Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session U55: Topological Superconductors in More Than One DimensionFocus
|
Hide Abstracts |
Sponsoring Units: DCMP Chair: Daniel Agterberg, University of Wisconsin - Milwaukee Room: Mile High Ballroom 2B |
Thursday, March 5, 2020 2:30PM - 3:06PM |
U55.00001: Superconductivity without time-reversal or inversion symmetries Invited Speaker: Daniel Agterberg To ensure a superconducting instability, key symmetries are required. In three dimensions (3D), these are time-reversal (T) and inversion (I) symmetries. In two dimensions, T and I are joined by TMz and IMz symmetries where Mz is a mirror reflection through the basal plane. Here we discuss unusual physics that arises when one or more of these key symmetries are broken. In particular, in 3D, we show that spontaneously breaking T symmetry allows for topologically protected nodal Bogoliubov Fermi surfaces and apply this idea to URu2Si2 and Sr2RuO4. In 2D, we develop an energetic and topological superconductor classification in the unfamiliar situation that both T and I symmetreis are absent. We apply this to Ising superconductors with in-plane magnetic fields and monolayer FeSe coexisting with antiferromagnetic order. |
Thursday, March 5, 2020 3:06PM - 3:18PM |
U55.00002: Supercurrent-induced Majorana bound states in a planar geometry André Melo, Sebastian Rubbert, Anton Akhmerov We propose a new setup for creating Majorana bound states in a two-dimensional electron gas Josephson junction. Our proposal relies exclusively on a supercurrent parallel to the junction as a mechanism of breaking time-reversal symmetry. We show that combined with spin-orbit coupling, supercurrents induce a Zeeman-like spin splitting. Further, we identify a new conserved quantity---charge-momentum parity---that prevents the opening of the topological gap by the supercurrent in a straight Josephson junction. We propose breaking this conservation law by adding a third superconductor, introducing a periodic potential, or making the junction zigzag-shaped. By comparing the topological phase diagrams and practical limitations of these systems we identify the zigzag-shaped junction as the most promising option. |
Thursday, March 5, 2020 3:18PM - 3:30PM |
U55.00003: Dissipative chiral superconductors in 3D Meng Hua, Lingyu Yang, Jeffrey Teo Topological band theories that describe insulators, superconductors and (semi)metals are conventionally classified according to the Altland-Zirnbauer classification based on the presence or absence of local time-reversal and particle-hole symmetries. More recently, there are theoretical generalizations that incorporate non-local magnetic space group symmetries. There are more exotic combined antiunitary spatial symmetries that involve particle-hole conjugation and may be allowed in a non-Hermitian system. In this talk, we focus on the classification, model realization and topological-index characterization of dissipative topological superconductors in 3D and the corresponding dissipative chiral Majorana fermion on the boundary surface. |
Thursday, March 5, 2020 3:30PM - 3:42PM |
U55.00004: Topological Skyrmion Phases of Matter Ashley Cook We introduce two-dimensional topological phases of matter defined by non-trivial homotopy groups into the literature, characterized either by a single Skyrmion number, known as the chiral topological Skymion insulator, or a pair of Skyrmion numbers, known as the helical topological Skyrmion insulator, which generalize and extend the concepts of the Chern insulator and quantum spin Hall insulator, respectively. We show each topological phase of matter is protected by a combination of a mirror symmetry and a generalized particle-hole symmetry equal to the product of particle-hole symmetry and spatial inversion symmetry. Despite these phases being protected in part by crystalline point group symmetries, the phases introduced here are very different from all others known: we characterize three kinds of phase transitions by which a Skyrmion number can change. One kind of topological phase transition occurs without the closing of energy gaps, which has important consequences for study of topologically non-trivial phases of matter. We find that each phase is realized in a tight-binding model relevant to Sr2RuO4 and discuss experimental realization given that the chiral topological Skyrmion insulator phase is realized for a parameter set used to characterize Sr2RuO4. |
Thursday, March 5, 2020 3:42PM - 3:54PM |
U55.00005: Triplet-Superconductivity in Triple-Band Crossings GiBaik Sim, Moon Jip Park, SungBin Lee In this talk, we will introduce the triplet superconductivity in the newly discovered form of the topological semimetal, where the electrons with pseudospin J=1 form triple-band crossings. The peculiar property of J=1 electrons is that Fermi statistics rules out on-site spin singlet pairing. Performing the exact decoupling of on-site electron interactions into pairing channels, we adopt the Landau theory of spin-triplet pairings and study the Landau free energy functional to plot out the global phase diagram. We find two distinct phases; (i) time-reversal symmetric (Sz) state, (ii) time-reversal broken (Sx + i Sy) state. Remarkably, both of these states have gapless Bogoliubov quasiparticles and furthermore have topological invariants for each nodal line or Bogoliubov Fermi surface. |
Thursday, March 5, 2020 3:54PM - 4:06PM |
U55.00006: Higher-order topological superconductor in ${\mathcal P}, {\mathcal T}$-odd quadrupolar Dirac metal Bitan Roy Presence or absence of certain symmetries in the normal state (NS) also plays important role in determining the symmetry of the Cooper pairs. We here show that two- and three-dimensional Dirac metals, realized by doping parity (${\mathcal P}$) and time-reversal (${\mathcal T}$) odd topologically trivial Dirac insulators, sustain a local or inter-unit cell pairing that assumes the form of a time-reversal odd and mixed parity (due to the absence of ${\mathcal T}$ and ${\mathcal P}$ in the NS, respectively) pairing around the Fermi surface. When the NS additionally breaks discrete four-fold ($C_4$) symmetry (yielding a ${\mathcal P}$, ${\mathcal T}$-odd, quadrupolar Dirac metal), the system gives birth to a higher-order $p+id$ pairing, hosting corner (in $d=2$) or hinge (in $d=3$) modes of codimension $d_c=2$ of Majorana fermions. While the $p$-wave component stems from the Dirac nature of quasiparticles in the NS, appearance of the $d$-wave component is solely attributed to the lack of $C_4$ symmetry, as its restoration produces $p+is$ pairing. |
Thursday, March 5, 2020 4:06PM - 4:18PM |
U55.00007: Refined symmetry indicators for topological superconductors in all space groups Seishiro Ono, Hoi Chun Po, Haruki Watanabe Topological superconductors are exotic phases of matter featuring robust surface states that could be leveraged for topological quantum computation. A useful guiding principle for the search of topological superconductors is to relate the topological invariants with the behavior of the pairing order parameter on the normal-state Fermi surfaces. The existing formulas, however, become inadequate for the prediction of the recently proposed classes of topological crystalline superconductors. |
Thursday, March 5, 2020 4:18PM - 4:30PM |
U55.00008: Emergent spacetime and gravitational Nieh-Yan anomaly in chiral Weyl superfluids and superconductors Jaakko Nissinen Momentum transport in topological Weyl superfluids (and superconductors) is anomalous in the presence of textures and superflow. Using the semi-classical approximation and gradient expansion, the gauge and Galilean symmetries of the system induce via the superfluid hydrodynamics an emergent "Riemann-Cartan" spacetime with torsion and curvature for the low-energy Weyl quasiparticles. Furthermore, the background exhibits local conservation laws corresponding to emergent quasi-relativistic Lorentz symmetry. We show that the momentum anomaly can be given a consistent interpretation as the chiral Nieh-Yan gravitational anomaly experienced by Weyl fermions on the emergent curved spacetime. The coefficient of this anomaly term is not universally quantized and seems to be determined by the underlying non-relativistic Fermi-liquid and Galilean symmetries. Finally, the connection and extension of the emergent spacetime to finite temperature corrections and thermal effects will be discussed. |
Thursday, March 5, 2020 4:30PM - 4:42PM |
U55.00009: Weyl-superconductor phases in the multilayer model of Weyl semimetals and superconductors Ryota Nakai, Kentaro Nomura We study the superconducting proximity effect in the bulk of Weyl semimetals by considering a multilayer structure consisting of alternating Weyl-semimetal and superconductor (WSM-SC) layers. Our model realizes Weyl-superconductor phases with 2 or 4 nodes of the Majorana fermion, and (2d) topological-superconductor phases characterized by half-odd-integer and integer Chern numbers. We found that potential barriers at the interface and/or mismatch of the Fermi velocity favor topological-superconductor phases with half-odd integer Chern numbers, while Weyl-superconductor phases with 4 nodes and topological-superconductor phases with integer Chern numbers are realized when the Fermi velocity coincides. Topological-superconductor phases are characterized by the quantization of the thermal Hall conductivity, and Weyl-superconductor phases by continuously varying thermal Hall conductivity determined by the position of nodes. |
Thursday, March 5, 2020 4:42PM - 4:54PM |
U55.00010: Edge current and orbital angular momentum of chiral superfluids revisited Wenxing Nie, Wen Huang, Hong Yao Cooper pairs in chiral superfluids carry quantized units of orbital angular momentum (OAM). Various predictions of the total OAM of a chiral superfluid differ by several orders of magnitude. These constitute the so-called angular momentum paradox. Following several previous studies, we substantiate the semiclassical Bogoliubov-de Gennes theory of the edge current and OAM in two-dimensional chiral superfluids in the BCS limit. The analysis provides a simple intuitive understanding for the vanishing of OAM in a non-p-wave chiral superfluid (such as d+id) confined by a rigid potential. When generalized to anisotropic chiral superconductors and three-dimensional (3D) chiral superfluids, the theory similarly returns an accurate description. We will also present a numerical study of the chiral phases in the BEC limit, where the OAM density is found to vanish in the bulk and arise purely from the boundary effects. |
Thursday, March 5, 2020 4:54PM - 5:06PM |
U55.00011: Vortices in a Monopole Superconducting Weyl Semi-metal Shu-Ping Lee, Canon Sun, Yi Li A monopole superconductor is a novel topological phase of matter with topologically protected gap nodes that result from the non-trivial Berry phase structure of Cooper pairs. In this work we study the zero-energy vortex bound states in a model of a monopole superconductor based on a time-reversal broken Weyl semi-metal with proximity-induced superconductivity. The zero modes exhibit a non-trivial phase winding in real space as a result of the non-trivial winding of the order parameter in momentum space. By mapping the Hamiltonian to the $(1+1)$d Dirac Hamiltonian, it is shown that the zero modes, analogous to the Jackiw-Rebbi mode, are protected by the index theorem. |
Thursday, March 5, 2020 5:06PM - 5:18PM |
U55.00012: Majorana Kramers Pairs in Higher-Order Topological Insulators Chen-Hsuan Hsu, Peter Stano, Jelena Klinovaja, Daniel Loss We propose a tune-free scheme to realize Majorana bound states using higher-order topological insulators [1]. When two hinges of a higher-order topological insulator nanowire are brought into the proximity of an s-wave superconductor, two types of pairings arise, one being local (intra-hinge) pairing and the other nonlocal (inter-hinge) pairing. We find that, the competition between these pairings leads to a topological phase transition. The energy band is inverted in the regime where the nonlocal pairing dominates over the local one, leading to the formation of a Kramers pair of Majorana bound states at each end of the nanowire. We show that this topological condition can be fulfilled in the presence of moderate electron-electron interactions, without fine-tuning system parameters such as external magnetic fields. |
Thursday, March 5, 2020 5:18PM - 5:30PM |
U55.00013: Topology-Bounded Superfluid Weight in Twisted Bilayer Graphene Fang Xie, Zhida Song, Biao Lian, Andrei Bernevig We derive the superfluid weight (phase stiffness) of the TBLG superconducting flat bands with a uniform pairing, and show that it can be expressed as an integral of the Fubini-Study metric of the flat bands. This mirrors results [1] already obtained for nonzero Chern number bands even though the TBLG flat bands have zero Chern number. We further show the metric integral is lower bounded by the topological C2zT Wilson loop winding number of the TBLG flat bands, which renders the superfluid weight has a topological lower bound proportional to the pairing gap. In contrast, trivial flat bands have a zero superfluid weight. The superfluid weight is crucial in determining the BKT transition temperature of the superconductor. Based on the transition temperature measured in TBLG experiments, we estimate the topological contribution of the superfluid weight in TBLG. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700