Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session L45: Defects in Topological MaterialsLive
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Sponsoring Units: DCMP Chair: Yulia Maximenko, National Institute of Standards and Technology |
Wednesday, March 17, 2021 8:00AM - 8:12AM Live |
L45.00001: Resolving topological classification through topological defects Abhay Nayak, Jonathan Reiner, Raquel Queiroz, Huixia Fu, Chandra Shekhar, Binghai Yan, Claudia Felser, Nurit Avraham, Haim Beidenkopf Bulk boundary correspondence has been the cornerstone in the study of topological quantum materials. It has enabled the exploration of electronic bulk properties through the investigation of topological boundary modes. However, the growing diversity and profusion of topological classes has lead to ambiguity between classes sharing similar boundary phenomenology. This is the current status of bismuth, for which recent studies have suggested nontrivial classifications like strong or higher order TI, both of which hosts 1D helical modes on their boundaries. Here, we use a novel approach to resolve the topological classification of bismuth by spectroscopically mapping the response of a topological lattice defect like screw dislocation using scanning tunneling microscope. We find a 1D edge mode, bound to the step edges of bismuth, extending to the core of the screw dislocation without gapping out. This signifies that the edge mode binds to the topological defect, characteristic of a material with nonzero weak indices. This work paves the way for the identification of novel electronic topological phases through the study of boundary modes associated with topological defects. |
Wednesday, March 17, 2021 8:12AM - 8:24AM Live |
L45.00002: Anomalous and normal dislocation modes in Floquet topological insulators Tanay Nag, Bitan Roy Electronic band structure featuring nontrivial bulk topological invariant manifest through robust gapless modes at the boundaries, e.g., edges and surfaces. As such this bulk-boundary correspondence is also operative in driven quantum materials. For example, a suitable periodic drive can convert a trivial state into a Floquet topological one, accommodating nondissipative dynamic gapless modes at the interfaces with vacuum. Here we theoretically demonstrate that dislocations, ubiquitous lattice defects in crystals, can probe Floquet topological and an unconventional $\pi$-trivial insulators in the bulk of a driven quantum system by supporting normal and anomalous modes at its core. Respectively they reside at the Floquet zone center and boundary. We exemplify these outcomes specifically for two-dimensional Floquet Chern insulator and $p_x+ip_y$ superconductor, where these localized modes are respectively constituted by charged and neutral Majorana fermions. Our findings should be instrumental in probing Floquet topological phases in the state-of-the-art experiments in driven quantum crystals and metamaterials through bulk topological lattice defects. |
Wednesday, March 17, 2021 8:24AM - 8:36AM Live |
L45.00003: Tuning Fermi Levels in Intrinsic Antiferromagnetic Topological Insulators MnBi2Te4 and MnBi4Te7 by Defect Engineering and Chemical Doping Mao-Hua Du, Jiaqiang Yan, Valentino Cooper, Markus Eisenbach MnBi2Te4 and MnBi4Te7 are intrinsic antiferromagnetic topological insulators, offering a promising materials platform for realizing exotic topological quantum states. However, high densities of intrinsic defects in these materials not only cause bulk metallic conductivity, preventing the measurement of quantum transport in surface states, but may also affect magnetism and topological properties. Here, systematic density functional theory calculations reveal specific material chemistry and growth conditions that determine the defect formation and dopant incorporation in MnBi2Te4 and MnBi4Te7. The large strain induced by the internal heterostructure promotes the formation of large-size-mismatched antisite defects and substitutional dopants. Our results show that the abundance of antisite defects is responsible for the observed n-type metallic conductivity. We predict that a Te-rich growth condition should reduce the bulk free electron density, which is confirmed by experimental synthesis and transport measurements in MnBi2Te4. Furthermore, Na doping is proposed to be an effective acceptor dopant to pin the Fermi level within the bulk band gap to enable the observation of surface quantum transport. |
Wednesday, March 17, 2021 8:36AM - 8:48AM Live |
L45.00004: Band structure and end states in InAs/GaSb core-shell-shell nanowires Florinda Viñas Boström, Athanasios Tsintzis, Michael Hell, Martin Leijnse Quantum wells in InAs/GaSb heterostructures can be tuned to a topological regime associated with the quantum spin Hall (QSH) effect, which arises due to an inverted bandgap and hybridized electron and hole states. We investigate electron-hole hybridization and the fate of the QSH effect in a quasi-1D geometry, realized in a core-shell-shell (CSS) nanowire with an insulator core and InAs,GaSb shells. We calculate the band structure using kp theory within the Kane model and the envelope function approximation, then map the result onto a BHZ model which is used to investigate finite-length wires. QSH edge states cannot appear in the CSS nanowires, but we find that the finite-length wires host localized states at the wire ends. These end states are not topologically protected, they are four-fold degenerate and split into two Kramers pairs in the presence of potential disorder along the axial direction. However, there is some remnant of the topological protection of the QSH edge states in the sense that the end states are fully robust to angular disorder, as long as the bulk bandgap is not closed. |
Wednesday, March 17, 2021 8:48AM - 9:00AM Live |
L45.00005: Atom by atom: realization of robust corner states in an electronic breathing Kagome lattice Marlou Slot, Sander Kempkes, Jette van den Broeke, Pierre Capiod, Wladimir Benalcazar, Dario Bercioux, Daniel Vanmaekelbergh, Cristiane Morais Smith, Ingmar Swart Quantum simulators are a versatile platform to study the behavior of novel quantum matter in a controlled way. Platforms based on ultracold atoms in optical lattices and photonic devices have led the field so far, but electronic quantum simulators are proving to be equally relevant. A tunable electronic platform is provided by the 2D electron gas at specific metal surfaces. By atomic-scale patterning of the surface in a scanning tunneling microscope, the electrons can be corralled into electronic lattices with nearly any geometry [1]. In this way, a variety of lattices defined by lattice geometry and bond strengths can be experimentally created and characterized. In particular, we will show the realization of an electronic breathing Kagome lattice. Alternating weak and strong bonds were engineered such that robust zero-energy states emerged at the corners of the lattice [2], determined by the chiral-symmetric edges of the structure [3]. By introducing defects, the zero-energy states were manipulated at will. This tunable approach holds promise for investigating other novel quantum phases of matter. |
Wednesday, March 17, 2021 9:00AM - 9:12AM Live |
L45.00006: Localized spin-orbit polaron in magnetic Weyl semimetal Co3Sn2S2 Yuqing Xing, Shen Jian Lei, Hui Chen, Li Huang, Yuxiang Gao, Qi Zheng, Yu-Yang Zhang, Geng Li, Bin Hu, Hechang Lei, Wei Ji, Shixuan Du, Xiao Lin, Enke Liu, Baogen Shen, Ziqiang Wang, Hongjun Gao The transition metal based kagome lattice compounds have merged as a novel platform for exploring the physics of geometric frustration, correlation and magnetism, and the topological behaviors of quantum electronic states. The kagome lattice Co3Sn2S2 exhibits the quintessential topological phenomena of a magnetic Weyl semimetal such as chiral anomaly and Fermi-arc surface states. Probing its magnetic properties is crucial for understanding this correlated topological state. Here, using spin-polarized scanning tunneling microscopy/spectroscopy (STM/S) and non-contact atomic force microscopy (nc-AFM) combined with first-principle calculations, we report the discovery of localized spin-orbit polarons (SOPs) with three-fold rotation symmetry nucleated around single S-vacancies in Co3Sn2S2. The SOPs carry a magnetic moment and a large diamagnetic orbital magnetization of a possible topological origin relating to the diamagnetic circulating current around the S-vacancy. Appreciable magneto-elastic coupling of the SOP is detected by nc-AFM and STM. Our findings suggest the SOPs can enhance magnetism and more robust time-reversal-symmetry-breaking topological phenomena. Controlled engineering of the SOPs may pave the way toward practical applications in functional quantum devices. |
Wednesday, March 17, 2021 9:12AM - 9:24AM Live |
L45.00007: Robust electronic states due to inhomogeneous spin-orbit couplings in graphene heterostructure Jean-Baptiste Touchais, Pascal Simon, Andrej Mesaros Recent experimental progress in designing electronic states in layered heterostructures raises questions about inducing topological states in inhomogeneous systems. In particular, we investigate numerically and analytically several types of inhomogeneous spin-orbit coupling terms in graphene. We begin by studying domain walls in Kane-Mele spin-orbit coupling, addressing the robustness of gapless helical electronic modes and the emergence of gapped Landau-level-like modes bound to the domain wall. Our numerical analysis shows that the domain-wall modes are gapped by Rashba spin-orbit coupling of any strength, even while the Kane-Mele topological bulkgap remains open. The domain-wall states are also gapped by randomness in the Kane-Mele term even without breaking the conserved spin component. |
Wednesday, March 17, 2021 9:24AM - 9:36AM Live |
L45.00008: Nanoscale Conducting and Insulating Domains on YbB6 Aaron Coe, Zhihuai zhu, Yang He, Dae-Jeong Kim, Zachary Fisk, Jenny E. Hoffman Hexaborides possess aexhibit diverse set of exotic properties, including f-state magnetism and, in some cases, non-trivial topology. Hhowever, the lack of a natural neutral cleavage plane, along with surface dependent polarity, has obscured their study. YbB6 has beenwas predicted to host topological surface states, and indeed conducting surface states have beenwere observed experimentally, but their origin has yet to be determined. We use scanning tunneling microscopy and spectroscopy to provide the first real-space atomically resolved picture of the surface electronic structure in YbB6. We observe several coexisting nanoscale surface terminations that can be identified by the relative downward or upward band bending determined by the surface polarity. Sharp metal-to-insulating transitions are observed on the surface of YbB6 along with the onset of a van Hove singularity identifying spin-split Rashba surface states. Our study suggests the utility possibility of engineering YbB6 for creating spin-polarized p-n junctions at the atomic scale on YbB6. |
Wednesday, March 17, 2021 9:36AM - 9:48AM Live |
L45.00009: Bulk Electrical Transport studies of Magnetically doped SmB6 Jarryd Horn, Yun Suk Eo, Shanta Saha, Wesley Furhman, Samantha O'Sullivan, Dmitri Mihaliov, Cagliyan Kurdak, Michael S Fuhrer The past decade of samarium hexaboride (SmB6) studies has challenged our fundamental understanding of the bulk gap of Kondo insulators. The non-trivial topology and the existence of possible chargeless fermions are current exciting directions in studying intrinsic properties of SmB6. The extrinsic nature, including disorder, of SmB6, is another exciting direction but a mystery at the same time. Using the inverted resistance technique to separate the bulk and surface conduction, the previous report of a 10-order-of-magnitude exponential increase of resistivity upon lowering the temperature suggests that the bulk gap is very robust against disorder. This questions whether an impurity band exists in the bulk gap of SmB6 and whether the transport response is identical to that of doped semiconductors like silicon. This also relates to one of the most fundamental questions: what is the true bulk gap dispersion and where is the chemical potential located? In this talk, we present bulk transport studies using the inverted resistance method and Hall effect on various Fe-, Gd- and Nd-doped SmB6 single crystals to partially answer these questions. |
Wednesday, March 17, 2021 9:48AM - 10:00AM Live |
L45.00010: Mn/Bi site mixing and complex magnetic structure in MnBi2Te4 Yaohua Liu, Jiaqiang Yan Intrinsic magnetic topological insulators (TIs) provide a fertile playground to pursuit exotic quantum states of matter from the nontrivial band topology, such as the quantum anomalous Hall insulators and axion insulators. Recently, the van der Waals magnet MnBi2Te4, with the stacking of septuple-layers, is the first instance of an intrinsic antiferromagnetic TI, which attracts much interest. However, MnBi2Te4 and its related compounds tend to have a significant amount of structural imperfections, notably the Mn/Bi site mixing. In this work, we have performed a detailed study on the chemical and magnetic structures of MnBi2Te4 via single-crystal neutron diffraction at beamline CORELLI, SNS. In contrast to the previous reports of a simple A-type antiferromagentic structure, we found that the Mn ions on both Mn and Bi sites are magnetically ordered at low temperatures, forming a ferrimagnetic structure. This work calls for further studies on the ferrimagnetic structure's role in the band topology and the surface state of this novel material. |
Wednesday, March 17, 2021 10:00AM - 10:12AM Live |
L45.00011: Soliton Dynamics in Interacting Double SSH chains JinHyung Choi, Sangmo Cheon The phi^4 model is a model that describes Su-Schrieffer-Heeger (SSH) model and kinks therein. The dynamics of SSH model such as kink-antikink solitons also can be studied with phi^4 model. In this work, we considered a double SSH chain model having an interchain coupling and studied kink-antikink collisions using double phi^4 model. In this model, there are four degenerate groundstates and three types of chiral solitons; right chiral, left chiral and achiral solitons. We investigated various types of soliton collisions using these three types of chiral solitons. We found that a soliton pair can switch their residing chain after the collision when the initial collision velocity is larger than a critical velocity. Using the collective coordinates method, we explained the collision mechanism as well as the various types of soliton collision operation. We expect that our work will help to understand the experimental soliton collision reported in atomic indium chains on the silicone substrate and give an insight to the collisions of topological objects in general topological systems. |
Wednesday, March 17, 2021 10:12AM - 10:24AM Live |
L45.00012: Dislocation as a bulk probe of higher-order topological insulators Vladimir Juricic, Bitan Roy Higher-order topological (HOT) states extend this usual bulk-boundary correspondence, so they host the modes localized at lower-dimensional boundaries, such as corners and hinges, which may hinder their experimental detection. We theoretically demonstrate that dislocations, ubiquitous defects in crystalline materials, can probe higher-order topology [1]. We show that both two- and three-dimensional HOT insulators respond to the dislocations through the protected finite-energy in-gap electronic modes, localized at the defect core, which originate from the interplay between the orientation of the HOT mass domain wall and the Burgers vector of the dislocation. The protection mechanism is based on non-crystalline symmetries, such as time-reversal and particle-hole, the crystalline ones, and may also involve their combination. Finally, we will discuss the consequences of our results for the systematic probing of the extended bulk-boundary correspondence in a broad range of HOT crystals and metamaterials through the lattice dislocation defects, controllable in state-of-the-art experiments. |
Wednesday, March 17, 2021 10:24AM - 10:36AM Live |
L45.00013: Identifying Non-centrosymmetric Electronic Structures of Defect in Tungsten Ditelluride Wan-Hsin Chen, Naoya Kawakami, Jhe-Jhih Lin, Hsiang-I Huang, Ryuichi Arafune, Noriaki Takagi, Chunliang Lin WTe2 is a transition metal dichalcogenide with novel electronic structures. Because defects in WTe2 can affect its properties, a precise classification of defects is required to help understand the possible influence beyond them. By using scanning tunneling microscope (STM) and density functional theory (DFT) calculation, we investigated both geometric and electronic structures of the defects in WTe2. We revealed non-centrosymmetric electronic structures near the Te defects. Furthermore, each type of defect show unique interference patterns, significantly identifying four types of defects at both the bottom and top sides. These findings confirm that the observation of interference pattern is a feasible method to characterize the defects in layered materials. |
Wednesday, March 17, 2021 10:36AM - 10:48AM On Demand |
L45.00014: Spin-polarized STM study of topological magnet and quantum impurity Nana Shumiya, Jiaxin Yin, Yuxiao Jiang, Huibin Zhou, Gennevieve Macam, Hano Omar Mohammad Sura, Ziqiang Wang, Shuang Jia, Zahid Hasan We have developed vector magnetic field based scanning tunneling microscopy/spectroscopy (STM/S) technique to probe and discover topological magnets [1-2]. Here we report spin-polarized STM/S to study of the engineered quantum impurity in a topological magnet Co3Sn2S2 [2]. We find that each substituted In impurity introduces a striking localized bound state. Our systematic magnetization-polarized probe reveals that this bound state is spin-down polarized, in lock with a negative orbital magnetization. Moreover, the magnetic bound states of neighboring impurities interact to form quantized orbitals, exhibiting an intriguing spin-orbit splitting, analogous to the splitting of the topological fermion line. Our work collectively demonstrates the strong spin-orbit effect of the single-atomic impurity at the quantum level, suggesting that a nonmagnetic impurity can introduce spin-orbit coupled magnetic resonance in topological magnets. |
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