Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
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 electron-doped oxide superconductor Invited Speaker: Jason Robinson The Bardeen–Cooper–Schrieffer theory of superconductivity describes the condensation of electron pairs with antiparallel spins in a so-called singlet state with s-wave symmetry. Unconventional superconductivity is predicted at certain superconductor interfaces with non-superconducting materials [1], including magnets, materials with strong spin-orbit coupling such as topological insulator and single-layer graphene (SLG). For the case of SLG, the electron pairs are predicted to stabilize to a p-wave or chiral d-wave symmetry, depending on the position of the Fermi energy with respect to the Dirac point. By placing SLG on an electron-doped (non-chiral) oxide high temperature d-wave superconductor and performing local scanning tunnelling microscopy and spectroscopy, we have observed evidence for a p-wave 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 iron-based 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 non-Abelian statistics, can be used for low-decoherence quantum computations. Most of the proposed topological superconductors are realized with spin-helical 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 hetero-structures. Here by using high-resolution spin-resolved and angle-resolved photoelectron spectroscopy, we discover that the iron-based superconductor FeTe1-xSex (x = 0.45, Tc = 14.5 K) hosts Dirac-cone type spin-helical surface states at Fermi level, which open an s-wave 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 high-Tc platform for realizing Majorana fermions. |
Tuesday, March 6, 2018 12:03PM - 12:15PM |
F13.00003: Proximity-Induced Superconductivity in a Topological Insulator using an Fe-based Superconductor He Zhao, Bryan Rachmilowitz, Zheng Ren, Ruobin Han, J. A. Schneeloch, Ruidan Zhong, Genda Gu, Ziqiang Wang, Ilija Zeljkovic
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Tuesday, March 6, 2018 12:15PM - 12:27PM |
F13.00004: Topological Surface State on a Fe-based High Tc Superconductor Nader Zaki, Jonathan Rameau, Genda Gu, Michael Weinert, Peter Johnson Laser-based ARPES is used to examine FeTe0.5Se0.5. We observe the presence of a Dirac cone with helical spin structure as expected for a topological surface state and as previously reported1. 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 Iron-based High-Temperature 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 p-wave superconductor such as Sr2RuO4, 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 iron-based high-temperature superconductors. Using first-principles many-body-theory calculations, we find that several iron-based superconductors are non-topological 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 FeTe0.55Se0.45 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 quasi-particle excitations in vortex core of p-wave superconductors. Inspired by heterostructure proximity effect, Fu and Kane [1] proposed a realistic way to construct effective p-wave conditions with ordinary superconductor, which needs to induce full superconducting gap on non-degenerate Dirac surface states. Recently, we found a new platform (FeTe0.55Se0.45 single crystal) supporting pure Majorana-type excitations mimic Fu-Kane Model, which benefit from interband k-space proximity effect and strong correlation of materials. By combining ARPES [2] and STM [3] measurements, we, previously, established topological non-trivial 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 FeTe0.55Se0.45, 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 d-wave 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, s-wave superconductors (SCs) with Rashba spin-orbit coupling (SOC) are shown to be TSC under Zeeman field, although centrosymmetric s-wave 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 translation-symmetry breaking in the pseudogap phase of the hole-doped 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 mean-field theory and quantum Monte Carlo calculations on the strongly-correlated, hole-doped Hubbard model on the square lattice admit a natural explanation in terms of fractionalization. We study an effective low-energy 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 self-energy. |
Tuesday, March 6, 2018 1:15PM - 1:27PM |
F13.00009: p-wave Superconductivity in Weakly Repulsive 2D Hubbard Model with Zeeman Splitting and Weak Rashba Spin-orbit Coupling Henning Hugdal, Asle Sudbo We study the superconducting order in a two-dimensional square lattice Hubbard model with weak repulsive interactions, subject to a Zeeman field and weak Rashba spin-orbit interactions. Diagonalizing the non-interacting Hamiltonian leads to two separate bands, and by deriving an effective low-energy 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 Kohn-Luttinger type interaction, while the pairing symmetry of the bands are indirectly affected by the spin-orbit 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 px±ipy type states for out-of-plane fields, to nodal p-wave type states for purely in-plane fields. |
Tuesday, March 6, 2018 1:27PM - 1:39PM |
F13.00010: Signatures of the topological s+- superconducting order parameter in the type-II Weyl semimetal Td-MoTe2 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 Td polymorph, MoTe2 is a type-II Weyl semimetal, where the Weyl fermions emerge at the boundary between electron and hole pockets. Non-saturating magnetoresistance (MR) and superconductivity were also observed in Td-MoTe2. Understanding the superconductivity in Td-MoTe2, which was proposed to be topologically non-trivial, is of eminent interest. I will report high-pressure muon spin rotation experiments probing the temperature-dependent magnetic penetration depth in Td-MoTe2 [1]. |
Tuesday, March 6, 2018 1:39PM - 1:51PM |
F13.00011: The Double-Edged Sword of Weak Disorder in Topological Superconductivity Arbel Haim, Ady Stern Majorana bound states are zero-energy 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>> Eg, where τ is the disorder mean-free time and Eg is the bulk energy gap. Our results are particularly relevant to a recent experiment [S. Hart et al. Nat. Phys. 13, 87-93 (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 spin-singlet 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 d-Wave Superconductors Laura Ortiz, Santiago Varona, Oscar Viyuela, Miguel Martin-Delgado We study the localization and oscillation properties of the Majorana fermions that arise in a two-dimensional electron gas (2DEG) with spin-orbit coupling (SOC) and a Zeeman field coupled with a d-wave superconductor. Despite the angular dependence of the d-wave pairing, localization and oscillation properties are found to be similar to the ones seen in conventional s-wave 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 next-to-nearest-neighbors 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 high-Tc superconductors. |
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