Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session H13: New Theoretical Proposals for Topological SuperconductivityFocus
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Sponsoring Units: DMP Chair: Fan Zhang, University of Texas at Dallas Room: LACC 304A |
Tuesday, March 6, 2018 2:30PM - 3:06PM |
H13.00001: Novel Superconducting Phases in Monolayer Transition Metal Dichacolgenides Invited Speaker: Kam Tuen Law It was recently discovered that transition metal dichacolgenides (TMDs), such as NbSe2 and MoS2, can be superconducting down to monolayer thickness. Importantly, due to the breaking of in-plane mirror symmetry and strong orbital spin-orbit coupling (SOC), electrons in monolayer TMDs experience Ising SOC which pins electron spins to the out-of-plane directions. It has been shown experimentally and theoretically that Ising SOC can strongly enhance the in-plane upper critical field of superconducting TMDs several times beyond the Pauli limit [1-3]. In this talk, we will further explore how Ising SOC can induce novel superconducting phases in monolayer TMDs including a spin-triplet nodal superconducting phase and a spin-singlet topological nodal superconducting phase. |
Tuesday, March 6, 2018 3:06PM - 3:18PM |
H13.00002: Topological Larkin-Ovchinnikov phase and Majorana zero mode chain in bilayer superconducting topological insulator films Lun-Hui Hu, Chao-Xing Liu, Fu-Chun Zhang We theoretically study bilayer superconducting topological insulator film, in which superconductivity exists for both top and bottom surface states. We show that an in-plane magnetic field can drive the system into Larkin-Ovchinnikov (LO) phase, where electrons are paired with finite momenta. The LO phase is topologically non-trivial and characterized by a Z2 topological invariant, leading to a Majorana zero mode chain along the edge perpendicular to in-plane magnetic fields. |
Tuesday, March 6, 2018 3:18PM - 3:30PM |
H13.00003: Topological superconductivity from ferroelectric fluctuations Minseong Lee, Hyun-Jae LEE, Suk Bum Chung, Jun Hee Lee A topological superconductor features at its boundary Majorana fermions, which are potentially applicable for topological quantum computations. Research on the topological superconductivity has been hampered by the scarcity of the real systems. Therefore, there is a strong demand for such an unconventional superconductivity. In this research, we study a heterostructure consisting of a two-dimensional electron gas (2DEG) from 5d transition metals sandwiched by ferroelectric materials using first-principle DFT calculations. This system possesses suitable conditions for the topological superconductivity: it has a strong spin-orbit coupling, and with odd-parity phonon modes at low energy due to ferroelectric fluctuations. The band structure calculations show the Rashba type spin splitting, which results from the spin-orbit coupling and inversion symmetry breaking. Setting the substrate lattice constant at an appropriate range softens the odd-parity phonon mode to induce the strong electron-phonon coupling. Our results indicate that appropriately strained substrates with an extremely soft phonon mode may give rise to the topological superconductivity on 2DEG. We will suggest several candidate substrates and experimental probing techniques. |
Tuesday, March 6, 2018 3:30PM - 3:42PM |
H13.00004: Monopole Superconductivity with Half-integer Partial-wave Symmetries Yi Li The pairing symmetries of superconducting gap functions are usually of integer partial-waves (s,p,d, and etc). In this work, we study a new class of topological superconducting states whose gap functions possess half-integer partial-wave symmetries. These exotic states can exist in systems with topological Fermi surfaces possessing non-zero Chern numbers. When pairing occurs between Fermi surfaces whose Chern numbers are differed by an odd integer, the corresponding superconducting gap function is represented by monopole harmonics with half-integer monopole charges, and thus carries spinor partial-wave symmetries. Topological properties of this class of states, including the surface Majorana modes are studied. We also discuss the possible experimental realizations in Weyl semi-metal system and ultra-cold atom systems. |
Tuesday, March 6, 2018 3:42PM - 3:54PM |
H13.00005: Pseudo-Landau Levels of Bogoliubov Quasiparticles in Strained Nodal Superconductors Geremia Massarelli, Gideon Wachtel, John Wei, Arun Paramekanti Motivated by theory and experiments on strain-induced pseudo-Landau levels of Dirac fermions in graphene and topological materials, we consider its extension for Bogoliubov quasiparticles in a nodal superconductor (SC). We show, using an effective low energy description and numerical lattice calculations for a d-wave SC, that a spatial variation of the electronic hopping amplitude or a spatially varying s-wave pairing component can act as a pseudo-magnetic field for the Bogoliubov quasiparticles, leading to the formation of pseudo-Landau levels. We propose realizations of this phenomenon in the cuprate SCs, via strain engineering in films or nanowires, or s-wave proximity coupling in the vicinity of a nematic instability, and discuss its signatures in tunneling experiments. |
Tuesday, March 6, 2018 3:54PM - 4:06PM |
H13.00006: The 10 phases of spin chains with two Ising symmetries Bat-el Friedman, Atanu Rajak, Angelo Russomanno, Emanuele Dalla Torre We explore the topological properties of quantum spin-1/2 chains with two Ising symmetries. This class of models does not possess any of the symmetries that are required to protect the Haldane phase. Nevertheless, we show that there are 4 symmetry-protected topological phases, in addition to 6 phases that spontaneously break one or both Ising symmetries. By mapping the model to one-dimensional interacting fermions with particle-hole and time-reversal symmetry, we obtain integrable parent Hamiltonians for the conventional and topological phases of the spin model. We use these Hamiltonians to characterize the physical properties of all 10 phases, identify their local and nonlocal order parameters, and understand the effects of weak perturbations that respect the Ising symmetries. Our study provides the first explicit example of a class of spin chains with several topologically non-trivial phases, and binds together the topological classifications of interacting bosons and fermions. |
Tuesday, March 6, 2018 4:06PM - 4:18PM |
H13.00007: From one-dimensional charge conserving superconductors to the gapless Haldane phase Anna Keselman, Patrick Azaria, Erez Berg We discuss the existence of topological superconducting phases in one-dimensional, charge conserving systems. We develop a general approach to treat one-dimensional systems with multiple flavors and attractive pairing interactions. For a single flavor, i.e. a spinless system, we recover the result that a distinct topological phase does not exist due to absence of a gap to single particles in the bulk. For N flavors, we study the case of SO(N) symmetric interactions, and show that for N > 1, a distinct topological phase, with protected, exponentially localized end-modes, can exist. We discuss in detail the case of N = 3, and show that such a system can realize a gapless analogue of the Haldane phase in spin-1 chains. In this phase, although the bulk is gapless to single particle excitations, the ends host spin-1/2 degrees of freedom which are exponentially localized and protected by the spin gap in the bulk. We introduce a lattice model that realizes this phase and analyze it both analytically, using Bosonization and RG, and numerically, using DMRG. |
Tuesday, March 6, 2018 4:18PM - 4:30PM |
H13.00008: Non-adiabatic Berry phase description of Floquet topological states Ian Mondragon, Aris Alexandradinata, Ivar Martin, Meng Cheng We study Floquet topological states using non-adiabatic Berry phases. We show that these geometric quantities correspond to the centers of Wannier states in the frequency domain, in complete analogy with how conventional Berry phases in equilibrium systems correspond to Wannier centers in position space. This link reveals a natural way to understand the topology of periodically driven phases in terms of the behavior of Wannier centers in the frequency domain. We illustrate the versatility of this approach with a model of a driven Majorana wire. We show that the bulk topology is characterized by a Chern number computed using a non-adiabatic version of the Berry curvature. Furthermore, we find that the corresponding Majorana modes at the edge carry a quantized non-adiabatic polarization in the frequency domain. We discuss higher-dimensional systems, such as Floquet Chern insulators, as well as systems with interactions and disorder. |
Tuesday, March 6, 2018 4:30PM - 4:42PM |
H13.00009: Topological Superconductivity with Spin Valves Tong Zhou, Alex Matos Abiague, Jong E Han, Igor Zutic Recent advances in fabricating two-dimensional (2D) epitaxial superconductor/semiconductor heterostructures [1] support the feasibility of implementing a new platform for topological superconductivity employing spin valves, a key building block in magnetic computer hard drives and magnetic random access memory [2].By controlling magnetization states in an array of spin valves, the resulting tunable magnetic textures can modify a proximity-induced superconductivity in the nearby 2D electron gas to provide a confinement and synthetic spin-orbit coupling, supporting the formation of Majorana bound states (MBS) [3,4]. From a detailed analysis of these magnetic textures we discuss their optimization for the control of MBS and demonstration of the non-Abelian statistics. In fact, magnetic textures can already provide topologically-nontrivial effects in the normal state. |
Tuesday, March 6, 2018 4:42PM - 4:54PM |
H13.00010: Abstract Withdrawn
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Tuesday, March 6, 2018 4:54PM - 5:06PM |
H13.00011: Geometric percolation at the surface of a topological superconductor Sayed Ali Akbar Ghorashi, Yunxiang Liao, Matthew S. Foster We present evidence that strongly suggests the equivalence between disordered surface states of topological superconductors (TSCs) and geometric percolation. Percolation is known to play a role in quantum Hall systems with magnetic fields. Our unexpected result implies that percolation applies to TSC surface states, in the absence of time-reversal symmetry breaking. Moreover, the usual "even-odd" effect that occurs in such a system (as identified by Pruisken in the integer quantum Hall effect and by Haldane for spin chains) is found to be absent. |
Tuesday, March 6, 2018 5:06PM - 5:18PM |
H13.00012: Commuting-projector Hamiltonians for Chiral Topological Phases Built from Parafermions Jun Ho Son, Jason Alicea
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Tuesday, March 6, 2018 5:18PM - 5:30PM |
H13.00013: Supersymmetry in Closed Chains of Coupled Majorana Modes Zhao Huang, Shinji Shimasaki, Muneto Nitta We consider a closed chain of even number of Majorana zero modes with nearest-neighbour couplings which are different site by site generically, thus no any crystal symmetry. Instead, we demonstrate the possibility of an emergent supersymmetry (SUSY), which is accompanied by gapless Fermionic excitations. In particular, the condition can be easily satisfied by tuning only one coupling, regardless of how many other couplings are there. Such a system can be realized by four Majorana modes on two parallel Majorana nanowires with their ends connected by Josephson junctions and bodies connected by an external superconducting ring. By tuning the Josephson couplings with a magnetic flux Φ through the ring,we get the gapless excitations at Φ_SUSY =±fΦ_0 with Φ_0 = hc/2e, which is signaled by a zero-bias conductance peak in tunneling conductance. We find this f generally a fractional number and oscillating with increasing Zeeman fields that parallel to the nanowires, which provide a unique experimental signature for the existence of Majorana modes. |
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