Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session F13: High Tc superconductor based topological superconductivityFocus

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Sponsoring Units: DMP Chair: Vic Law, Hong Kong University of Science and Technology Room: LACC 304A 
Tuesday, March 6, 2018 11:15AM  11:51AM 
F13.00001: Unconventional superconductivity in graphene on an electrondoped oxide superconductor Invited Speaker: Jason Robinson The Bardeen–Cooper–Schrieffer theory of superconductivity describes the condensation of electron pairs with antiparallel spins in a socalled singlet state with swave symmetry. Unconventional superconductivity is predicted at certain superconductor interfaces with nonsuperconducting materials [1], including magnets, materials with strong spinorbit coupling such as topological insulator and singlelayer graphene (SLG). For the case of SLG, the electron pairs are predicted to stabilize to a pwave or chiral dwave symmetry, depending on the position of the Fermi energy with respect to the Dirac point. By placing SLG on an electrondoped (nonchiral) oxide high temperature dwave superconductor and performing local scanning tunnelling microscopy and spectroscopy, we have observed evidence for a pwave triggered superconducting density of states in SLG at temperatures as high as 4.2 K [2]. The nature of the superconducting state is also shown to depend on the underlying facet orientation of the superconductor. 
Tuesday, March 6, 2018 11:51AM  12:03PM 
F13.00002: Observation of topological superconductivity on the surface of ironbased superconductor Peng Zhang, Koichiro Yaji, Takahiro Hashimoto, Yuichi Ota, Takeshi Kondo, Kozo Okazaki, Zhijun Wang, Jinsheng Wen, Genda Gu, Hong Ding, Shik Shin Topological superconductors, whose edge hosts Majorana bound states or Majorana fermions that obey nonAbelian statistics, can be used for lowdecoherence quantum computations. Most of the proposed topological superconductors are realized with spinhelical states through proximity effect to BCS superconductors. However, such approaches are difficult for further studies and applications because of the low transition temperatures and complicated heterostructures. Here by using highresolution spinresolved and angleresolved photoelectron spectroscopy, we discover that the ironbased superconductor FeTe1xSex (x = 0.45, Tc = 14.5 K) hosts Diraccone type spinhelical surface states at Fermi level, which open an swave SC gap below Tc. Our study proves that the surface states of FeTe0.55Se0.45 are 2D topologically superconducting, and thus provides a simple and possibly highTc platform for realizing Majorana fermions. 
Tuesday, March 6, 2018 12:03PM  12:15PM 
F13.00003: ProximityInduced Superconductivity in a Topological Insulator using an Febased Superconductor He Zhao, Bryan Rachmilowitz, Zheng Ren, Ruobin Han, J. A. Schneeloch, Ruidan Zhong, Genda Gu, Ziqiang Wang, Ilija Zeljkovic

Tuesday, March 6, 2018 12:15PM  12:27PM 
F13.00004: Topological Surface State on a Febased High Tc Superconductor Nader Zaki, Jonathan Rameau, Genda Gu, Michael Weinert, Peter Johnson Laserbased ARPES is used to examine FeTe_{0.5}Se_{0.5}. We observe the presence of a Dirac cone with helical spin structure as expected for a topological surface state and as previously reported^{1}. These experimental studies are compared with theoretical studies that take account of the disordered local magnetic effects related to the paramagnetism observed in this system. Including magnetic contributions in the theoretical description of the normal state is necessary to bring the chemical potential of the calculated electronic band structure into alignment with the experimental observations. Our studies performed above the superconducting transition temperature show evidence for a gap around the Dirac point reflecting mass acquisition associated with the breaking of time reversal symmetry. This points to the possibility of some form of surface magnetic order. 
Tuesday, March 6, 2018 12:27PM  12:39PM 
F13.00005: Intrinsic Topological Superconductivity in Ironbased HighTemperature Superconductors Zhiping Yin, Yundi Quan Looking for Majorana fermions and realizing quantum computing have been the forefront of condensed matter physics in recent years. Topological superconductivity plays a key role to this end. However, topological superconductivity is believed to be a rare phenomenon whose realization requires strict conditions. Previous proposals invoke pwave superconductor such as Sr_{2}RuO_{4}, or proximity effect induced by putting topological insulators on top of a superconductor. Another route is to find intrinsic topological superconductors whose presence is still very limited. Here we show that intrinsic topological superconductivity can be realized in a number of ironbased hightemperature superconductors. Using firstprinciples manybodytheory calculations, we find that several ironbased superconductors are nontopological according to band theory but become topological when the strong electronic correlation effects are properly taken into account. Our results provide a new route to achieve and tune topological superconductivity and shed new lights on realizing topological quantum computing. 
Tuesday, March 6, 2018 12:39PM  12:51PM 
F13.00006: Bulk effect and scaling behavior of Majorana Bound States in FeTe_{0.55}Se_{0.45} Lingyuan Kong, Dongfei Wang, Shiyu Zhu, Peng Fan, Hui Chen, Wenyao Liu, Lu Cao, Yujie Sun, Shixuan Du, Hong Ding, H.J. Gao, John Schneeloch, Ruidan Zhong, Genda Gu, Liang Fu Majorana bound states (MBS) can arise as quasiparticle excitations in vortex core of pwave superconductors. Inspired by heterostructure proximity effect, Fu and Kane [1] proposed a realistic way to construct effective pwave conditions with ordinary superconductor, which needs to induce full superconducting gap on nondegenerate Dirac surface states. Recently, we found a new platform (FeTe_{0.55}Se_{0.45} single crystal) supporting pure Majoranatype excitations mimic FuKane Model, which benefit from interband kspace proximity effect and strong correlation of materials. By combining ARPES [2] and STM [3] measurements, we, previously, established topological nontrivial properties and shown strong evidences of existence of MBS on vortex cores. In this talk, we will report temperature and tunneling barrier dependent measurements of MBS in FeTe_{0.55}Se_{0.45}, whose behaviors support Mojorana explanations. [1] L. Fu and C. L. Kane, Phys. Rev. Lett. 100, 096407 (2008) [2] P. Zhang et al., arXiv:1706.05163 (2017) [3] D.F. Wang^{*}, L.Y. Kong^{*}, P. Fan^{*} et al., arXiv:1706.06074 (2017) 
Tuesday, March 6, 2018 12:51PM  1:03PM 
F13.00007: Topological dwave Superconductivity in Cuprate Thin Films Akito Daido, Youichi Yanase Topological superconductivity (TSC) is an intriguing phase of matter. In particular, TSC in noncentrosymmetric (NCS) systems attract much attention. For instance, swave superconductors (SCs) with Rashba spinorbit coupling (SOC) are shown to be TSC under Zeeman field, although centrosymmetric swave SCs are topologically trivial [1]. Further research on NCS TSC may yield novel topological phenomena, and is an urgent issue. 
Tuesday, March 6, 2018 1:03PM  1:15PM 
F13.00008: Evidence for Topological Order in the Pseudogap Metal Mathias Scheurer, Shubhayu Chatterjee, Michel Ferrero, Antoine Georges, Subir Sachdev, Wei Wu The violation of the Luttinger theorem in the absence of translationsymmetry breaking in the pseudogap phase of the holedoped cuprates is a strong indication for the emergence of topological order and fractionalization. We provide further, independent evidence by showing that cluster extensions of dynamical meanfield theory and quantum Monte Carlo calculations on the stronglycorrelated, holedoped Hubbard model on the square lattice admit a natural explanation in terms of fractionalization. We study an effective lowenergy model of fluctuating antiferromagnetism, exhibiting topological order and associated fractionalization of the electron’s spin and charge degrees of freedom, which already captures many essentials features found in our numerical calculations. These features include the momentum, energy, and hopping dependence of the electronic spectral function and selfenergy. 
Tuesday, March 6, 2018 1:15PM  1:27PM 
F13.00009: pwave Superconductivity in Weakly Repulsive 2D Hubbard Model with Zeeman Splitting and Weak Rashba Spinorbit Coupling Henning Hugdal, Asle Sudbo We study the superconducting order in a twodimensional square lattice Hubbard model with weak repulsive interactions, subject to a Zeeman field and weak Rashba spinorbit interactions. Diagonalizing the noninteracting Hamiltonian leads to two separate bands, and by deriving an effective lowenergy interaction we find the mean field gap equations for the superconducting order parameter on the bands. Solving the gap equations just below the critical temperature, we find that superconductivity is caused by the KohnLuttinger type interaction, while the pairing symmetry of the bands are indirectly affected by the spinorbit coupling. The dominating attractive momentum channel depends on the filling fraction n of the system, and the momentum dependence of the order parameter can thus be changed by tuning n. The filling fraction also determines which band has the highest critical temperature. Rotating the magnetic field changes the momentum dependence from states that for small momenta reduce to chiral p_{x}±ip_{y} type states for outofplane fields, to nodal pwave type states for purely inplane fields. 
Tuesday, March 6, 2018 1:27PM  1:39PM 
F13.00010: Signatures of the topological s^{+} superconducting order parameter in the typeII Weyl semimetal T_{d}MoTe_{2} Zurab Guguchia, Fabian von Rohr, Zurab Shermadini, Alex Taekyung Lee, Soham Banerjee, Andrew Wieteska, Chris Marianetti, Benjamin Frandsen, Hubertus Luetkens, Zizhou Gong, Sky Cheung, Chris Baines, Alexander Shengelaya, Grigol Taniashvili, Abhay Pasupathy, Elvezio Morenzoni, Simon Billinge, Alex Amato, Robert Cava, Rustem Khasanov, Yasutomo Uemura In its orthorhombic T_{d} polymorph, MoTe_{2} is a typeII Weyl semimetal, where the Weyl fermions emerge at the boundary between electron and hole pockets. Nonsaturating magnetoresistance (MR) and superconductivity were also observed in T_{d}MoTe_{2}. Understanding the superconductivity in T_{d}MoTe_{2}, which was proposed to be topologically nontrivial, is of eminent interest. I will report highpressure muon spin rotation experiments probing the temperaturedependent magnetic penetration depth in T_{d}MoTe_{2 }[1]. 
Tuesday, March 6, 2018 1:39PM  1:51PM 
F13.00011: The DoubleEdged Sword of Weak Disorder in Topological Superconductivity Arbel Haim, Ady Stern Majorana bound states are zeroenergy modes localized at the ends of a topological superconductor. As long as their localization length is smaller than their spatial separation, they are protected aginst splitting from zero energy. Introducing disorder usually tends to increase the Majorana localization length, until eventually inducing a topological phase transition to a trivial phase. In this work we show that in some cases weak disorder actually causes the Majorana localization length to decrease, making the topological phase more robust. Increaseing the disorder further eventually leads to a change of trend and to a phase transition to a trivial phase, however, interestingly the transition occurs at τ^{1}>> E_{g}, where τ is the disorder meanfree time and E_{g} is the bulk energy gap. Our results are particularly relevant to a recent experiment [S. Hart et al. Nat. Phys. 13, 8793 (2017)] in planar Josephson junctions, a setup predicted to host Majorana bound states [F. Pientka et al. Phys. Rev. X 7, 021032 (2017), M. Hell et al. Phys. Rev. Lett. 118, 107701 (2017)]. 
Tuesday, March 6, 2018 1:51PM  2:03PM 
F13.00012: Intrinsic Topological Superconductors Induced by Textured Magnetic Order: A Versatile Majorana Fermion Platform Daniel Steffensen, Morten Holm Christensen, BRIAN ANDERSEN, Panagiotis Kotetes Systems that inherently exhibit topological superconductivity are rare in nature and the highly coveted Majorana fermions are mainly pursued in engineered hybrid devices. Here we propose to harness the possible microscopic coexistence of superconductivity and magnetism as an alternative pathway to craft intrinsic topological superconductors [1]. We focus on materials with spontaneous textured magnetic order driven by Fermi surface nesting. Our work is motivated by the iron pnictides, in which such a coexistence has been shown experimentally, and a recent theoretical analysis [2] has revealed that textured magnetism is also accessible. We explore the arising topological superconducting phases in layered multiband materials with magnetic spiral, whirl or skyrmion order, coexisting with various types of spinsinglet superconductivity. The diverse magnetic phases lead to a variety of flat, unidirectional and chiral Majorana edge modes. We show that this multifaceted manifestation of Majorana fermion modes stems from the interplay of topological phases with both gapped and nodal bulk energy spectra. [1] P. Kotetes, New J. Phys. 15, 105027 (2013). [2] M. H. Christensen, B. M. Andersen and P. Kotetes, arXiv:1612.07633. 
Tuesday, March 6, 2018 2:03PM  2:15PM 
F13.00013: Localization and Oscillations in Majorana Fermions from dWave Superconductors Laura Ortiz, Santiago Varona, Oscar Viyuela, Miguel MartinDelgado We study the localization and oscillation properties of the Majorana fermions that arise in a twodimensional electron gas (2DEG) with spinorbit coupling (SOC) and a Zeeman field coupled with a dwave superconductor. Despite the angular dependence of the dwave pairing, localization and oscillation properties are found to be similar to the ones seen in conventional swave superconductors. In addition, we study a microscopic lattice version of the previous system that can be characterized by a topological invariant. We derive its real space representation that involves nearest and nexttonearestneighbors pairing. Finally, we show that the emerging chiral Majorana fermions are indeed robust against static disorder. This analysis has potential applications to quantum simulations and experiments in highTc superconductors. 
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