Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session C05: Topological Superconductivity: Twisted Layers, Heterojunctions, InterfacesFocus

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Sponsoring Units: DMP Chair: Arun Bansil Room: BCEC 108 
Monday, March 4, 2019 2:30PM  2:42PM 
C05.00001: Topological Chiral Superconductor with Spontaneous Vortices and Supercurrent in Twisted Bilayer Graphene Fengcheng Wu, Ivar Martin We present a study of dwave superconductivity in twisted bilayer graphene within meanfield theory, and demonstrate new phenomena that arise due to the moire superlattice. In the dwave pairing, the relative motion (RM) of two electrons in a Cooper pair can have either d_{+} or d_{} symmetry, which carry opposite angular momenta. Due to the enlarged moire superlattice, the centerofmass motion (COMM) can also carry a finite angular momentum without breaking the moire periodicity. By matching the total angular momentum, which has contributions from both the RM and the COMM, d_{+} and d_{} RMs are intrinsically coupled in a way such that the COMM associated with one of the RMs has a spontaneous vortexantivortex lattice configuration. Another new phenomenon is that the chiral dwave state carries spontaneous bulk supercurrent. We also discuss the superconductivity gap structure. The chiral dwave superconductors are gapped and also topological as characterized by quantized Chern number. Nematic dwave superconductors, which could be stabilized when the sixfold rotational symmetry of the twisted bilayer graphene is broken, for example by uniaxial strain, are gapless with point nodes. 
Monday, March 4, 2019 2:42PM  2:54PM 
C05.00002: Theory of Superconductivity in MagicAngle Twisted Bilayer Graphene Noah BrayAli Superconductivity in graphene bilayers occurs at certain "magic angles" of relative orientation between the layers. At these angles, the MoireBloch bandstructure becomes nearly flat at the Fermi energy. To describe superconductivity in these flatband graphene bilayers, we extend the theory of superconductivity previously developed for singlelayer graphene doped away from the Dirac point to the saddle point in the bandstructure on the edge of the Brillouin zone. The dominant channel for condensation is still spinsinglet Cooper pairing with d_{x}+id_{y} pair wavefunction, as we show using a perturbative renormalization group argument. Going beyond perturbation theory, we compute the entanglement spectrum using the BardeenCooperSchrieffer wavefunction and compare the result to the spectrum expected from the conformal field theory of a pair of free massless chiral Majorana fermions confined to the edge. 
Monday, March 4, 2019 2:54PM  3:06PM 
C05.00003: Flat bands and superconductivity in twisted bilayer graphene: insights from full scale Bogoliubovde Gennes solutions. Tomas Lothman, Johann Schmidt, Fariborz Parhizgar, Annica M BlackSchaffer The recently experimentally observed, possibly unconventional, superconducting state in twisted bilayer graphene (TBLG) opens up a highly tunable playground for correlated systems and exotic orders, where optimal conditions for exotic superconductivity are believed to result from a twist induced flattening of the electronic bands. Here we test the distinctive role of the flat bands in the formation and enhancement of superconductivity in TBLG and we detail the resulting implications for unconventional chiral d+id superconductivity. We show these results from the selfconsistent solutions of the complete Bogoliubovde Gennes (BdG) equations of the full tightbinding Hamiltonian of TBLG, which in the case of magic twist angle TBLG with its long wavelength moiré interference pattern poses a formidable computational challenge that we rigorously address with a novel numerical approach. 
Monday, March 4, 2019 3:06PM  3:18PM 
C05.00004: A unified numerical approach to topological semiconductorsuperconductor heterostructures Georg W. Winkler, Andrey Antipov, Bernard Van Heck, Alexey A Soluyanov, Leonid Glazman, Michael Wimmer, Roman Lutchyn We develop a unified numerical approach for modeling semiconductorsuperconductor heterostructures and apply it to topological Majorana nanowires. Our approach takes into account on equal footing important key ingredients: proximityinduced superconductivity, orbital and Zeeman effect of an applied magnetic field, spinorbit coupling as well as the electrostatic environment. As a model system, we consider indium arsenide (InAs) nanowires with epitaxial aluminum (Al) shell and demonstrate qualitative agreement of the obtained results with the existing experimental data. Finally, we characterize the topological superconducting phase emerging in a finite magnetic field and calculate the corresponding topological phase diagram. 
Monday, March 4, 2019 3:18PM  3:30PM 
C05.00005: Spinresolved Andreev transport in quantum Hall edges of a proximitized van der Waals heterostructure Önder Gül, Yuval Ronen, Brennan Dizdar, Si Young Lee, Young Jae Shin, Danial Haei Najafabadi, Zeyu Hao, Jonathan Zauberman, Kenji Watanabe, Takashi Taniguchi, Philip Kim Topological superconductivity is a phase of matter that hosts nonAbelian quasiparticles, the building blocks for errorprotected quantum computation. A wellknown example is the Majorana quasiparticle. While several materials have been demonstrated to support topological superconductivity, most of them do not allow for the implementation of a universal set of protected logic gates. Van der Waals materials present a unique playground for topological superconductivity because they readily provide the ingredients to realize the quasiparticles suitable for universal topological quantum computation. Here, we show experiments on van der Waals heterostructures towards engineering nonAbelian quasiparticles from the quantum Hall state with induced superconductivity. Local electrostatic control of the graphene device enables spinresolved Andreev transport. Advanced heterostructures including atomically flat (graphite) gates that fully encapsulate graphene significantly bring down the magnetic fields required for symmetrybroken and fractional quantum Hall states. 
Monday, March 4, 2019 3:30PM  3:42PM 
C05.00006: Electronic structure of semiconductor/superconductor interfaces from angleresolved photoemission experiments Sergej Schuwalow, Niels Schröter, Candice Thomas, Alla Chikina, Marco Caputo, Jonas Krieger, Geoffrey Gardner, Vladimir Strocov, Gabriel Aeppli, Matthias Troyer, Michael Manfra, Peter Krogstrup Hybrid epitaxial semiconductor/superconductor heterostructures have recently shown promise as a platform to realize Majorana zero modes. To pave the way towards the engineering of such heterostructures for quantum information applications, it is imperative to obtain information about their electronic properties at the semiconductor/superconductor interface. Here, we report on recent soft Xray angleresolved photoemission experiments to obtain key electronic material parameters for IIIV semiconductor/aluminium interfaces, such as effective mass, charge density, and band offset. 
Monday, March 4, 2019 3:42PM  4:18PM 
C05.00007: Topological Superconductivity on Moire Superlattices Invited Speaker: Cenke Xu Surprising correlated phenomena such as superconductor and insulator at fractional electron fillings have been observed in systems with Moire superlattice. We study the nature of these correlated phenomena, and explore the possibility of topological superconductivity in these systems using different approaches. The Dirac valley degree of freedom allows these system to form either spin triplet or spin singlet topological superconductor, depending on the sign of an effective "Hund's coupling". Experimental predictions such as magnetic flux quantization can be made based on the particular type of pairing. We will also investigate the nature of the observed insulators, and map the effective spinvalley physics of the insulator at both 1/2 and 1/4 filling to the boundary of a higher dimensional symmetry protected topological (SPT) state. The spinvalley physics of the insulators is governed by a LiebShultzMattis theorem which can be interpreted as the boundary 't Hooft anomaly of the higher dimensional SPT state. 
Monday, March 4, 2019 4:18PM  4:30PM 
C05.00008: Transparent Contacts between Quantum Anomalous Hall Insulators and Superconductors Morteza Kayyalha, Di Xiao, Ruoxi Zhang, Jaeho Shin, Jue Jiang, Fei Wang, Yifan Zhao, Ling Zhang, ChaoXing Liu, Qi Li, Moses H. W. Chan, Nitin Samarth, CuiZu Chang A quantum anomalous Hall (QAH) insulator coupled to an swave superconductor is predicted to harbor a chiral topological superconducting phase, the elementary excitations of which (i.e. chiral Majorana fermions) upon nonAbelian braiding operations can form topological quantum qubits. Here, we fabricated the QAH/Nb hybrid heterostructures and first studied their twoterminal conductance. We found that the twoterminal conductance constantly shows a halfquantized value in the entire range of the magnetic field where the magnetization is wellaligned. Next, we studied the contact transparency between the QAH and Nb films. When the QAH layer is tuned to the metallic regime by gating, we observed Andreev reflections, i.e., a large enhancement of the resistance when a DC bias voltage across the magnetic TI/Nb junction is increased above the Nb superconducting gap. This observation indicates a transparent interface between the QAH and Nb layers. Our study provides a more comprehensive understanding of the relation between the superconducting proximity effect and the observation of the half quantized twoterminal conductance in the QAH/Nb hybrid structure. 
Monday, March 4, 2019 4:30PM  4:42PM 
C05.00009: Coherent singleelectron transport in hybrid superconductorsemiconductor Coulomb islands Alexander Whiticar, Antonio Fornieri, Eoin C O'Farrell, Asbjorn C. C. Drachmann, Tian Wang, Candice Thomas, Sergei Gronin, Geoffrey Gardner, Michael Manfra, Charles M Marcus, Fabrizio Nichele The spatial separation of Majorana modes is expected to allow coherent transport of single electrons through a Coulomb blockaded onedimensional topological superconductor [1]. Here we investigate phase coherent transport in a hybrid AlInAs Coulomb island embedded in an AharonovBohm interferometer. When a magnetic field is applied, the energy of a discrete subgap state is lowered below the charging energy and conductance oscillations are observed with a magnetic flux period of h/e (h is the Planck constant and e is the elementary charge), which indicates coherent transport of single electrons. The oscillation amplitude is maximized when the Coulomb peaks are 1 electron (1e) periodic, while the interference signal is suppressed when transport is 2e periodic or the island is in the normal state. The observation of phasecoherent transport enables us to investigate the coupling of Majorana modes from two separated topological wires with the goal of creating a topological qubit. 
Monday, March 4, 2019 4:42PM  4:54PM 
C05.00010: Nonreciprocal charge transport at topological insulator/superconductor interface Kenji Yasuda, Hironori Yasuda, Tian Liang, Ryutaro Yoshimi, Atsushi Tsukazaki, Kei Takahashi, Naoto Nagaosa, Masashi Kawasaki, Yoshinori Tokura Topological superconductor (TSC) is attracting growing interest for its potential application to topological quantum computation. The superconducting proximity effect on the topological insulator (TI) surface state is one promising way to yield the topological superconductivity. The superconductivity realized at the interface between TI Bi_{2}Te_{3} and nonsuperconductor FeTe is one of such candidates because the mutual interaction between superconductivity and topological order is expected. Here, to detect the effect of spinmomentum locking in the Cooper pair, we investigate nonreciprocal transport; i.e. currentdirection dependent resistance, which is sensitive to the broken inversion symmetry of the electronic state. The largely enhanced nonreciprocal phenomenon is detected in the Bi_{2}Te_{3}/FeTe heterostructure associated with the superconducting transition. The emergent nonreciprocal signal at low magnetic fields is attributed to the currentinduced modulation of supercurrent density under inplane magnetic fields perpendicular to current, exemplifying the close connection between the superconductivity and the topological electronic state at the interface. 
Monday, March 4, 2019 4:54PM  5:06PM 
C05.00011: Transport in SuperconductorTopological InsulatorSuperconductor 2D Arrays Vincent Humbert, Greg MacDougall, Nadya Mason Once coupled to swave superconducting material, threedimensional topological insulators (TIs) – providing spin momentum locking of the electrons and spin polarized current at the surface – are expected to show novel properties and unconventional superconductivity. To probe the interplay between TI surface properties and superconductivity, we fabricated 2D superconducting island arrays on exfoliated flakes of the 3D TI Bi_{2}Se_{3}. Such twodimensional junction arrays have been shown to undergo KosterlitzThouless transitions toward a superconducting state. Transport and Fraunhofer spectroscopy measurements carried out on our devices show unusual, asymmetric behavior. In particular, the measurements provide evidence of charged vortices on the topological surface. 
Monday, March 4, 2019 5:06PM  5:18PM 
C05.00012: Uniaxial Strain Effect on Superconductivity in LaAlO_{3}/SrTiO_{3} Nanostructures Xinyi Wu, Megan Briggeman, Joseph Albro, Jianan Li, Jungwoo Lee, Hyungwoo Lee, ChangBeom Eom, Patrick Irvin, Jeremy Levy We investigate the effects of uniaxial strain on superconductivity in nanowires created at the LaAlO_{3}/SrTiO_{3} interface using conductive atomic force microscope (cAFM) lithography ^{[1]}. CAFMwritten areas are associated with Zoriented ferroelastic domains, surrounded by inplane insulating regions ^{[2]}. Application of external uniaxial stress is expected to displace the ferroelastic domain boundaries, either inward or outward, depending on the sign. Our initial experiments indicate that tensile and compressive strains profoundly affect the superconducting state at milliKelvin temperatures. Uniaxial stretching of the nanowire in the parallel direction is found to completely suppress the superconducting state, while reversal of the applied strain restores superconductivity. We discuss implications for understanding possible role of ferroelastic domain walls in electronpairing mechanisms. 
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