Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session R02: Topological Materials -- New Materials IIFocus
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Sponsoring Units: DMP Chair: Suchitra Sebastian, Univ of Cambridge Room: BCEC 107A |
Thursday, March 7, 2019 8:00AM - 8:12AM |
R02.00001: Towards Topological States in Silver Bismuthates Synthesized under High-Pressure Mohamed Oudah, Minu Kim, Douglas Andrew Bonn, Bernhard Keimer, Hidenori Takagi The stoichiometric silver bismuthate Ag2BiO3 typically contains equal amounts of bismuth in the Bi+3 and Bi+5 states, and this charge ordering renders Ag2BiO3 insulating [1]. Recent theoretical predictions of metal-insulator transition and Weyl semimetal state in Ag2BiO3 have revived the interest in this material [2]. However, such novel properties are only expected in the absence of the charge ordering of bismuth. Here, we examine the possibility of achieving a topologically nontrivial phase and suppressing the charge ordering in Ag2BiO3 via chemical substitution/doping. We present a new high-pressure synthesis route for Ag2BiO3, which we utilize for substituting/doping at the Ag site. Also, we explore the possibility of making Ag-deficient Ag2-xBiO3 phases using high pressure synthesis. In the presentation, we will discuss the specific synthesis conditions, Rietveld refinement, and some magnetic and transport properties in detail. |
Thursday, March 7, 2019 8:12AM - 8:24AM |
R02.00002: Controlled layer growth and topological states in stanene studied by STM Xiaohu Zheng, Jianfeng Zhang, Rui-Rui Du It is proposed that stanene (a single layer of Sn) is a two-dimensional topological insulator with a bandgap of more than 0.3 eV [1]. Moreover, its band structure and properties may be modulated with increasing thickness (layers), ranging from topological insulator, topological Dirac semimetal to zero gap semimetal [2-3]. We have grown high quality few layer Sn on B-faced InSb (111) through MBE and measured their structural and electronic properties by 400 mK STM. The topological properties of various trivial/non-trivial phases are also investigated in-situ in magnetic field. Main results and discussions will be presented. |
Thursday, March 7, 2019 8:24AM - 8:36AM |
R02.00003: Revealing Optical Transitions and Carrier Dynamics within the Bulk Band Structure of Chiral Tellurium Nanosheets Giriraj Jnawali, Samuel M Linser, Seyyedesadaf Pournia, Iraj Abbasian Shojaei, Howard E Jackson, Leigh Smith, Ruoxing Wang, Gang Qiu, Wenzhuo Wu, Peide (Peter) Ye Tellurium is a chiral crystal consisting of helical chains arranged in a hexagonal array along the c-axis. The bulk band structure due to the spin-orbit interaction exhibits Weyl nodes with opposite chirality at the H and H’ points in the Brillouin zone. Here we use polarized mid-infrared pump-probe spectroscopy on nanosheets of tellurium to map the band-edge electronic structure and interrogate carrier relaxation processes over a wide energy range (0.3 to 1.2 eV). We observe a series of transitions between all three valence bands at H-point of the Brillouin zone (H4, H5 and H6 symmetries) with the doubly degenerate conduction band with Weyl points (H6 Symmetry) with dynamic response of polarization anisotropy. The main decay of photoexcited carriers occurs within the first 50 ps, followed by a weak long-lived decay of carriers. The carrier thermalization process varies with polarization state of the probe beam at different bands. Such knowledge of electronic structure and carrier dynamics provides a foundation for understanding the topological and anisotropic nature of the material. |
Thursday, March 7, 2019 8:36AM - 8:48AM |
R02.00004: Effects of Cu intercalation in Single Crystals of CuxBi2Te2Se Yanan Li, Christian Parsons, Nathaniel Smith, Prasenjit Guptasarma Intercalation of Cu in Topological Insulators such as Bi2Se3 and Bi2Te3 is known to yield superconductivity. The fundamental origin of superconductivity in these systems, whose parent compounds have non-trivial topology, remains under intense scrutiny. Bi2Te2Se is believed to be a topological insulator with structure similar to Bi2Se3 and Bi2Te3, but with much higher bulk resistivity. Here, we report structure-property relationships of Cu-intercalation in single crystals of CuxBi2Te2Se (0<x<0.5) using X-ray Diffraction, Raman Spectroscopy and X-ray Photoemission Spectroscopy. For x<0.2, Cu intercalates in Bi2Te2Se as Cu1+. Increasing x results an increase in c-axis length along with a softening of A1g2 and Eg2 Raman modes. We observe a double-mode behavior in both A1g2 and Eg2 modes corresponding to the outer atoms of Bi and Te(1)/Se(1). Thus, Bi-Te(1) and Bi-Se(1) vibrations are decoupled from each other. For higher x, we observe a triple mode behavior in both A1g2 and Eg2 modes. We ascribe this to additional Cu atoms partially substituting at Bi sites and creating additional modes such as Cu-Se(1) and Cu-Te(1). |
Thursday, March 7, 2019 8:48AM - 9:00AM |
R02.00005: Collective magnon excitations observed in a dilute ferromagnetic topological insulator Robert McQueeney, Daniel Pajerowski, Deborah Schlagel, David Vaknin Inelastic neutron scattering measurements are presented on ferromagnetic Mn-doped Bi2Te3 with a concentration of 0.1 Mn per formula unit. Susceptibility and neutron diffraction measurements find a Curie temperature (TC) of 12 K and ordered moments oriented perpendicular to the quintuple-layer structure. Sharp magnetic diffraction peaks suggest long-ranged and homogeneous ferromagnetism. Despite the dilute nature of the magnetic constituents, collective ferromagnetic magnon excitations are observed below TC with a bandwidth of ~3 meV, although the dispersive features are broadened by site disorder. The magnons are two-dimensional in character, indicating that the strongest magnetic interactions occur within the quintuple-layer with weaker interlayer interactions. In addition, sharp, low-energy (< 1 meV) localized excitations are observed which are consistent with free Mn pairs or small clusters separated from the main percolated ferromagnetic Mn network. |
Thursday, March 7, 2019 9:00AM - 9:12AM |
R02.00006: Topological Insulator Superlattices via Spinodal Decomposition Demet Usanmaz, Pinku Nath, Cormac Toher, Jose Javier Plata, Rico Friedrich, Marco Fornari, Marco Buongiorno Nardelli, Stefano Curtarolo Advanced thermodynamic and electronic structure concepts are combined to define a design strategy for topological insulator superlattices – an alternative to the costly and time-consuming experimental artificial growth methods [1]. Stabilizing self-assembled interfaces between iso-structural and iso-valent topological insulators is possible through spinodal decomposition. To investigate the composition range guaranteeing the topologically protected gapless metallic states, various thermodynamically driven boundaries are designed between constituent materials. The dimensions and topological nature of the metallic channels are tracked by following the spatial distribution of the charge density and spin-texture. The results validate the proof of concept for obtaining spontaneously forming two-dimensional topologically protected metallic states embedded in a three-dimensional insulating environment without any vacuum interfaces. |
Thursday, March 7, 2019 9:12AM - 9:24AM |
R02.00007: Hybrid Quantum Anomalous Hall Effect at Graphene-Oxide Interfaces Zeila Zanolli Interfaces are ubiquitous in materials science, and devices in particular. As device dimensions are constantly shrinking, understanding the physical properties emerging at interfaces is crucial to exploit them for applications. Using first principles techniques and Monte Carlo simulations, we investigate the mutual magnetic interaction at the interface between graphene and an antiferromagnetic semiconductor, BaMnO3 [1]. We find that graphene deeply affects the magnetic state of the substrate, down to several layers below the interface, by inducing an overall magnetic softening, and switching the in-plane magnetic ordering from anti- to ferromagnetic. The graphene-BaMnO3 system presents a Rashba gap 300 times larger than in pristine graphene, leading to a new flavor of Quantum Anomalous Hall effect (QAHE), a hybrid QAHE, characterized by the coexistence of metallic and topological insulating states. These findings could be exploited to fabricate novel devices that use graphene to control the magnetic configuration of a substrate [2]. |
Thursday, March 7, 2019 9:24AM - 9:36AM |
R02.00008: Magneto transport properties of iron-containing Cobalt Monosilicide Lauritz Schnatmann, Christine Damm, Dr.Ruben Hühne, Bernd Rellinghaus, Sergey Novikov, Alexander Burkov, Heiko Reith, Gabi Schierning, Kornelius Nielsch A new strategy in thermoelectrics focussed in this work arise from the interaction of topological electronic states on the thermoelectric properties of materials with nontrivial band structures. Recently, transition metal silicides with B20 crystal structure have been found to belong to the class of topologically nontrivial materials. The electronic structure of CoSi contains linear dispersion branches with nodal points located near the Fermi level. Therefore, CoSi is an interesting candidate to study the correlation between electronic structure and thermoelectric properties. |
Thursday, March 7, 2019 9:36AM - 10:12AM |
R02.00009: Discovery and Categorization of Topological Materials Invited Speaker: Maia Vergniory Topological Quantum Chemistry (TQC) links the chemical and symmetry structure of a given material with its topological properties. This field tabulates the data of the 10398 real-space atomic limits of materials, and solves the compatibility relations of electronic bands in momentum space. A material that is not an atomic limit or whose bands do not satisfy the compatibility relations, is a topological insulator/semimetal. We use TQC to find the topological stoichiometric non-magnetic, ``high-quality'' materials in the world. We develop several code additions to VASP which can compute all characters of all symmetries at all high-symmetry points in the Brillouin Zone (BZ). Using TQC we then develop codes to check which materials in ICSD are topological. Out of 26938 stoichiometric materials in our filtered ICSD database, we find around 7300 topological materials. For the majority of the "high-quality'' topological materials, we compute: the topological class (equivalence classes of TQC elementary band representations - equivalent to the topological index), the symmetry(ies) that protects the topological class, the representations at high symmetry points and the direct gap (for insulators), and the topological index. For topological semimetals we then compute whether the system becomes a topological insulator (whose index/class we compute) upon breaking symmetries - useful for experiments. Our exhaustive results show that a large proportion of all materials in nature are topological. We confirm the topology of several new materials by Wilson loop calculations. I will also explain an open-source code and end-user button on the Bilbao Crystallographic Server (http://www.cryst.ehu.es/cgi-bin/cryst/programs/topological.pl) which checks the topology of any material. |
Thursday, March 7, 2019 10:12AM - 10:24AM |
R02.00010: High-throughput discovery of topological materials using spin-orbit spillage Kamal Choudhary, Kevin Garrity, Francesca Tavazza We present the results of a high-throughput, first principles search for topological materials based on identifying materials with band inversion induced by spin-orbit coupling. Out of the currently available 30000 materials in our database, we investigate more than 4507 non-magnetic materials having heavy atoms and low bandgaps. We compute the spillage between the spin-orbit and non-spin-orbit wave functions, resulting in more than 1699 high-spillage candidate materials. We demonstrate that in addition to Z2 topological insulators, this screening method successfully identifies many semimetals and topological crystalline insulators. Our approach is applicable to the investigation of disordered or distorted materials, because it is not based on symmetry considerations, and it can be extended to magnetic materials. After our first screening step, we use Wannier-interpolation to calculate the topological invariants and to search for band crossings in our candidate materials. We discuss some individual example materials, as well as trends throughout our dataset, that is available at JARVIS-DFT website: http://jarvis.nist.gov |
Thursday, March 7, 2019 10:24AM - 10:36AM |
R02.00011: Electrides as a New Platform of Topological Materials Motoaki Hirayama, Satoru Matsuishi, Hideo Hosono, Shuichi Murakami Research on the topological properties of the wave function attracts a great deal of attention [1]. In our presentation, we show that electrides are suitable for achieving various topological insulating and topological semimetal phases [2]. In electrides, some electrons reside in the interstitial regions and act as anions to stabilize the structure. Since interstitial electrons have small work function, band inversion around the Fermi level is likely to occur. We find that Sc2C shows nontrivial insulating phase characterized by the π Zak phase. This π Zak phase appears as a surface polarization charge, and we propose that this surface charge is useful for carrier doping by using the electride. We find various topological electrides such as Y2C, Sr2Bi (nodal-line semimetal), HfBr (quantum spin Hall system), and LaBr (quantum anomalous Hall insulator). We also discuss one-dimensional electride materials as topological electrides with corner charges. |
Thursday, March 7, 2019 10:36AM - 10:48AM |
R02.00012: Restoring E vs k band structure dispersion in low symmetry disordered systems Zhi Wang, Qihang Liu, Alex Zunger Many target properties of materials are not readily available for pure components AX or BX but do exist in alloys of (AX)x(BX)1-x. Substitutional alloy disorder generally results in the distribution of local environments as well as symmetry-breaking atomic displacements as seen in EXAFS, leading to the removal of band degeneracies and meaning of sharp wavevectors k hence E vs k band dispersion. These effects are not treatable by single-site disorder theories such as CPA. Supercells do retain atomic resolution of disorder but do not readily produce E vs k dispersion needed to judge topological properties. We combine supercells with band unfolding [1] in alloys, leading to Effective Band Structure which recognizes local symmetry yet informs about the extent to which the long-range translational symmetry is retained. We show how the scale of disorders in alloy systems, such as PbS-PbTe, PbSe-SnSe, CdTe-HgTe, PM phase Mott insulators and ABO3 perovskites affects the topological properties, and how the restored band dispersion helps us to understand experimental observations. |
Thursday, March 7, 2019 10:48AM - 11:00AM |
R02.00013: Flat Energy Bands within Antiphase and Twin Boundaries and at Open Edges in Topological Materials Linghua Zhu, Emil Prodan, Keun Hyuk Ahn We present a model of two-dimensional electronic, photonic, and mechanical metamaterial systems, which has flat one-dimensional zero-mode energy bands and stable localized states of a topological origin confined with twin boundaries, antiphase boundaries, and at open edges. Topological origins of these flat bands are analyzed for an electronic system as a specific example, using a two-dimensional extension of the Su-Schrieffer-Heeger Hamiltonian with alternating shift of the chains. It is demonstrated that the slow group velocities of the localized flat band states are sensitively controlled by the distance between the boundaries and the propagation can be guided through designed paths of these boundaries. We also discuss how to realize this model in metamaterials. |
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