Surface States around a Vortex in Topological Superconductors: Intersection of a Surface and a Vortex

Abstract Topological superconductors should be able to provide essential ingredients for quantum computing, but are very challenging to realize. Spin–orbit interaction in iron-based superconductors opens the energy gap between the p-states of pnictogen and d-states of iron very close to the Fermi level, and such p-states have been recently experimentally detected. Density-functional theory predicts existence of topological surface states within this gap in FeTe1−xSex making it an attractive candidate material. Here we use synchrotron-based angle-resolved photoemission spectroscopy and band structure calculations to demonstrate that FeTe1−xSex (x = 0.45) is a superconducting 3D Dirac semimetal hosting type-I and type-II Dirac points and that its electronic structure remains topologically trivial. We show that the inverted band gap in FeTe1−xSex can possibly be realized by further increase of Te content, but strong correlations reduce it to a sub-meV size, making the experimental detection of this gap and corresponding topological surface states very challenging, not to mention exact matching with the Fermi level. On the other hand, the p–d and d–d interactions are responsible for the formation of extremely flat band at the Fermi level pointing to its intimate relation with the mechanism of high-Tc superconductivity in IBS.

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Topological Superconductors and Majorana Fermions

10.1093/oxfordhb/9780198738169.013.6 ◽

2017 ◽

Author(s):

Y.Y. Li ◽

J.F. Jia

Keyword(s):

Quantum State ◽

Topological Insulators ◽

Surface States ◽

Proximity Effects ◽

Majorana Fermions ◽

Topological Superconductors ◽

Atomic Chains ◽

Recent Developments ◽

State Of Matter ◽

Topological Superconductivity

This article discusses recent developments relating to the so-called topological superconductors (TSCs), which have a full pairing gap in the bulk and gapless surface states consisting of Majorana fermions (MFs). It first provides a background on topological superconductivity as a novel quantum state of matter before turning to topological insulators (TIs) and superconducting heterostructures, with particular emphasis on the vortices of such materials and the Majorana mode within a vortex. It also considers proposals for realizing TSCs by proximity effects through TI/SC heterostructures as well as experimental efforts to fabricate artificial TSCs using nanowires, superconducting junctions, and ferromagnetic atomic chains on superconductors.

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Superconducting topological surface states in the noncentrosymmetric bulk superconductor PbTaSe2

Science Advances ◽

10.1126/sciadv.1600894 ◽

2016 ◽

Vol 2 (11) ◽

pp. e1600894 ◽

Cited By ~ 64

Author(s):

Syu-You Guan ◽

Peng-Jen Chen ◽

Ming-Wen Chu ◽

Raman Sankar ◽

Fangcheng Chou ◽

...

Keyword(s):

Bound States ◽

Dirac Point ◽

Fault Tolerant ◽

Bulk Material ◽

Surface States ◽

Atomic Scale ◽

Edge States ◽

Quasi Particle ◽

Topological Superconductors ◽

Topological Surface

The search for topological superconductors (TSCs) is one of the most urgent contemporary problems in condensed matter systems. TSCs are characterized by a full superconducting gap in the bulk and topologically protected gapless surface (or edge) states. Within each vortex core of TSCs, there exists the zero-energy Majorana bound states, which are predicted to exhibit non-Abelian statistics and to form the basis of the fault-tolerant quantum computation. To date, no stoichiometric bulk material exhibits the required topological surface states (TSSs) at the Fermi level (EF) combined with fully gapped bulk superconductivity. We report atomic-scale visualization of the TSSs of the noncentrosymmetric fully gapped superconductor PbTaSe2. Using quasi-particle scattering interference imaging, we find two TSSs with a Dirac point atE≅ 1.0 eV, of which the inner TSS and the partial outer TSS crossEF, on the Pb-terminated surface of this fully gapped superconductor. This discovery reveals PbTaSe2as a promising candidate for TSC.

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Refined symmetry indicators for topological superconductors in all space groups

Science Advances ◽

10.1126/sciadv.aaz8367 ◽

2020 ◽

Vol 6 (18) ◽

pp. eaaz8367 ◽

Cited By ~ 5

Author(s):

Seishiro Ono ◽

Hoi Chun Po ◽

Haruki Watanabe

Keyword(s):

Order Parameter ◽

Surface States ◽

Topological Invariants ◽

Inversion Symmetry ◽

Fermi Surfaces ◽

Space Groups ◽

Guiding Principle ◽

Topological Superconductors ◽

Topological Quantum ◽

Topological Quantum Computation

Topological superconductors are exotic phases of matter featuring robust surface states that could be leveraged for topological quantum computation. A useful guiding principle for the search of topological superconductors is to relate the topological invariants with the behavior of the pairing order parameter on the normal-state Fermi surfaces. The existing formulas, however, become inadequate for the prediction of the recently proposed classes of topological crystalline superconductors. In this work, we advance the theory of symmetry indicators for topological (crystalline) superconductors to cover all space groups. Our main result is the exhaustive computation of the indicator groups for superconductors under a variety of symmetry settings. We further illustrate the power of this approach by analyzing fourfold symmetric superconductors with or without inversion symmetry and show that the indicators can diagnose topological superconductors with surface states of different dimensionalities or dictate gaplessness in the bulk excitation spectrum.

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Helical spin texture of surface states in topological superconductors

New Journal of Physics ◽

10.1088/1367-2630/17/1/013016 ◽

2015 ◽

Vol 17 (1) ◽

pp. 013016 ◽

Cited By ~ 7

Author(s):

P M R Brydon ◽

Andreas P Schnyder ◽

Carsten Timm

Keyword(s):

Surface States ◽

Topological Superconductors ◽

Spin Texture

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Helical Majorana surface states of strongly disordered topological superconductors with time-reversal symmetry

Physical Review B ◽

10.1103/physrevb.91.014202 ◽

2015 ◽

Vol 91 (1) ◽

Cited By ~ 10

Author(s):

Raquel Queiroz ◽

Andreas P. Schnyder

Keyword(s):

Time Reversal ◽

Surface States ◽

Time Reversal Symmetry ◽

Topological Superconductors

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Weyl nodes in periodic structures of superconductors and spin-active materials

Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences ◽

10.1098/rsta.2015.0151 ◽

2018 ◽

Vol 376 (2125) ◽

pp. 20150151

Author(s):

Ahmet Keles ◽

Erhai Zhao

Keyword(s):

Bound States ◽

Periodic Structures ◽

Surface States ◽

Calculated Spectrum ◽

Active Materials ◽

Nodal Structure ◽

Topological Superconductors ◽

Fermi Arcs ◽

Spin Texture ◽

Weyl Fermions

Motivated by recent progress in epitaxial growth of proximity structures of s -wave superconductors (S) and spin-active materials (M), in this paper we show that certain periodic structures of S and M can behave effectively as superconductors with pairs of point nodes, near which the low-energy excitations are Weyl fermions. A simple model, where M is described by a Kronig–Penney potential with both spin–orbit coupling and exchange field, is proposed and solved to obtain the phase diagram of the nodal structure, the spin texture of the Weyl fermions, as well as the zero-energy surface states in the form of open Fermi lines (Fermi arcs). As a second example, a lattice model with alternating layers of S and magnetic Z 2 topological insulators is solved. The calculated spectrum confirms previous predictions of Weyl nodes based on the tunnelling Hamiltonian of Dirac electrons. Our results provide further evidence that periodic structures of S and M are well suited for engineering gapless topological superconductors. This article is part of the theme issue ‘Andreev bound states’.

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Observation of topological superconductivity in a stoichiometric transition metal dichalcogenide 2M-WS2

Nature Communications ◽

10.1038/s41467-021-23076-1 ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

Y. W. Li ◽

H. J. Zheng ◽

Y. Q. Fang ◽

D. Q. Zhang ◽

Y. J. Chen ◽

...

Keyword(s):

Transition Metal ◽

Surface States ◽

Superconducting Gap ◽

Transition Metal Dichalcogenide ◽

Superconducting Transition ◽

Natural Form ◽

Angle Resolved Photoemission Spectroscopy ◽

Topological Superconductors ◽

Device Applications ◽

Fundamental Research

AbstractTopological superconductors (TSCs) are unconventional superconductors with bulk superconducting gap and in-gap Majorana states on the boundary that may be used as topological qubits for quantum computation. Despite their importance in both fundamental research and applications, natural TSCs are very rare. Here, combining state of the art synchrotron and laser-based angle-resolved photoemission spectroscopy, we investigated a stoichiometric transition metal dichalcogenide (TMD), 2M-WS2 with a superconducting transition temperature of 8.8 K (the highest among all TMDs in the natural form up to date) and observed distinctive topological surface states (TSSs). Furthermore, in the superconducting state, we found that the TSSs acquired a nodeless superconducting gap with similar magnitude as that of the bulk states. These discoveries not only evidence 2M-WS2 as an intrinsic TSC without the need of sensitive composition tuning or sophisticated heterostructures fabrication, but also provide an ideal platform for device applications thanks to its van der Waals layered structure.

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