mn3X is crucial to Weyl physics, but is still under debate due to the lack of experimental studies. To determine the structure, we report polarized neutron diffraction studies on Mn3X (X=Sn,Ge). In Mn3Ge, a k=0 antichiral structure was determined, which naturally explains the origin of a net in-plane magnetization along the [110] direction. In Mn3Sn, an additionnal incommensurate phase was discovered and further characterized with polarization analysis. The magnetic ground state selection of Mn3X (X=Sn,Ge) will be discussed in terms of exchange, Dzyaloshinskii-Moriya and crystal electric field interactions. |
Tuesday, March 5, 2019 12:15PM - 12:27PM |
F01.00006: A topological classification of molecules and chemical reactions with and without interactions Lukas Muechler In this talk, we propose a topological classification of molecules and their chemical reactions with and without many-body interactions. We consider 0-dimensional molecular Hamiltonians in a real-space tight-binding basis with time-reversal symmetry and an additional spatial reflection symmetry. On a single particle level, the reflection symmetry gives rise to a perplectic structure which can be probed by a Wilson loop after a flux-insertion. The classification in terms of Wilson loops remains stable in the presence of many-body interactions, which can be explained by the presence of zeros of the interacting single particle Green's function. |
Tuesday, March 5, 2019 12:27PM - 12:39PM |
F01.00007: Spin-wave excitations in the antiferromagnetic Weyl semimetal candidates Mn3Ge and Mn3Sn Jonathan Gaudet, Youzhe Chen, Guy G Marcus, Ikhlas Muhammad, Naoki kiyohara, Agustinus A. Nugroho, Satoru Nakatsuji, Matthew Brandon Stone, Minoru Soda, Takatsugu Masuda, Collin Broholm Mn3X ( with X = Ge or Sn) are semimetal antiferromagnets with large anomalous Hall and Nernst effects. These anomalous transport properties have been attributed to the presence of electronic Weyl nodes near the Fermi energy of these materials [1,2]. The magnetism of Mn3X remains relatively unexplored. For example, a detail knowledge of their magnetic excitations is still lacking. In this talk, we report the inelastic neutron scattering spectra of Mn3Ge and Mn3Sn. Our analysis reveals linear dispersive modes that are gapped by ~ 5 meV and have an energy bandwidth of ~ 80 meV. Spin Hamiltonians for both Mn3Sn and Mn3Ge have been derived and will be presented along with a detailed comparison between the two sister compounds. [1] S. Nakasutji et al., Nature 527, p.212–215 (2015) [2] M. Iklhas et al., Nature Physics 13, p. 1085–1090 (2017) |
Tuesday, March 5, 2019 12:39PM - 12:51PM |
F01.00008: Field theory of incompressible topological quantum liquids in higher dimensions Predrag Nikolic I will discuss a derivation of field theories that describe quantum dynamics of topological defects in a broad range of topologically ordered phases. Beyond quantum Hall liquids in two dimensions, the obtained Lagrangians hint at the existence of stable topologically ordered states with fractional excitations in higher dimensions - involving defects such as monopoles and hedgehogs. I will outline some properties of these topological phases (e.g. fractionalized magnetoelectric effect), and touch upon the prospects for their observation in three-dimensional topological materials. |
Tuesday, March 5, 2019 12:51PM - 1:03PM |
F01.00009: Possible 3-dimensional Topological Excitonic Insulator in a Semimetal under Magnetic Field Zhiming Pan, Ryuichi Shindou Motivated by a recent graphite experiment under high magnetic field, we introduce an interacting electron model with a pair of electron pocket and hole pocket under magnetic field. We use a fermionic renormalization group (RG) method and study a parquet RG equation of the model for various types of effective electron-electron interactions. We found that, for a screened Coulomb interaction, the one-loop level RG equation exhibit an instability of the excitonic pairing, while, for a short-ranged repulsive interaction case, the excitonic instability and a charge-density wave instability appear simultaneously. We further argue that, depending on the sign of the interaction, the excitonic insulator phase can be either a symmetry-protected topological excitonic insulator or topologically trivial excitonic insulator. In the former case, an odd-parity excitonic pairing in the bulk reconstruct a chiral Fermi arc state of electron type and that of hole type into a helical surface state with gapless Dirac cone. The reconstructed surface state may provide a simple explanation for a mysterious in-plane transport behaviour observed in the graphite experiment under high magnetic field. |
Tuesday, March 5, 2019 1:03PM - 1:15PM |
F01.00010: Coexistence of metallic edge states and antiferromagnetic ordering in correlated topological insulators Giorgio Sangiovanni, Adriano Amaricci, Angelo Valli, Massimo Capone, Jan Carl Budich, Björn Trauzettel We investigate the emergence of antiferromagnetic ordering and its effect on the helical edge states in a quantum spin Hall insulator, in the presence of strong Coulomb interaction [1-5]. Using dynamical mean-field theory, we show that the breakdown of lattice translational symmetry favors the formation of magnetic ordering with nontrivial spatial modulation. The onset of a nonuniform magnetization enables the coexistence of spin-ordered and topologically nontrivial states. An unambiguous signature of the persistence of the topological bulk property is the survival of bona fide edge states. We show that the penetration of the magnetic order is accompanied by the progressive reconstruction of gapless states in subperipheral layers, redefining the actual topological boundary within the system. |
Tuesday, March 5, 2019 1:15PM - 1:27PM |
F01.00011: Kondo Hole Scattering in the Strongly Correlated Topological Insulator SmB6 Harris Pirie, Yu Liu, Sagen C Cocklin, Eric Mascot, Pengcheng Chen, Shanta Saha, Xiangfeng Wang, Johnpierre Paglione, Mohammad H Hamidian, Dirk Morr, Jennifer Hoffman Quantum materials combining strong correlations and spin-orbit coupling are predicted to generate multiple exotic ground states. For example, in topological Kondo insulators, interactions within a lattice of local moments open a hybridization gap in the conduction band, within which topologically protected heavy Dirac surface states emerge. When a lattice moment is removed, the resulting Kondo hole is predicted to create oscillations in the electron screening cloud, with accompanying magnetic fluctuations. Here we use scanning tunneling microscopy and spectroscopy to image the interaction between a Kondo hole and the topological surface state in SmB6. We show that Sm vacancies induce oscillations in the hybridization gap and electrochemical potential, matching predictions for Kondo holes [1], while B-site defects do not. Furthermore, we find that only Sm-site defects cause significant scattering of the topological surface state. Our results demonstrate how intrinsic, nominally nonmagnetic defects can generate magnetic fluctuations that provide a new mechanism for scattering topological surface states. |
Tuesday, March 5, 2019 1:27PM - 1:39PM |
F01.00012: Pulsed field studies of Kondo insulator YbB12 Ziji Xiang, Yuichi Kasahara, Tomoya Asaba, Benjamin Lawson, Colin Tinsman, Lu Chen, Yuki Sato, Fumitoshi Iga, John Singleton, Yuji Matsuda, Lu Li The Shubnikov-de Haas (SdH) effect, quantum oscillations in the electrical conductivity, have been detected in Kondo insulator YbB12 [1]. Sunch an observation suggests a highly exotic ground state in this material. Here we show the results of magnetoresistance and proximity detector oscillator (PDO) measurements in YbB12 in pulsed magnetic fields up to 65 T. SdH oscillations are observed both above and below the field-induced insulator-metal (I-M) transition. An abrupt increase in the oscillation frequency takes place at the transition. A unique angular dependence of the SdH frequencies is also revealed, which is related to the angle-dependent I-M transition field. The origin of the SdH effect in the insulating phase of YbB12 is discussed based on the pulsed field studies. |
Tuesday, March 5, 2019 1:39PM - 1:51PM |
F01.00013: Coupled-wire models of non-Abelian topological orders in 3D Syed Raza, Jeffrey C.Y. Teo Topological order corresponds to patterns of long-range entanglement in ground-states. The point-like and line-like excitations in 3D topologically ordered states can have fractional charge and spin degrees of freedom giving rise to fractional quantum statistics. In this work, we study 3D non-Abelian topological order via coupled-wire construction. By putting the wire configuration on a closed manifold, we study the properties of ground states and the Wilson algebra of various excitations. We study the topological order of previously proposed coupled-wire models of symmetry-preserving gapped Dirac (semi)metal and Dirac superconductor. We also study new non-Abelian topologically ordered states that inherit their topological properties from conformal field theories like SO(3)3. |
Tuesday, March 5, 2019 1:51PM - 2:03PM |
F01.00014: Study of hysteretic magnetotransport of SmB6 using local and non-local Corbino disk techniques. Yun Suk Eo, Tristin E Metz, Hyunsoo Kim, Wesley T. Fuhrman, Shanta Saha, Xiangfeng Wang, Juan Chamorro, Seyed Koohpayeh, Tyrel McQueen, Michael Fuhrer, Johnpierre Paglione Recent experiments that have been reported during the past several years suggest that samarium hexaboride (SmB6) is a true topological Kondo insulator (TKI). Particularly, the hysteretic magnetotransport at low temperatures is a remarkable feature that seems to be consistent with the TKI picture, but distinct from other topological insulators. Motivated by the growing interest in the role of disorder and impurities in bulk SmB6, we revisit the surface transport, including the hysteresis features, using Corbino disk structures on flux- and floating zone-grown samples with small impurity concentrations (e.g., Gd, Ni, and Fe). Also, using bulk probing techniques such as non-local (inverted) Corbino transport and heat capacity, we investigate if the bulk channel has an influence on the hysteresis signals we observe in standard transport configurations. |
Tuesday, March 5, 2019 2:03PM - 2:15PM |
F01.00015: Correlation Driven Topological Insulator to Weyl Semimetal Transition in Actinide System UNiSn Vsevolod Ivanov, Sergey Savrasov We use a modern electronic structure method combining density functional theory of band electrons with dynamical self-energies of strongly correlated states to predict that two well known phases of actinide compound UNiSn, a paramagnetic semiconducting and antiferromagnetic metallic, correspond to Topological Insulator (TI) and Weyl semimetal (WSM) phases of topological quantum matter. Thus, the famous unconventional insulator-metal transition observed in UNiSn is also a TI to WSM transition. Driven by a strong hybridization between U f-electron multiplet transitions and band electrons, multiple energy gaps open up in the single-particle spectrum whose topological physics is revealed using the calculation of Z2 invariants in the strongly correlated regime. A simplified physical picture of these phenomena is provided based on a periodic Anderson model of strong correlations and multiple band inversions that occur in this fascinating compound. Studying the topology of interacting electrons reveals interesting opportunities for finding new exotic phase transitions in strongly correlated systems. |
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