Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session L60: One and TwoDimensional Topological SuperconductivityFocus

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Sponsoring Units: DMP Chair: Javad Shabani, New York University Room: Mile High Ballroom 4A 
Wednesday, March 4, 2020 8:00AM  8:36AM 
L60.00001: Towards Material realizations of 2D Topological Superconductivities in Pb_{1x}Bi_{x} Systems Invited Speaker: Wei Qin New material realization of twodimensional (2D) topological superconductors (TSCs) can provide new platforms for experimentally detecting and manipulating Majorana quasiparticles, which in pairs serve as an exotic entity for nonlocal encoding of quantum information [1]. In this talk, I will focus possibilities of realizing 2D intrinsic topological superconductivity in a new material family, namely atomiclayerthin Pb_{1x}Bi_{x} alloys. Based on firstprinciples calculations, three nearly energetically degenerate structural configurations of the prototypical system Pb_{3}Bi grown on a Ge substrate are identified and labeled respectively as T_{1}, H_{3} and T_{4 }[2]. All three structures possess large Rashba energy band splittings and van Hove singularities (VHS) in the density of states. With proper tensile strains, H_{3} and T_{4 }configurations are demonstrated to have nontrivial band topology characterized by topological edge states. To investigate the superconductivity, an effective formalism that respects hexagonal symmetry, Rashba splitting, VHS and electronelectron interactions is developed with parameters obtained from firstprinciples calculations. Our renormalization group analysis shows that a chiral pwave superconducting phase dominates over other competing orders. Given these results, we identify holedoped Pb_{3}Bi/Ge(111) as an appealing platform for realizing intrinsic 2D topological superconductivity [3]. 
Wednesday, March 4, 2020 8:36AM  8:48AM 
L60.00002: VolkovPankratov states in twodimensional topological superconductors David Alspaugh, Daniel E Sheehy, Mark Oliver Goerbig, Pascal Simon We study the ingap states that appear at the boundaries of twodimensional topological superconductors. While the massless chiral Majorana quasiparticles are guaranteed to arise by the bulkedge correspondence, we find that they could be accompanied by massive VolkovPankratov (VP) states which are present only when the interface is sufficiently smooth. These predictions can be tested in an swave superconductor with strong Rashba spinorbit coupling placed on top of a magnetic domain wall. We calculate the spinresolved local density of states of the VP states about the band inversion generated by a magnetic domain wall and find that they are oppositely spinpolarized on either side of the topological phase boundary. We also demonstrate that the spatial position, energylevel spacing, and spin polarization of the VP states can be modified by the introduction of inplane electric fields. 
Wednesday, March 4, 2020 8:48AM  9:00AM 
L60.00003: Unconventional superconductivity in a doped quantum spin Hall insulator Domenico Di Sante, Xianxin Wu, Mario Fink, Werner R Hanke, Ronny Thomale A monolayer of jacutingaite (Pt_{2}HgSe_{3}) has recently been identified as a novel quantum spin Hall insulator. By firstprinciples calculations, we study its Fermiology in the doped regime and unveil a typeI and typeII van Hove singularity for hole and electron doping, respectively. We find that the common link between the propensity for a topological band gap at pristine filling and unconventional superconductivity at finite doping roots in the longer ranged hybridization integrals on the honeycomb lattice. In a combined effort of random phase approximation and functional renormalization group, we find chiral dwave order for the typeI and oddparity fwave order for the typeII regime. 
Wednesday, March 4, 2020 9:00AM  9:12AM 
L60.00004: Majorana zero modes by engineering topological kink states in two dimensional
electron gas Shuguang Cheng, Jie Liu, Haiwen Liu, Hua Jiang, QingFeng Sun, Xincheng Xie Majorana zero modes (MZMs)–bearing potential applications for topological quantum computing are verified in quasionedimensional (1D) Fermion systems, including semiconductor nanowires, magnetic atomic chains, planar Josephson junctions. However, the existence of multibands in these systems makes the MZMs fragile to the influence of disorder. Moreover, in practical perspective, the proximity induced superconductivity may be difficult and restricted for 1D systems. Here, we propose a flexible route to realize MZMs through 1D topological kink states by engineering a 2D electron gas with antidot lattices, in which both the aforementioned issues can be avoided owing to the robustness of kink states and the intrinsically attainable superconductivity in highdimensional systems. The MZMs are verified to be quite robust against disorders and the bending of kink states, and can be conveniently tuned by varying the Rashba spinorbit coupling strength. Our proposal provides an experimental feasible platform for MZMs with systematic manipulability and assembleability based on the present techniques in 2D electron gas system. 
Wednesday, March 4, 2020 9:12AM  9:24AM 
L60.00005: Transport properties of MajoranaJosephson interferometer ChangAn Li, Jian Li, ShunQing Shen We study the transport properties of MajoranaJosephson interferometer, which is consisted of a twoterminal quantum anomalous Hall insulator strip with two superconducting bars grown over it, to showcase the interference nature of chiral Majorana modes. In this MajoranaJosephson interferometer, the interference effect is caused by a Majorana valve between two topological superconductors that form a Josephson junction. Its transport observables exhibit interference patterns depending on the Josephson phase as well as the junction length. We also show that the interference patterns are robust against weak disorder while strong dephasing effect destroys them. Interestingly, a MajoranaJosephson interferometer can work as two effectively separated MachZennder interferometers, and it is even able to detect the Z2 phase of quantum vortices in topological superconductors. Experimental observation of these interference patterns will probe the topological superconducting phase in related systems and may pave a path for their further manipulations in topological quantum computation. 
Wednesday, March 4, 2020 9:24AM  9:36AM 
L60.00006: Josephson Interferometry of ZigZag Junctions Demonstrating Enhanced Localization Andrew Saydjari, Andrew Pierce, Hechen Ren, Michael Kosowsky, Christopher Ames, Martin Stenho, Laurens W Molenkamp, Amir Yacoby Josephson junctions on mercury telluride quantum wells combine strong spinorbit effects, swave superconductivity, and arbitrary lithographic design. This platform facilitates both the study of induced superconductivity in the quantum well and topological states in quasi1D systems. While zero bias peaks have been observed in these quasi1D systems, small topological gaps and poor confinement of the 0D topological states makes their assignment as Majorana fermions challenging. Recent theoretical work suggests an order of magnitude enhancement in the topological gap and localization when the Josephson junction is fabricated in a zigzag geometry [1]. In this work, we provide the first characterization of zigzag geometry junctions and demonstrate enhanced localization of electrons within the junction. 
Wednesday, March 4, 2020 9:36AM  9:48AM 
L60.00007: Magnetic Skyrmion Induced Topological Superconductivity Eric Mascot, Dirk Klaus Morr Magnetic skyrmions are a stable topological spin texture found in magnetic materials with DzyaloshinskiiMoriya interactions. These skyrmions are highly controllable and can be written and deleted using spin polarized currents [1] and varied in size from 1 nm to 100 nm [2] by controlling material properties such as thickness or doping [3]. 
Wednesday, March 4, 2020 9:48AM  10:00AM 
L60.00008: Tuning Shiba states hybridization on proximitized superconducting surface Hao Ding, Yuwen Hu, Silas Hoffman, Mallika Randeria, Oindrila Deb, Jelena Klinovaja, Daniel Loss, Ali Yazdani Magnetic atoms on a superconductor induce ingap Shiba states that can be hybridized to create a topological superconducting phase when placed in onedimensional chains, with Majorana zero modes (MZMs) localized at their ends. Here we study magnetic Gd adatoms deposited on the surface of epitaxially grown Bi(110) thin films on a superconducting Nb(110) surface using a dilution refrigerator scanning tunneling microscope (STM). Our spectroscopic measurements show a hard superconducting gap of 1.5 meV on Bi surface and the presence of Shiba states near Gd atoms. By manipulating Gd atoms with the STM tip, we find the Shiba states start hybridizing once two Gd atoms are closer than Fermi wavelength of Bi(110) surface states (~ 20 Å), which indicates the critical role of RKKY interaction mediated by Bi electrons. Through systematically changing the configurations of two Gd atoms, we show the hybridization can be dramatically strengthened as the distance between Gd atoms decreases. Our work provides a promising platform for realizing MZMs because it allows fine control over Shiba states hybridization. 
Wednesday, March 4, 2020 10:00AM  10:12AM 
L60.00009: RKKYinduced magnetic transition for pairs of magnetic atoms on proximitized superconducting surface Yuwen Hu, Hao Ding, Silas Hoffman, Mallika Randeria, Oindrila Deb, Jelena Klinovaja, Daniel Loss, Ali Yazdani An atomic chain with helical spin texture proximitized by an swave superconductor can harbor topological superconductivity that gives rise to Majorana zero modes (MZMs) localized at its both ends. Understanding the exchange interaction between magnetic atoms on a superconducting surface is the key to realizing helical spin chains and MZMs. Here we use atomic manipulation to build magnetic Gd dimers on proximitized superconducting Bi(110) surface with a dilution refrigerator scanning tunneling microscope (STM). By measuring the ingap Shiba states induced by magnetism of Gd dimers, we find the magnetic ground state of Gd dimers changes from outofplane ferromagnetic to inplane antiferromagnetic and then to inplane ferromagnetic phase as a function of the distance between two Gd atoms. The Shiba states splitting behavior in the inplane ferromagnetic state suggests the exchange interaction between two Gd atoms is dominated by RKKY interaction. Finally, by taking magnetic anisotropy, spinorbit coupling and RKKY interaction into account, we propose a theoretical model that can describe this system. 
Wednesday, March 4, 2020 10:12AM  10:24AM 
L60.00010: Topological superconductivity of adatom chains as probe for unconventional pairing Andreas Kreisel, Timo Hyart, Bernd Rosenow Chains of magnetic atoms on the surface of swave superconductors, have been established as a laboratory for the study of Majorana bound states. In such systems, the breaking of time reversal due to magnetic moments makes the system a onedimensional topological superconductor. However, in unconventional superconductors even nonmagnetic impurities can induce ingap states since scattering of Cooper pairs changes their momentum but not their phase. Here, we propose a novel paradigm for creating topological superconductivity, which is based on an unconventional superconductor with a chain of nonmagnetic impurities on its surface. The system can be driven into the topological phase by tuning magnitude and direction of an external Zeeman field. We show that the topological energy gap can approach the minimum of the bulk energy gap. We develop a general mapping of films with impurity chains in multiband superconductors to onedimensional lattice Hamiltonians. This allows us to illustrate the feasibility of our proposal in the case of the material Sr_{2}RuO_{4}, and to demonstrate that the study of impurity chains can be employed as a diagnostic tool for the nature of the pairing symmetry. This enables to distinguish competing proposals for the pairing symmetry in Sr_{2}RuO_{4}. 
Wednesday, March 4, 2020 10:24AM  10:36AM 
L60.00011: New classification of topological superconductors in 1D magnetic groups Jinyu Zou, Zhida Song, Gang Xu Topological superconductors (TSCs) with Majorana zero modes (MZMs) and their potential application in topological quantum computation have become an important area in condensed matter physics. Their topological classification with crystalline symmetries is an important and useful question for finding the possible TSC materials. We have classified the TSC phase in 1D system with local crystalline time reversal symmetry (LCTRS), where the time reversal symmetry (TRS) is broken, but the combination of TRS with some local crystalline symmetry is preserved. The LCTRS M_{x}T or C_{2z}T will lead to an effective BDI symmetry class which has a Z classification. The case of LCTRS C_{4z}T (C_{6z}T) are not included in the Altland and Zirnbauer (AZ) symmetry classes, and exhibit the Z (Z + Z) classification TSC phase with the robust MZMs protected by the unitary chiral symmetry, which is different from the previously studied TSCs in BDI, DIII and D classes. We also present a minimal BdG Hamiltonian with C_{4}T or C_{6}T symmetry to illustrate the novel TSC phase. 
Wednesday, March 4, 2020 10:36AM  10:48AM 
L60.00012: Topological Superconductivity Intertwined with Broken Symmetries HuiKe Jin, Yi Zhou, DongHui Xu Recently the superconductor and topological semimetal PbTaSe2 was experimentally found to exhibit surfaceonly lattice rotational symmetry breaking below Tc . We exploit the GinzburgLandau free energy and propose a microscopic twochannel model to study possible superconducting states on the surface of PbTaSe2 . We identify two types of topological superconducting states. One is timereversal invariant and preserves the lattice hexagonal symmetry while the other breaks both symmetries. We prove that for a C3v lattice with spatial inversion symmetry breaking and strong spinorbit coupling, the time reversal symmetry must be broken and the only way to lead a nematic superconductor is the mixing of 1D and 2D irreducible representation superconducting states. 
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