Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session L13: Dirac Semimetal Based Topological superconductivityFocus

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Sponsoring Units: DMP Chair: Vincent Humbert, University of Illinois UrbanaChampaign Room: LACC 304A 
Wednesday, March 7, 2018 11:15AM  11:51AM 
L13.00001: Supercurrent in the quantum Hall regime Invited Speaker: Gleb Finkelstein One of the promising routes towards creating novel topological states and excitations is to combine superconductivity and quantum Hall (QH) effect. However, signatures of superconductivity in the QH regime remain scarce, and a superconducting current through a QH weak link has so far eluded experimental observation. By utilizing high mobility graphene/boron nitride heterostructures we demonstrate the existence of a novel type of supercurrentcarrying states in a QH regime at magnetic fields as high as 2 Tesla. At low magnetic fields, devices demonstrate the Fraunhoffer pattern and FabriPerot oscillations, confirming their uniformity and ballisticity. In the QH regime, when Landau quantization is fully developed, regions of superconductivity can be observed on top of the conventional QH fan diagram. The measured supercurrent is very small, on a few nA scale, and periodic in magnetic field. We discuss possible mechanisms that could mediate supercurrent along the QH edge states. 
Wednesday, March 7, 2018 11:51AM  12:03PM 
L13.00002: Tunneling signature of superconducting coupling in graphene quantum Hall edge states Jing Shi, GilHo Lee, Seung Hyun Park, Onder Gul, Kenji Watanabe, Takashi Taniguchi, Philip Kim Recent development in graphene device fabrication has allowed the observation of quantum Hall (QH) edge states at very low magnetic fields. By employing graphite back gate, we can produce broken symmetry QH states in graphene in a low magnetic field, while still preserving the superconductivity of a typeII superconductor NbN. The induced superconducting coupling of counterpropagating QH edge states spaced within the coherence length of NbN can be probed by quasiparticle tunneling. To measure the tunneling spectra, we build a local top gate to bring the normal QH edge state to proximity of the proximitized QH edge state. In addition, we will also present Josephson junction based on a pair of proximitized counter propagating QH edge states. 
Wednesday, March 7, 2018 12:03PM  12:15PM 
L13.00003: InsulatortoSuperconductor Transition in the KaneMele Model with Attractive Interactions Tamaghna Hazra, Kyungmin Lee, Mohit Randeria, Nandini Trivedi We investigate the onset of pairing and the nature of superconducting (SC) states in a KaneMele model with attractive interactions. The noninteracting state at half filling evolves from a trivial to a topological insulator as a function of the relative strength of the spinorbit coupling to the sublattice potential, with a massless Dirac dispersion at the topological transition. At halffilling, there is a finite threshold for superconductivity and we find a particularly rich phase diagram for the case of nearestneighbor attraction. On either side of the topological transition, we find within mean field theory the same SC state with a condensate that spontaneously breaks translational invariance but is topologically trivial. The insulatortosuperconductor transition is continuous on the topological side and firstorder on the trivial side. Near the topological transition, the system is unstable to a different SC state with equalspin pairing. This state harbors topological superconductivity with a nontrivial Z_{2} invariant. Our work illuminates the transitions between topological and trivial insulators and superconductors by capturing all four phases in one model. 
Wednesday, March 7, 2018 12:15PM  12:27PM 
L13.00004: Pairing Instabilities at the Edge and in the Bulk of a Topological Insulator Kyungmin Lee, Tamaghna Hazra, Nandini Trivedi, Mohit Randeria The gapless edge modes of a topological insulator are hallmarks of its bulk topological nature. The fate of these edge modes in presence of interactions driving symmetry breaking is not well understood. Motivated by this, we study pairing instability of KaneMele model on a strip geometry with shortranged attractive interaction within the Bogoliubovde Gennes framework. We explore how the instabilities at the edge and the bulk are linked. In particular, we find that when the superconducting transition at the edge precedes the bulk transition, the edge modes may be gapped even when the bulk is topologically nontrivial. Additionally, we find that the pairing instability is strongly affected by the type of the edge (zigzag or armchair). Notably, the edge mode on an armchair edge is surprisingly robust against pairing. We also discuss the difference between the paired states from onsite and nearest neighbor interactions on both the edge and the bulk. 
Wednesday, March 7, 2018 12:27PM  12:39PM 
L13.00005: Topological Superconductivity in the Extended KitaevHeisenberg Model Johann Schmidt, DANIEL SCHERER, Annica BlackSchaffer We discover new superconducting pairing symmetries in the doped Kitaev 
Wednesday, March 7, 2018 12:39PM  12:51PM 
L13.00006: Robust semiDirac points and unconventional topological phase transitions in superconductor/metal heterostructures Timo Hyart, Mats Horsdal SemiDirac fermions are known to exist at the critical points of topological phase transitions requiring finetuning of the parameters. We show that robust semiDirac points can appear in twodimensional superconductor/metal heterostructures without finetuning if one of the layers supports a large intraionic spinorbit coupling [1]. These semiDirac points are topologically stable in the presence of timereversal and twofold rotational symmetries, and a single active band in the superconductor. If the metallic layer is tunnel coupled to two different superconductors (effectively a multiband superconductor) in a threelayerstructure the semiDirac points can split into two stable Dirac points with opposite chiralities. As an example we consider a heterostructure consisting of layers of superconducting Sr_{2}IrO_{4} and a metallic t_{2g} electron system, and we predict a rich topological phase diagram as a function of various parameters. 
Wednesday, March 7, 2018 12:51PM  1:03PM 
L13.00007: Impurity and vortexcore states in superconducting spinhelical Dirac fermions Hiroki Isobe, Michal Papaj, Liang Fu We study the bound states at an impurity and a vortex core in superconducting spinhelical Dirac fermions in two dimensions. The energy of the YuShibaRusinov state, or the impurity bound state, approaches zero with a strongly spinanisotropic coupling, both at the Dirac point and with large doping. The resulting zero energy states are doubly degenerate in contrast to the Majorana zero modes at vortices, and their spatial distributions and spin structures are also distinct. Such differences can be measured by tunneling experiments, which distinguishes impurity bound states from vortexcore states including Majorana zero modes. 
Wednesday, March 7, 2018 1:03PM  1:15PM 
L13.00008: Topological DiracNodalLine Semimetal Phase in HighTemperature Superconductor MgB_{2} KyungHwan Jin, Huaqing Huang, JiaWei Mei, Zheng Liu, Lih King Lim, Feng Liu Significant research effort has been devoted to searching for materials that host both superconducting and topological state, known as topological superconductor. Unfortunately, all the known topological superconductors to date have a very low transition temperature, which severely limits experimental measurement of Majorana fermions. We discover the existence of a topological Diracnodalline (DNL) state in MgB_{2} which has the highest transition temperature (~40 K) for a conventional BCS superconductor using firstprinciples calculations. The DNL structure in MgB_{2} exhibits unique onedimensional dispersive DNL, which connects the electron and hole Dirac states. Since the experimental samples and superconductivity measurement of MgB_{2} is well established, confirmation of our theoretical prediction is highly feasible. 
Wednesday, March 7, 2018 1:15PM  1:27PM 
L13.00009: DoubleHelix Nodal Line Superconductors XiaoQi Sun, Biao Lian, Shoucheng Zhang Timereversal invariant superconductors in three dimensions may contain nodal lines in the Brillouin zone, which behave as Wilson loops of 3d momentumspace ChernSimons theory of the Berry connection. Here we study the conditions of realizing linked nodal lines (Wilson loops), which yield a topological contribution to the thermal magnetoelectric coefficient that is given by the ChernSimons action. We find the essential conditions are the existence of torus or higher genus fermi surfaces and spiral spin textures. We construct such a model with two torus fermi surfaces, where a generic spindependent interaction leads to doublehelixlike linked nodal lines as the superconductivity is developed. 
Wednesday, March 7, 2018 1:27PM  1:39PM 
L13.00010: Topological crystalline superconductivity from doped nodalloop semimetals Yuxuan Wang, Hassan Shapourian, Rahul Nandkishore, Shinsei Ryu We study the intrinsic fullygapped oddparity superconducting order in the doped nodalloop materials with a torusshaped Fermi surface. We show that the mirror symmetry that protects the nodal loop in the normal state also protects the superconducting state as a topological crystalline superconductor. As a result, the surfaces preserving the mirror symmetry host gapless Majorana cones. Moreover, for a Weyl loop system (twofold degenerate at the nodal loop), the surfaces that breaks mirror symmetry (those parallel to the bulk nodal loop) contribute a Chern (winding) number to the quasitwodimensional system in a slab geometry, which leads to a quantized thermal Hall effect and a single Majorana zero mode bound at a vortex line penetrating the system. 
Wednesday, March 7, 2018 1:39PM  1:51PM 
L13.00011: Superconductivity in Fewlayer Stanene Menghan Liao, Yunyi Zang, Zhaoyong Guan, Haiwei Li, Yan Gong, Kejing Zhu, xiaopeng hu, ding zhang, Yong Xu, Yayu Wang, Ke He, XUCUN MA, Shoucheng Zhang, Qikun Xue

Wednesday, March 7, 2018 1:51PM  2:03PM 
L13.00012: A FirstPrinciples Study on Magnetic Silicene and graphene Quantum Dots BiRu Wu Embedded graphene quantum dots in hydrogenated graphene was found having magnetic characters with triangular and parallelogram. We investigate the magnetic properties of graphene and silicene quantum dots with triangular and parallelogram shapes for both the pure dots and embedded ones in hydrogenated graphene (silicene). It was found that the magnetic properties of the graphene (silicene) dots are determined by the shapes of dots as well as the embedded dots in the hydrogenated graphene. The total magnet moment (in Bohr magneton) of triangular graphene (silicene) dot is nearly root square root of the number of atoms in the dot for both the pure and embedded types. The parallelogram graphene (silicene) dots for both types are antiferromagnetic, except for the graphene dot with eight atoms, it shows as nonmagnetic semiconductor. The HOMO LUMO gap (E_{g}) of the pure silicene dots is larger than that of embedded ones, nevertheless, oppositely, the E_{g} of pure graphene dots is about half of that of the embedded one. 
Wednesday, March 7, 2018 2:03PM  2:15PM 
L13.00013: Electronic structure of topological superconductor candidate Au2Pb Yun Wu, Gil Drachuck, LinLin Wang, Duane Johnson, Daixiang Mou, Przemyslaw Swatek, Lunan Huang, Benjamin Schrunk, Sergey Bud'ko, Paul Canfield, Adam Kaminski We use magnetization measurements, ARPES, and DFT calculations to study the electronic properties of topological superconductor Au2Pb. Magnetization data show discontinuities at 40, 51 and 99K, consistent with the reported structural transitions. ARPES data of Au2Pb (111) surface shows a hole pocket at the center and flowerpetallike surface states at the corners of the Brillouin zone, which match relatively well with the DFT calculations. Interestingly, these surface states seem to originate from a Dirac like dispersion close to the zone corner. Further ARPES data of Au2Pb (001) surface shows an electron pocket between G and X, consistent with the predicted location of the 3D Dirac states. However, a significant gap separates the conduction and valence bands, which is probably due to the off kz value probed by 21.2 eV photons. These results provide evidence for the possible existence of Dirac state in this material. 
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