Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session P13: Topological Insulator based Topological SuperconductivityFocus

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Sponsoring Units: DMP Chair: Yi Li, Johns Hopkins University Room: LACC 304A 
Wednesday, March 7, 2018 2:30PM  3:06PM 
P13.00001: Rotational symmetry breaking in the superconducting state of doped bismuth selenides Invited Speaker: Tomoya Asaba Topological superconductors (TSC) have been attracting huge interest due to their potential applications to topological quantum computation. Particularly, theories predicted that superconducting doped Bi_{2}Se_{3} is a TSC and shows a nematic order in the TSC state. Recently, Cu, Nb and Srdoped Bi_{2}Se_{3} have shown the rotational symmetry breaking by means of NMR, heat capacity, torque magnetometry and transport H_{c2.} We probed the rotational symmetry of doped Bi_{2}Se_{3} samples in both normal and superconducting states by torque magnetometry. The magnetic field was applied inplane and the symmetry of magnetic anisotropic susceptibility as well as hysteresis loop was measured. We observed that the superconducting hysteresis loop, as well as the anisotropic magnetic susceptibility, was enhanced along one direction, indicating the rotational symmetry breaking. The relationship between the TSC state and rotational symmetry breaking will be discussed. 
Wednesday, March 7, 2018 3:06PM  3:18PM 
P13.00002: Lowenergy quasiparticle excitations in superconducting doped Topological Insulator Sr_{x}Bi_{2}Se_{3} studied by Penetration depth measurements Takaaki Takenaka, Yijie Miao, Yuta Mizukami, Kazumune Tachibana, Takao Sasagawa, Yuichi Kasahara, Yuji Matsuda, Takasada Shibauchi Superconducting doped bismuth selenide (Bi_{2}Se_{3}) is a promising candidate of topological superconductors. Recent studies in the superconducting state reveal that this material shows spontaneous rotational symmetry breaking in the gap function, which is termed as nematic superconductivity. Among pairing symmetries proposed for superconducting doped Bi_{2}Se_{3}, only the oddparity pairings with E_{u} representation generate nematicity in the superconducting gap. E_{u} pairing states allow two kinds of gap structures, Δ_{4x} state with point nodes along k_{y} direction and Δ_{4y} state with gap minima along k_{x} direction. These two states can be distinguished from the measurements which is sensitive to lowenergy quasiparticle excitations. Here we report the magnetic penetration depth in Srdoped Bi_{2}Se_{3} (T_{c}~2.8K) down to 50 mK measured by tunneldiode oscillator technique. All samples show powerlaw behavior λ(T)∝T^{n} (n=1~2) down to 0.05T_{c}, indicating strong momentum dependence of the gap. However, the data give much higher exponent n>3 at the lowest temperature region below 0.05T_{c}. Our data can be interpreted as tiny but finite gap minima exists on Fermi surfaces, which is consistent with Δ_{4y} state. 
Wednesday, March 7, 2018 3:18PM  3:30PM 
P13.00003: Nematicity of the superconducting gap in Cu_{x}Bi_{2}Se_{3 }studied by scanning tunneling microscopy Yajun Yan, Ran Tao, Tong Zhang, Donglai Feng The search for unconventional superconductor is always a hotspot in condensed matter physics. Cu_{x}Bi_{2}Se_{3} is a strong candidate given both the topological insulator nature and superconductivity. Nematic superconducting state has been observed in Cu_{x}Bi_{2}Se_{3 }by NMR study, and subsequently confirmed by thermodynamic study, torque magnetometry and transport measurements. Here by using ultralow temperature and vector magnetic field equipped STM, we directly measured the inplane magnetic field angular dependence of the superconducting gap in Cu_{x}Bi_{2}Se_{3}. Fully gapped superconducting gap is observed, which is insensitive to defects and step edges, excluding the existence of nodes. Under the inplane magnetic fields of 0.5 T and 1.0 T, the angular dependence of superconducting gap size exhibits twofold symmetry. The orientation of the twofold symmetry of SC gap is the same in one domain, but differs in different SC domains. In addition, the magnitude of magnetic field may alter the orientation of the twofold symmetry of SC gap. The direct observation of the anisotropy of the superconducting gap and its response to magnetic field may lay a foundation for further understanding the superconducting state of Cu_{x}Bi_{2}Se_{3}. 
Wednesday, March 7, 2018 3:30PM  3:42PM 
P13.00004: Topological Superconductivity Formed by Intersurface Electron Pairing in Sb_{2}Te_{3} YangYuan Chen, m.n. ou, ChungYu Mou, TingKuo Lee Topological superconductivity is an exotic state of matter that supports Majorana zeromodes, which have been predicted to occur in the surface states of threedimensional systems. Recently the foreseeing was experimentally observed in the topological insulator Sb_{2}Te_{3} with an emergent surface superconductivity [1]. The surface superconductivity on top and bottom surfaces of a bulk are individual and irrelevant, however as the thickness of a bulk reduces to a nanometer range, the top and bottom surfaces approach to each other, they should become an integral part of the whole system. To demonstrate the idea, nanoflakes of single crystal Sb_{2}Te_{3} with various thickness d were grown. Experimental data and theoretical analyses show that the superconductivity in Sb_{2}Te_{3} nanoflakes for d < 9 nm a new class of superconductivity origins from pairing between top and bottom surfaces Dirac fermions. The funding of emergent intersurface superconductivity dominated by the spinful pairing in specimens below 9 nm illuminate a new path to the study of topological superconductivity. 
Wednesday, March 7, 2018 3:42PM  3:54PM 
P13.00005: Normalstate and superconducting anisotropy in the doped topological insulator Sr_{0.1}Bi_{2}Se_{3} Matthew Smylie, Kristin Willa, Helmut claus, Alexei Koshelev, Kok Wee Song, Ulrich Welp, WaiKwong Kwok, Zahirul Islam, John Schneeloch, Ruidan Zhong, Genda Gu The topological insulator Bi_{2}Se_{3} becomes superconducting when doped with Cu, Nb (Tc~3.5 K) or Sr (Tc~3.0K); these materials are candidate topological superconductors. We observe twofold inplane anisotropy in H_{c2} in Sr_{0.1}Bi_{2}Se_{3} via magnetotransport and thermodynamic measurements despite the trigonal crystal structure. We show that the direction of current does not affect the axis of anisotropy and that the normal state electronic and magnetic properties are isotropic in the basal plane. Synchrotron xray diffraction shows no distortions from the ideal crystal structure. We conclude that the anisotropy arises from the superconducting gap structure itself, consistent with the recently proposed oddparity nematic superconducting state characterized by a nodal gap of E_{u} symmetry. 
Wednesday, March 7, 2018 3:54PM  4:06PM 
P13.00006: Asymmetric Transport in SuperconductorTopological InsulatorSuperconductor 2D Arrays Vincent HUMBERT, Greg MacDougall, Nadya Mason Threedimensional topological insulators (TIs) are Dirac materials having spin momentumlocking of the electrons and showing evidence of spinpolarized currents at their surfaces. TIs proximitycoupled by swave superconducting materials are expected to display unconventional superconductivity. To probe the interplay between TI surface properties and superconductivity, we fabricated twodimensional (2D) superconducting island arrays on exfoliated flakes of the threedimensional TI Bi_{2}Se_{3}. Such 2D arrays have been shown to undergo KosterlitzThouless transitions toward a superconducting state. Transport and Fraunhofer spectroscopy measurements carried out on our devices show unusual behavior. They exhibit strong asymmetry in the field oscillations of the magnetoresistance suggestive of a spinlocked supercurrent. 
Wednesday, March 7, 2018 4:06PM  4:18PM 
P13.00007: Measurement of the currentphase relation in superconductortopological insulatorsuperconductor Josephson junctions Morteza Kayyalha, Aleksandr Kazakov, I. Miotkowski, Sergei Khlebnikov, Leonid Rokhinson, Yong Chen The spinhelical topological surface states (TSS) of topological insulators in proximity with an swave superconductor are predicted to demonstrate signatures of topological superconductivity and host Majorana fermions. Here, we report highly skewed currentphase relation (CPR) in gatetunable Josephson junctions made from topological insulator BiSbTeSe_{2} with superconducting Nb electrodes. Our observations provide strong evidences for the existence of lowenergy states and the ballistic transport in the TIbased junctions. We also investigate the gate dependence of the CPR, where we observe that close to the Dirac point the transport becomes more diffusive, most likely due to the presence of electron/hole puddles. 
Wednesday, March 7, 2018 4:18PM  4:30PM 
P13.00008: Nanoscale Imaging of a Topological Insulator Grown on a Cuprate Superconductor using Molecular Beam Epitaxy Bryan Rachmilowitz, Zheng Ren, He Zhao, John Schneeloch, Ruidan zhong, Genda Gu, Ilija Zeljkovic When an swave superconductor (SC) is interfaced with a topological insulator (TI), the TI can become superconducting by proximity. The emergent phase of matter is predicted to host Majorana zero modes (Fu and Kane, PRL 100, 096407 (2008)), and has promising applications in quantum computation. In contrast to a wellunderstood theoretical framework of swave SC/TI heterostructures, it is unclear if Majorana zero modes are expected to appear in a dwave SC/TI heterostructure. Moreover, the experiments so far have reported conflicting observations of proximityinduced superconductivity in these systems (Wang et al., Nature Physics 9, 621 (2013), Yilmaz et al., PRL 113, 077003 (2014)). To shed light on this, we perform scanning tunneling microscopy/spectroscopy experiments of a prototypical topological insulator Bi_{2}Te_{3} grown on a cuprate highT_{c} superconductor Bi_{2}Sr_{2}CaCu_{2}O_{8+x }using molecular beam epitaxy. We characterize the electronic band structure of the topmost TI surface using quasiparticle interference imaging, and discuss the implications of our measurements on previous observations of proximityinduced superconductivity in this system. 
Wednesday, March 7, 2018 4:30PM  4:42PM 
P13.00009: Topological crystalline insulators and topological superconductors in the PbSnInTe Genda Gu, Ruidan zhong, J. A. Schneeloch, qiang Li, Tonica Valla, John Tranquada The discovery of 3D topological insulator materials and topological superconductor open up a new research field in the condensed matter physics. In order to search for the ideal topological insulator, topological crystalline insulator and topological superconductor, we have grown a large number of the single crystals of Pbsystem ( PbSnInTe) topological crystalline insulator and their topological superconductor . We have measured the physical properties on these single crystals by various techniques. We have studied the effect of crystal growth condition, impurity and composition on the bulk electrical conductivity of these single crystals. We try to find out which composition and crystal growth condition is the best for the ideal topological insulator, topological crystalline insulator and topological superconductor. We have got the bulk topological superconductor with Tc=5K. 
Wednesday, March 7, 2018 4:42PM  4:54PM 
P13.00010: Fermiology and Superconductivity of Topological Surface States in PdTe_{2} Oliver Clark, Matthew Neat, K. Okawa, Lewis Bawden, Igor Marković, F. Mazzola, Jiagui Feng, Takao Sasagawa, Peter Wahl, M. Bahramy, Philip King The recent discovery of a k_{z}dependent series of band inversions has been shown to stabilise a rich array of bulk Dirac fermions and topological surface states in the chalcogen manifolds of transition metal dichalcogenides [1], and has led to speculation that these compounds could hold the necessary prerequisites to host topological superconductivity [2]. Here, we use spin and angleresolved photoemission, scanningtunnelling microscopy and firstprinciples calculations to investigate PdTe_{2}, an intrinsic bulk superconductor. We uncover a new set of topological surface states which cross the Fermi level resulting in rich, multivalley Fermi surfaces possessing an unusual and complex spintexture. Despite this, our spectroscopy measurements reveal conventional typeII surface superconductivity with no ingap states. This demonstrates that the presence of Fermilevel pinned topological surface states in an intrinsic superconductor is not a sufficient criterion to realise surface topological superconductivity. [1] Bahramy, Clark et al., Nature Mater. in press (arXiv:1702.08177); [2] Soluyanov. APS Viewpoint, Physics 10, 74. 
Wednesday, March 7, 2018 4:54PM  5:06PM 
P13.00011: Point contact Andreev reflection on topological insulatorsuperconductor SmB_{6}/YB_{6} heterostructures Seunghun Lee, Xiaohang Zhang, Valentin Stanev, Joshua Higgins, Victor Galitski, Richard Greene, Ichiro Takeuchi Topological insulatorsuperconductor (TISC) heterostructures have attracted great interest for exploring topological superconductivity which has been predicted to creation of Majorana Fermions as well as exotic superconductivity. We have previously demonstrated the superconducting proximity effect in insitu deposited SCTI Nb/SmB_{6} heterostructures as a result of Nb superconductivity induced in the surface state of SmB_{6}. Here, to probe the proximityinduced superconductivity in the surface state of SmB_{6}, we fabricated inverted TISC SmB_{6}/YB_{6} heterostructures insitu and investigated the superconductivity with point contact spectroscopy measurements. In the limit of SmB_{6} layer thickness going to zero, the YB_{6} thin film exhibits tunnelinglike spectra with a finite Z (barrier parameter), whereas a thin SmB_{6}/YB_{6} bilayer exhibits clear doubling of the conductance within the gap of YB_{6}, indicating occurrence of near perfect Andreev reflection with Z ~ 0. We will discuss the results within the context of the Fermi velocity mismatch, helical Cooper pairing, and the topological proximity effect which can influence the observed transition from the tunnelinglike regime to the Andreev reflection regime. 
Wednesday, March 7, 2018 5:06PM  5:18PM 
P13.00012: Hybridization of Bound States in a TSC/TI/TSC Junction Konstantin Yavilberg, Eran Ginossar, Eytan Grosfeld It has been shown previously that a highly isolated and coherent qubit can be engineered by combining a topological superconducting Josephson junction with a microwave resonator into a device known as the MajoranaTransmon. Here we consider a related device where a topological insulator nanowire of type Bi2Se3 is proximitized to an swave Josephson junction. The resulting system is a topological superconducting junction with a topological insulator as the weak link. The Bi2Se3 nanowire is known for its significant surface states bandwidth and thus is an ideal candidate for a realization of the topologically protected Majorana edge states. We investigate the low energy Andreev bound states and the accompanying Majorana edge states residing in the junction. The hybridization between these states has a notable effect on the fermionic parity of the superconductors, and as a result on the dipole coupling between the junction and the cavity, which is evident in its electromagnetic signatures. 
Wednesday, March 7, 2018 5:18PM  5:30PM 
P13.00013: Majorana edge and zero modes at topological insulator/superconductor heterojunctions in the quantum Hall regime. Gaurav Chaudhary, Allan MacDonald One promising proposal to realize Majorana quasiparticles relies on the electronic structure of topological insulator (TI) surface states that are proximity coupled to an $s$wave superconductor, as described in the FuKane model, combined with broken timereversal symmetry. 
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