Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
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 Bi2Se3 is a TSC and shows a nematic order in the TSC state. Recently, Cu, Nb and Sr-doped Bi2Se3 have shown the rotational symmetry breaking by means of NMR, heat capacity, torque magnetometry and transport Hc2. We probed the rotational symmetry of doped Bi2Se3 samples in both normal and superconducting states by torque magnetometry. The magnetic field was applied in-plane 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: Low-energy quasiparticle excitations in superconducting doped Topological Insulator SrxBi2Se3 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 (Bi2Se3) 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 Bi2Se3, only the odd-parity pairings with Eu representation generate nematicity in the superconducting gap. Eu pairing states allow two kinds of gap structures, Δ4x state with point nodes along ky direction and Δ4y state with gap minima along kx direction. These two states can be distinguished from the measurements which is sensitive to low-energy quasiparticle excitations. Here we report the magnetic penetration depth in Sr-doped Bi2Se3 (Tc~2.8K) down to 50 mK measured by tunnel-diode oscillator technique. All samples show power-law behavior λ(T)∝Tn (n=1~2) down to 0.05Tc, indicating strong momentum dependence of the gap. However, the data give much higher exponent n>3 at the lowest temperature region below 0.05Tc. Our data can be interpreted as tiny but finite gap minima exists on Fermi surfaces, which is consistent with Δ4y state. |
Wednesday, March 7, 2018 3:18PM - 3:30PM |
P13.00003: Nematicity of the superconducting gap in CuxBi2Se3 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. CuxBi2Se3 is a strong candidate given both the topological insulator nature and superconductivity. Nematic superconducting state has been observed in CuxBi2Se3 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 in-plane magnetic field angular dependence of the superconducting gap in CuxBi2Se3. Fully gapped superconducting gap is observed, which is insensitive to defects and step edges, excluding the existence of nodes. Under the in-plane magnetic fields of 0.5 T and 1.0 T, the angular dependence of superconducting gap size exhibits two-fold symmetry. The orientation of the two-fold 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 two-fold 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 CuxBi2Se3. |
Wednesday, March 7, 2018 3:30PM - 3:42PM |
P13.00004: Topological Superconductivity Formed by Inter-surface Electron Pairing in Sb2Te3 Yang-Yuan Chen, m.n. ou, Chung-Yu Mou, Ting-Kuo Lee Topological superconductivity is an exotic state of matter that supports Majorana zero-modes, which have been predicted to occur in the surface states of three-dimensional systems. Recently the foreseeing was experimentally observed in the topological insulator Sb2Te3 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 Sb2Te3 with various thickness d were grown. Experimental data and theoretical analyses show that the superconductivity in Sb2Te3 nanoflakes for d < 9 nm a new class of superconductivity origins from pairing between top and bottom surfaces Dirac fermions. The funding of emergent inter-surface 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: Normal-state and superconducting anisotropy in the doped topological insulator Sr0.1Bi2Se3 Matthew Smylie, Kristin Willa, Helmut claus, Alexei Koshelev, Kok Wee Song, Ulrich Welp, Wai-Kwong Kwok, Zahirul Islam, John Schneeloch, Ruidan Zhong, Genda Gu The topological insulator Bi2Se3 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 in-plane anisotropy in Hc2 in Sr0.1Bi2Se3 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 x-ray 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 odd-parity nematic superconducting state characterized by a nodal gap of Eu symmetry. |
Wednesday, March 7, 2018 3:54PM - 4:06PM |
P13.00006: Asymmetric Transport in Superconductor-Topological Insulator-Superconductor 2D Arrays Vincent HUMBERT, Greg MacDougall, Nadya Mason Three-dimensional topological insulators (TIs) are Dirac materials having spin momentum-locking of the electrons and showing evidence of spin-polarized currents at their surfaces. TIs proximity-coupled by s-wave superconducting materials are expected to display unconventional superconductivity. To probe the interplay between TI surface properties and superconductivity, we fabricated two-dimensional (2D) superconducting island arrays on exfoliated flakes of the three-dimensional TI Bi2Se3. Such 2D arrays have been shown to undergo Kosterlitz-Thouless 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 spin-locked supercurrent. |
Wednesday, March 7, 2018 4:06PM - 4:18PM |
P13.00007: Measurement of the current-phase relation in superconductor-topological insulator-superconductor Josephson junctions Morteza Kayyalha, Aleksandr Kazakov, I. Miotkowski, Sergei Khlebnikov, Leonid Rokhinson, Yong Chen The spin-helical topological surface states (TSS) of topological insulators in proximity with an s-wave superconductor are predicted to demonstrate signatures of topological superconductivity and host Majorana fermions. Here, we report highly skewed current-phase relation (CPR) in gate-tunable Josephson junctions made from topological insulator BiSbTeSe2 with superconducting Nb electrodes. Our observations provide strong evidences for the existence of low-energy states and the ballistic transport in the TI-based 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 s-wave 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 well-understood theoretical framework of s-wave SC/TI heterostructures, it is unclear if Majorana zero modes are expected to appear in a d-wave SC/TI heterostructure. Moreover, the experiments so far have reported conflicting observations of proximity-induced 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 Bi2Te3 grown on a cuprate high-Tc superconductor Bi2Sr2CaCu2O8+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 proximity-induced superconductivity in this system. |
Wednesday, March 7, 2018 4:30PM - 4:42PM |
P13.00009: Topological crystalline insulators and topological superconductors in the Pb-Sn-In-Te 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 Pb-system ( Pb-Sn-In-Te) 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 PdTe2 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 kz-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 angle-resolved photoemission, scanning-tunnelling microscopy and first-principles calculations to investigate PdTe2, an intrinsic bulk superconductor. We uncover a new set of topological surface states which cross the Fermi level resulting in rich, multi-valley Fermi surfaces possessing an unusual and complex spin-texture. Despite this, our spectroscopy measurements reveal conventional type-II surface superconductivity with no in-gap states. This demonstrates that the presence of Fermi-level 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 insulator-superconductor SmB6/YB6 heterostructures Seunghun Lee, Xiaohang Zhang, Valentin Stanev, Joshua Higgins, Victor Galitski, Richard Greene, Ichiro Takeuchi Topological insulator-superconductor (TI-SC) 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 in-situ deposited SC-TI Nb/SmB6 heterostructures as a result of Nb superconductivity induced in the surface state of SmB6. Here, to probe the proximity-induced superconductivity in the surface state of SmB6, we fabricated inverted TI-SC SmB6/YB6 heterostructures in-situ and investigated the superconductivity with point contact spectroscopy measurements. In the limit of SmB6 layer thickness going to zero, the YB6 thin film exhibits tunneling-like spectra with a finite Z (barrier parameter), whereas a thin SmB6/YB6 bilayer exhibits clear doubling of the conductance within the gap of YB6, 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 tunneling-like 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 Majorana-Transmon. Here we consider a related device where a topological insulator nanowire of type Bi2Se3 is proximitized to an s-wave 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 Fu-Kane model, combined with broken time-reversal symmetry. |
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