scholarly journals Generalized Anderson’s theorem for superconductors derived from topological insulators

2020 ◽  
Vol 6 (9) ◽  
pp. eaay6502 ◽  
Author(s):  
Lionel Andersen ◽  
Aline Ramires ◽  
Zhiwei Wang ◽  
Thomas Lorenz ◽  
Yoichi Ando
Keyword(s):  
Degrees Of Freedom ◽  
Energy Gap ◽  
Energy Scale ◽  
Cooper Pairs ◽  
Topological Superconductors ◽  
Scattering Rate ◽  
Nonmagnetic Impurities ◽  
Superconducting Energy Gap ◽  
Strong Scattering ◽  
Internal Degrees Of Freedom

A well-known result in unconventional superconductivity is the fragility of nodal superconductors against nonmagnetic impurities. Despite this common wisdom, Bi2Se3-based topological superconductors have recently displayed unusual robustness against disorder. Here, we provide a theoretical framework that naturally explains what protects Cooper pairs from strong scattering in complex superconductors. Our analysis is based on the concept of superconducting fitness and generalizes the famous Anderson’s theorem into superconductors having multiple internal degrees of freedom with simple assumptions such as the Born approximation. For concreteness, we report on the extreme example of the Cux(PbSe)5(BiSe3)6 superconductor. Thermal conductivity measurements down to 50 mK not only give unambiguous evidence for the existence of nodes but also reveal that the energy scale corresponding to the scattering rate is orders of magnitude larger than the superconducting energy gap. This provides the most spectacular case of the generalized Anderson’s theorem protecting a nodal superconductor.

scholarly journals Conversion of a conventional superconductor into a topological superconductor by topological proximity effect

2020 ◽  
Vol 11 (1) ◽  
Author(s):  
C. X. Trang ◽  
N. Shimamura ◽  
K. Nakayama ◽  
S. Souma ◽  
K. Sugawara ◽  
...  
Keyword(s):  
Proximity Effect ◽  
Energy Gap ◽  
Majorana Fermions ◽  
Cooper Pairs ◽  
Superconducting Transition ◽  
Topological Superconductors ◽  
Alternative Approach ◽  
Properties Of Materials ◽  
Topological Proximity ◽  
Conventional Superconductor

AbstractRealization of topological superconductors (TSCs) hosting Majorana fermions is a central challenge in condensed-matter physics. One approach is to use the superconducting proximity effect (SPE) in heterostructures, where a topological insulator contacted with a superconductor hosts an effective p-wave pairing by the penetration of Cooper pairs across the interface. However, this approach suffers a difficulty in accessing the topological interface buried deep beneath the surface. Here, we propose an alternative approach to realize topological superconductivity without SPE. In a Pb(111) thin film grown on TlBiSe2, we discover that the Dirac-cone state of substrate TlBiSe2 migrates to the top surface of Pb film and obtains an energy gap below the superconducting transition temperature of Pb. This suggests that a Bardeen-Cooper-Schrieffer superconductor is converted into a TSC by the topological proximity effect. Our discovery opens a route to manipulate topological superconducting properties of materials.

Phase fluctuations in conventional superconductors

2021 ◽  
Author(s):  
Pratap Raychaudhuri ◽  
Surajit Dutta
Keyword(s):  
Energy Gap ◽  
Energy Scale ◽  
Bcs Theory ◽  
Superconducting Properties ◽  
Phase Fluctuations ◽  
Low Dimensions ◽  
Superconducting Energy Gap ◽  
Zero Resistance ◽  
Resistance State ◽  
Granular Superconductors

Abstract Within the Bardeen-Cooper-Schrieffer (BCS) theory, superconductivity is entirely governed by the pairing energy scale, which gives rise to the superconducting energy gap, Δ. However, another important energy scale, the superfluid phase stiffness, J, which determines the resilience of the superconductor to phase-fluctuations is normally ignored. The spectacular success of BCS theory owes to the fact that in conventional superconductors J is normally several orders of magnitude larger than Δ and thus an irrelevant energy scale. However, in certain situations such as in the presence of low carrier density, strong disorder, at low-dimensions or in granular superconductors, J can drastically come down and even become smaller than Δ. In such situations, the temperature and magnetic field evolution of superconducting properties is governed by phase fluctuations, which gives rise to novel electronic states where signatures of electronic pairing continue to exist even when the zero resistance state is destroyed. In this article, we will review the recent experimental developments on the study of phase fluctuations in conventional superconductors.

scholarly journals Josephson Currents and Gap Enhancement in Graph Arrays of Superconductive Islands

Entropy ◽  
2021 ◽  
Vol 23 (7) ◽  
pp. 811
Author(s):  
Massimiliano Lucci ◽  
Davide Cassi ◽  
Vittorio Merlo ◽  
Roberto Russo ◽  
Gaetano Salina ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Energy Gap ◽  
Quantitative Agreement ◽  
Josephson Coupling ◽  
Cooper Pairs ◽  
Coupling Energy ◽  
Qualitative And Quantitative ◽  
Superconducting Energy Gap ◽  
Theoretical Predictions ◽  
New Phase

Evidence is reported that topological effects in graph-shaped arrays of superconducting islands can condition superconducting energy gap and transition temperature. The carriers giving rise to the new phase are couples of electrons (Cooper pairs) which, in the superconducting state, behave as predicted for bosons in our structures. The presented results have been obtained both on star and double comb-shaped arrays and the coupling between the islands is provided by Josephson junctions whose potential can be tuned by external magnetic field or temperature. Our peculiar technique for probing distribution on the islands is such that the hopping of bosons between the different islands occurs because their thermal energy is of the same order of the Josephson coupling energy between the islands. Both for star and double comb graph topologies the results are in qualitative and quantitative agreement with theoretical predictions.

scholarly journals Out-of-equilibrium spin transport in mesoscopic superconductors

2018 ◽  
Vol 376 (2125) ◽  
pp. 20150342 ◽  
Author(s):  
C. H. L. Quay ◽  
M. Aprili
Keyword(s):  
Bound States ◽  
Degrees Of Freedom ◽  
Spin Transport ◽  
Energy Gap ◽  
Particle Density ◽  
Zeeman Splitting ◽  
Quasi Particle ◽  
Particle Spin ◽  
Superconducting Energy Gap ◽  
Mesoscopic Superconductors

The excitations in conventional superconductors, Bogoliubov quasi-particles, are spin- fermions but their charge is energy-dependent and, in fact, zero at the gap edge. Therefore, in superconductors (unlike normal metals) spin and charge degrees of freedom may be separated. In this article, we review spin injection into conventional superconductors and focus on recent experiments on mesoscopic superconductors. We show how quasi-particle spin transport and out-of-equilibrium spin-dependent superconductivity can be triggered using the Zeeman splitting of the quasi-particle density of states in thin-film superconductors with small spin-mixing scattering. Finally, we address the spin dynamics and the feedback of quasi-particle spin imbalances on the amplitude of the superconducting energy gap. This article is part of the theme issue ‘Andreev bound states’.

Superconducting Tunnel Junction Spectrometers for High Resolution Energy Dispersive Spectroscopy

1998 ◽  
Vol 4 (S2) ◽  
pp. 174-175
Author(s):  
Carl A. Mears
Keyword(s):  
High Resolution ◽  
High Efficiency ◽  
Energy Gap ◽  
Energy Dispersive Spectrometer ◽  
Energy Dispersive ◽  
Superconducting Tunnel Junction ◽  
Cooper Pairs ◽  
X Ray ◽  
Superconducting Energy Gap ◽  
The Difference

We have been developing superconducting tunnel junctions (STJs) for use as high-resolution energy dispersive spectrometers. STJ detectors simultaneously offer energy resolution better than 15 eV at 1 keV, count rates in excess of 10,000 counts per second, broad bandwidth and high efficiency. These attributes make them desirable detectors in a variety of applications, including x-ray microanalysis.When an x-ray photon is absorbed in a superconductor, about 60% of its energy is used to break the Cooper pairs that make up the superconducting ground state into excited electron-like and hole-like states called quasiparticles. This process is analogous to the creation of electron-hole pairs in a conventional energy dispersive spectrometer (EDS) based on silicon or germanium. The difference is that the superconducting energy gap Δ is on the order of a few millielectron volts, roughly a factor of 1000 less than the band gap in common semiconductors.

Role of superconducting energy gap in extended BCS-Bose crossover theory

2017 ◽  
Vol 31 (25) ◽  
pp. 1745004 ◽  
Author(s):  
I. Chávez ◽  
L. A. García ◽  
M. de Llano ◽  
M. Grether
Keyword(s):  
Energy Gap ◽  
Einstein Condensation ◽  
Cooper Pairs ◽  
Number Density ◽  
Superconducting Energy Gap ◽  
Special Cases ◽  
Bose Einstein ◽  
Hole Cooper Pairs ◽  
Theory Of Superconductivity

The generalized Bose–Einstein condensation (GBEC) theory of superconductivity (SC) is briefly surveyed. It hinges on three distinct new ingredients: (i) Treatment of Cooper pairs (CPs) as actual bosons since they obey Bose statistics, in contrast to BCS pairs which do not obey Bose commutation relations; (ii) inclusion of two-hole Cooper pairs (2hCPs) on an equal footing with two-electron Cooper pairs (2eCPs), thus making this a complete boson–fermion (BF) model; and (iii) inclusion in the resulting ternary ideal BF gas with particular BF vertex interactions that drive boson formation/disintegration processes. GBEC subsumes as special cases both BCS (having its 50–50 symmetry of both kinds of CPs) and ordinary BEC theories (having no 2hCPs), as well as the now familiar BCS-Bose crossover theory. We extended the crossover theory with the explicit inclusion of 2hCPs and construct a phase diagram of [Formula: see text] versus [Formula: see text], where [Formula: see text] and [Formula: see text] are the critical and Fermi temperatures, [Formula: see text] is the total number density and [Formula: see text] that of unbound electrons at [Formula: see text]. Also, with this extended crossover one can construct the energy gap [Formula: see text] versus [Formula: see text] for some elemental SCs by solving at least two equations numerically: a gap-like and a number equation. In 50–50 symmetry, the energy gap curve agrees quite well with experimental data. But ignoring 2hCPs altogether leads to the gap curve falling substantially below that with 50–50 symmetry which already fits the data quite well, showing that 2hCPs are indispensable to describe SCs.

Reversibly Sampling Conformations and Binding Modes Using Molecular Darting

2020 ◽  
Author(s):  
Samuel C. Gill ◽  
David Mobley
Keyword(s):  
Monte Carlo ◽  
Ligand Binding ◽  
Binding Sites ◽  
Degrees Of Freedom ◽  
Dynamics Simulation ◽  
Hiv Integrase ◽  
Binding Modes ◽  
System A ◽  
Multiple Binding ◽  
Internal Degrees Of Freedom

<div>Sampling multiple binding modes of a ligand in a single molecular dynamics simulation is difficult. A given ligand may have many internal degrees of freedom, along with many different ways it might orient itself a binding site or across several binding sites, all of which might be separated by large energy barriers. We have developed a novel Monte Carlo move called Molecular Darting (MolDarting) to reversibly sample between predefined binding modes of a ligand. Here, we couple this with nonequilibrium candidate Monte Carlo (NCMC) to improve acceptance of moves.</div><div>We apply this technique to a simple dipeptide system, a ligand binding to T4 Lysozyme L99A, and ligand binding to HIV integrase in order to test this new method. We observe significant increases in acceptance compared to uniformly sampling the internal, and rotational/translational degrees of freedom in these systems.</div>

scholarly journals A novel class of translationally invariant spin chains with long-range interactions

2020 ◽  
Vol 2020 (8) ◽  
Author(s):  
B. Basu-Mallick ◽  
F. Finkel ◽  
A. González-López
Keyword(s):  
Long Range ◽  
Degrees Of Freedom ◽  
Spin Chains ◽  
Open Chain ◽  
New Class ◽  
Spin Interactions ◽  
Long Range Interactions ◽  
Spin Reversal ◽  
Twisted Yangian ◽  
Internal Degrees Of Freedom

Abstract We introduce a new class of open, translationally invariant spin chains with long-range interactions depending on both spin permutation and (polarized) spin reversal operators, which includes the Haldane-Shastry chain as a particular degenerate case. The new class is characterized by the fact that the Hamiltonian is invariant under “twisted” translations, combining an ordinary translation with a spin flip at one end of the chain. It includes a remarkable model with elliptic spin-spin interactions, smoothly interpolating between the XXX Heisenberg model with anti-periodic boundary conditions and a new open chain with sites uniformly spaced on a half-circle and interactions inversely proportional to the square of the distance between the spins. We are able to compute in closed form the partition function of the latter chain, thereby obtaining a complete description of its spectrum in terms of a pair of independent su(1|1) and su(m/2) motifs when the number m of internal degrees of freedom is even. This implies that the even m model is invariant under the direct sum of the Yangians Y (gl(1|1)) and Y (gl(0|m/2)). We also analyze several statistical properties of the new chain’s spectrum. In particular, we show that it is highly degenerate, which strongly suggests the existence of an underlying (twisted) Yangian symmetry also for odd m.

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