Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session B23: Disorder and Topological Defects in Topological Phases |
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Sponsoring Units: DCMP Chair: Raquel Queiroz, Columbia University Room: Room 215 |
Monday, March 6, 2023 11:30AM - 11:42AM |
B23.00001: Codimension-2 defects and higher symmetries in (3+1)D topological phases Ryohei Kobayashi (3+1)D topological phases of matter can host a broad class of non-trivial topological defects of codimension-1, 2, and 3, of which the well-known point charges and flux loops are special cases. The complete algebraic structure of these defects defines a higher category, and can be viewed as an emergent higher symmetry. This plays a crucial role both in the classification of phases of matter and the possible fault-tolerant logical operations in topological quantum error correcting codes. In this paper, we study several examples of such higher codimension defects from distinct perspectives. We mainly study a class of invertible codimension-2 topological defects, which we refer to as twist strings. We provide a number of general constructions for twist strings, in terms of gauging lower dimensional invertible phases, layer constructions, and condensation defects. We study some special examples in the context of Z2 gauge theory with fermionic charges, in Z2 x Z2 gauge theory with bosonic charges, and also in non-Abelian discrete gauge theories based on dihedral (Dn) and alternating (A6) groups. The intersection between twist strings and Abelian flux loops sources Abelian point charges, which defines an H4 cohomology class that characterizes part of an underlying 3-group symmetry of the topological order. The equations involving background gauge fields for the 3-group symmetry have been explicitly written down for various cases. We also study examples of twist strings interacting with non-Abelian flux loops (defining part of a non-invertible higher symmetry), examples of non-invertible codimension-2 defects, and examples of interplay of codimension-2 defects with codimension-1 defects. We also find an example of geometric, not fully topological, twist strings in (3+1)D A6 gauge theory. |
Monday, March 6, 2023 11:42AM - 11:54AM Author not Attending |
B23.00002: Geometric response approach to the filling anomaly Pranav Rao, Barry Bradlyn The filling anomaly two-dimensional higher order topological insulators without chiral symmetry feature an excess charge at corners even in the absence of discrete zero modes. These systems exhibit a similar effect in the presence of rotational defects (disclinations), where additional bound charge is localized at disclination cores. In this work we provide a unifying field-theoretic description to these effects in the language of geometric response. By focusing on the low-energy theory of the gapped boundary, we derive the Wen-Zee response familiar from continuum topological phases, taking care to include the anomaly-canceling Gromov-Abanov-Jensen (GJA) boundary term, which gives rise to the filling anomaly. As a result we provide a concrete through-line between higher order topological insulators (HOTIs) and geometric effective action descriptions of topological phases. |
Monday, March 6, 2023 11:54AM - 12:06PM |
B23.00003: Solitons in Rhombohedral Graphite James Muten, Edward McCann The vertical stacking of rhombohedral graphite resembles the Su-Schrieffer-Heeger (SSH) model, with alternating intra- and interlayer hopping mimicking the role of alternating hopping in the SSH model. Surface states with flat bands at low energy in rhombohedral graphite correspond to edge states in the SSH model. Structural stacking faults in rhombohedral graphite have additional bands at low energy which are localized at the faults [1], in analogy to solitons (domain walls) in the SSH model. However, such stacking faults in rhombohedral graphite always create a pair of energy bands, being equivalent to a combination of a soliton and anti-soliton. Here we discuss the possibility of realizing the equivalent of a single soliton with a single energy band in rhombohedral graphite. We discuss the parameter range under which such solitons exist and we described their topological properties. |
Monday, March 6, 2023 12:06PM - 12:18PM |
B23.00004: Ring states in topological materials Raquel Queiroz, Roni Ilan, Zhida Song, Andrei B Bernevig, Ady L Stern Topological obstructions in quantum matter prevent the construction of symmetric and exponentially localized Wannier states from Bloch bands. We show that topological obstructions always lead to bound states at finite ingap energies for local symmetric potentials that exceed the Bloch Hamiltonian's bandwidth, such as lattice vacancies. The resulting ingap states, which we call ring states can serve as a local probe of topology, and are the precursors of topological boundary modes. We demonstrate the origin of ring states as well a boundary states from the pole structure of the local Green's function, present in topological phases protected by any internal or spatial symmetry. |
Monday, March 6, 2023 12:18PM - 12:30PM |
B23.00005: Relocating a topogical state using non-Hermitian defects Hamidreza Ramezani Topological midgap states are expected to appear at the interface between structurs with trivial and non-trivial topologies. Here, I show that a single embedded non-Hermitian defect in a one-dimensional topological system at certain degrees of non-Hermiticity can remove the topological mode from the edge and restore it inside the lattice at the same place where the non-Hermitian defect is placed. I relate this unexpected phenomenon to the wave-matching condition and continuity of the wave function at different sites in the lattice. These findings pave the way for controlling the position of topological states at will. |
Monday, March 6, 2023 12:30PM - 12:42PM |
B23.00006: Topological field-effect transistor with quantized ON/OFF conductance of helical/chiral dislocation states Xiaoyin Li, Feng Liu Topology is a key ingredient driving the emergence of quantum devices. Topological field-effect transistor (TFET) has been proposed to outperform the conventional FET by replacing the ON state with topology-protected quantized conductance, while the OFF state remains the same normal insulating characteristics and hence bears similar drawbacks. Here, we demonstrate a proof-of-concept TFET having both ON and OFF quantized conductance, by switching between helical and chiral topological screw dislocation (SD) states in three-dimensional topological insulators. A pair of SDs are configured with one acting as channel and the other as gate controlled by local magnetic field. A reversible field-switching is achieved with the ON and OFF conductance of 2e2/h and e2/h, respectively, as shown by tight-binding quantum transport calculations. Furthermore, BaBiO3 is shown as a candidate material having the desired topological SD states, based on first-principles calculations. Our findings open a new route to high-fidelity topological quantum devices. |
Monday, March 6, 2023 12:42PM - 12:54PM |
B23.00007: Defect-controlled Fermi-level tuning in half-Heusler topological semimetals Shoaib Khalid, Hadass S Inbar, Shouvik Chatterjee, Christopher J Palmstrom, Bharat K Medasani, Anderson Janotti Three-dimensional topological semimetals host a range of interesting quantum phenomena related to band crossing and band touching that give rise to Dirac or Weyl fermions, that could be engineered into novel technological applications. Harvesting the full potential of these materials in applications will depend on our ability to position the Fermi level near the symmetry-protected band crossings or touchings so that exotic spin and charge transport properties are manifest. Using first-principles calculations based on density functional theory, we investigate how point defects impact the Fermi level position in two representative half-Heusler topological semimetals, PtLuSb and PtLuBi; we explore how intrinsic defects can be used to tune the Fermi level, and explain recent observations based on Hall measurements in bulk and thin films. Under typical growth conditions we show that Pt vacancies are the most abundant intrinsic defects, leading to excess hole densities that place the Fermi level significantly below the expected position in the pristine material. Suggestions for tuning the Fermi level by tuning chemical potentials are discussed. |
Monday, March 6, 2023 12:54PM - 1:06PM |
B23.00008: Healing non-linear Se vacancy defect states by ambient gases in topological Bi2Se3: a first-principles study Sharmila Shirodkar, Pratibha Dev In this work, we elucidate how the most common non-magnetic intrinsic defects, Se vacancies, perturb the topologically protected states of Bi2Se3, and further explore a method to quench these defects by ambient gases. Our study shows that Se vacancies don’t merely n-dope the system but give rise to a vacancy induced non-linear band pinned near the Fermi level, in addition to splitting the Dirac cone into the valence and conduction bands. These non-trivial features protecting the topological nature of Bi2Se3 emerge from the hybridization between the surface, defect, and quantum well states, which we confirm using a numerical model [1]. Furthermore, we show that these effects are reversible, i.e. the Dirac cone can be reverted back to the Fermi level by the adsorption of ambient gases that are either isoelectronic to Se or highly electronegative to counter-dope the slab. We distinguish the possible features of the adsorbates that can be used to a priori predict their effects on the electronic structure of the Bi2Se3 slab after adsorption [2]. Our results provide a foundation for a general strategy to in situ engineer the band structure of the Bi2Se3 family of topological insulators by healing chalcogen vacancies. |
Monday, March 6, 2023 1:06PM - 1:18PM |
B23.00009: Topological Edge States and Winding Numbers in Graphene with Vacancies Amit Goft, Yuval Abulafia, Nadav Orion, Eric Akkermans We show that graphene with a vacancy, namely the removal of a neutral carbon atom, is described by a chiral massive Dirac Hamiltonian. We prove that it is topological according to the tenfold classification generalized to defects by Teo and Kane. We calculate the associated winding number and show that it is equal to 1. In agreement with the Atiyah Singer index theorem, this winding number equals the Hamiltonian's analytical index which counts the number of zero modes. This index corresponds to the topological edge state (zero mode) created around the vacancy location. |
Monday, March 6, 2023 1:18PM - 1:30PM |
B23.00010: Defect scattering in the Ultraclean Dirac nodal arc semimetal PtSn4 Dong Chen, Ashley N Warner, Seokhwan Choi, Samikshya Sahu, Mohamed Oudah, Markus Altthaler, Niclas P Heinsdorf, Alannah M Hallas, Sarah Burke, Douglas A Bonn PtSn4 was reported to be a Dirac nodal arc semimetal with a high residual resistivity ratio as high as 3000 and extremely large magnetoresistance (XMR). While the former feature is indicative of a scarcity of scattering by defects at low temperatures, the XMR has been attributed to the compensation of electron and hole charge carriers on its Fermi surface. |
Monday, March 6, 2023 1:30PM - 1:42PM |
B23.00011: Structural disorder driven Metal-Insulator transition in BixTeI thin films Paul Corbae Strong disorder has a crucial effect on the electronic structure in quantum materials by increasing localization, interactions, and modifying the local environment as well as the density of states [1]. Understanding how structural disorder can be used to tune the electronic structure in quantum materials is of great importance for technological applications where disorder is likely present. In this work we grow BixTeI thin films and measure their transport properties. The BixTeI family of materials have interesting electronic properties such as Rashba splitting, topological states, and superconductivity [2]. We find by changing the growth temperature and Bismuth concentration there is increased disorder which leads to a metal-insulator transition. This is accompanied by linear magnetoresistance, weak-antilocalization, and evidence for electron-electron interactions. We provide a clear picture how the electronic properties change with disorder. By using disorder as a tuning parameter we open up a new materials space to search for interesting quantum materials. |
Monday, March 6, 2023 1:42PM - 1:54PM |
B23.00012: Interplay between Rashba Edge States and Topological Surface States in 3D Topological Insulators SEOUNGHUN KANG, Wonhee Ko, Matthew Brahlek, An-Ping Li, Robert G Moore, Mina Yoon Bismuth selenide (Bi2Se3) is a well-known material as a quantum spin Hall insulator with Dirac surface states protected by time-reversal symmetry. Recently, some screw dislocations in Bi2Se3 films and their electronic properties were observed by scanning tunneling microscopy. The thick, bulk-like films display increased local density of states confined at step edges, while the edge states disappear when the film thickness is reduced to three quintuples. We investigated the origin of experimentally measured thickness-dependent (topological) quantum states on Bi2Se3 using first-principles based modeling: They arise from the interaction between topological surface states and trivial states of Bi2Se3 with embedded 2D islands on the films. Our results illustrate that the charge and spin transport in the topological materials can be strongly influenced by the interaction between the topological and trivial states. |
Monday, March 6, 2023 1:54PM - 2:06PM |
B23.00013: Bound states, spin polarization, and currents due to magnetic line defects at topological insulator surfaces Eklavya Thareja, Ilya Vekhter We show that a magnetic line defect on the surface of a topological Insulator generically supports two branches of spin-polarized and current carrying one-dimensional bound states. The velocity, and hence spin texture, of each of those branches can be independently tuned by a magnetic field rotated in the plane of the surface. We compute the local spin-resolved density of states as well as the net spin polarization and current due to both bound and scattering states as a function of potential and magnetic components of the scattering potential, and show that it varies stepwise. This work lays the foundation for defect engineering of spin textures and macroscopic currents at topological insulator surfaces. |
Monday, March 6, 2023 2:06PM - 2:18PM |
B23.00014: Computation of topological phase diagram of amorphous BixSb1-x 2d alloys using artificial neural networks Alejandro J Uria Alvarez, Juan José Palacios The entanglement spectrum of a solid is known to contain information regarding the topological behaviour of the material. On the other hand, the Wilson loop is the standard technique to assess if a material is topological or not, provided that it is gapped. For supercells this approach quickly becomes unpractical, or even unusable if the gap disappears, which is why we turn to the entanglement spectrum for the study of disordered systems. In a previous work, we showed that it is possible to train an artificial neural network to identify the topological invariant of disodered systems, using the entanglement spectrum. |
Monday, March 6, 2023 2:18PM - 2:30PM |
B23.00015: Structural spillage: an efficient method to screen amorphous topological materials Adolfo G Grushin, Paul Corbae, Daniel Varjas, Sinead M Griffin, Daniel Muñoz-Segovia While topological phases of matter are not restricted to crystals, there is no efficient method for predicting which amorphous solids are topological. In order to enable a high-throughput screening of amorphous topological materials, it is desirable to find a computationally efficient quantity, compatible with first-principle calculations. In this work, we introduce the structural spillage, a quantity that predicts the topological state of an unknown amorphous state by comparing it to a known reference crystal. To illustrate its potential, we benchmark it using tight-binding and first-principles calculations on two-dimensional amorphous bismuth and its bilayer, showing a transition between trivial and topological phases. Our work sets the basis to predict topological amorphous solids efficiently, opening up a large material class where to explore topological phenomena. |
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