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.

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 Proximity effect in [Nb(1.5 nm)/Fe(x)]10/Nb(50 nm) superconductor/ferromagnet heterostructures

2020 ◽  
Vol 11 ◽  
pp. 1254-1263
Author(s):  
Yury Khaydukov ◽  
Sabine Pütter ◽  
Laura Guasco ◽  
Roman Morari ◽  
Gideok Kim ◽  
...  
Keyword(s):  
Intermediate State ◽  
Proximity Effect ◽  
Intermediate Phase ◽  
Ferromagnetic Layer ◽  
Residual Resistivity ◽  
Neutron Reflectometry ◽  
Superconducting Transition ◽  
Residual Resistivity Ratio ◽  
Resistivity Ratio ◽  
Weak Ferromagnetic

We have investigated the structural, magnetic and superconduction properties of [Nb(1.5 nm)/Fe(x)]10 superlattices deposited on a thick Nb(50 nm) layer. Our investigation showed that the Nb(50 nm) layer grows epitaxially at 800 °C on the Al2O3(1−102) substrate. Samples grown at this condition possess a high residual resistivity ratio of 15–20. By using neutron reflectometry we show that Fe/Nb superlattices with x < 4 nm form a depth-modulated FeNb alloy with concentration of iron varying between 60% and 90%. This alloy has weak ferromagnetic properties. The proximity of this weak ferromagnetic layer to a thick superconductor leads to an intermediate phase that is characterized by a suppressed but still finite resistance of structure in a temperature interval of about 1 K below the superconducting transition of thick Nb. By increasing the thickness of the Fe layer to x = 4 nm the intermediate phase disappears. We attribute the intermediate state to proximity induced non-homogeneous superconductivity in the structure.

scholarly journals Fidelity in topological superconductors with end Majorana fermions

2013 ◽  
Vol 377 (38) ◽  
pp. 2653-2658 ◽  
Author(s):  
Zhi Wang ◽  
Qi-Feng Liang ◽  
Dao-Xin Yao
Keyword(s):  
Majorana Fermions ◽  
Topological Superconductors

Materials (II)

2021 ◽  
pp. 309-362
Author(s):  
Vladimir Z. Kresin ◽  
Sergei G. Ovchinnikov ◽  
Stuart A. Wolf
Keyword(s):  
High Pressure ◽  
Energy Spectrum ◽  
Energy Gap ◽  
Ambient Pressure ◽  
Record Values ◽  
Tungsten Oxides ◽  
Gap Energy ◽  
Topological Superconductors ◽  
New Family ◽  
Pressure Record

This chapter describes the properties of a number of interesting superconducting materials. The study of phonon-mediated superconductors, such as A-15 materials and MgB2, flourished after the discovery of the high-Tc hydrides. At present, this family displays, under high pressure, record values of Tc close to room temperature. Other interesting systems, such as pnictides, heavy fermions, and ruthenates, with their peculiar interplay of superconductivity and magnetism, are also described. Fe-based superconductors, which were recently discovered, have relatively high Tc at ambient pressure. They display a two-gap energy spectrum. Pairing in intercalated nitrides is mainly provided by acoustic plasmons. Tungsten oxides represent a new family of oxides containing elements other than copper; they form filamentary structures. A special class is formed by topological superconductors; usually their properties are caused by odd-parity pairing. The presence of the states inside of the energy gap make these superconductors similar to topological insulators.

Unrelated BCS-like pairing pseudogap and critical superconducting transition temperature in various cuprate compounds

2018 ◽  
Vol 32 (18) ◽  
pp. 1850195
Author(s):  
S. Dzhumanov ◽  
E. X. Karimboev ◽  
Sh. S. Djumanov
Keyword(s):  
Transition Temperature ◽  
Superconducting Transition Temperature ◽  
Order Parameter ◽  
Cuprate Superconductors ◽  
Energy Gap ◽  
Characteristic Temperature ◽  
Superconducting Order Parameter ◽  
Superconducting Transition ◽  
Gap Formation ◽  
Arpes Data

The smooth evolution of the energy gap observed in the tunneling and angle-resolved photoemission spectra (ARPES) of high-[Formula: see text] cuprates with lowering the temperature from a pseudogap state above the critical temperature [Formula: see text] to a superconducting state below [Formula: see text], has been poorly interpreted as the evidence that the pseudogap must have the same origin as the superconducting order parameter, and therefore, must be related to [Formula: see text]. We argue that such an explanation of the tunneling gap and ARPES data is misleading. We show that the BCS-like energy gap (or pseudogap) opening in the electronic excitation spectrum of underdoped-to-overdoped cuprates at a characteristic temperature [Formula: see text] and the true superconducting order parameter appearing only at [Formula: see text] are unrelated. The superconducting phenomenon in unconventional cuprate superconductors is fundamentally different from the BCS-like pairing of fermionic quasiparticles, and the superconducting transition temperature [Formula: see text] is not determined by the BCS-like gap formation. The unusual superconducting order parameter in these high-[Formula: see text] materials appears at [Formula: see text] and coexists with the BCS-like gap (or pseudogap) below [Formula: see text].

Entropy and heat capacity in the generalized Bose–Einstein condensation theory of superconductors

2019 ◽  
Vol 33 (26) ◽  
pp. 1950311
Author(s):  
L. A. García ◽  
M. de Llano
Keyword(s):  
Heat Capacity ◽  
Energy Gap ◽  
Coupling Parameter ◽  
Bcs Theory ◽  
Einstein Condensation ◽  
Cooper Pairs ◽  
Total Electron ◽  
Bose Einstein Condensation ◽  
Unpaired Electrons ◽  
Bose Einstein

The new generalized Bose–Einstein condensation (GBEC) quantum-statistical theory starts from a noninteracting ternary boson-fermion (BF) gas of two-hole Cooper pairs (2hCPs) along with the usual two-electron Cooper pairs (2eCPs) plus unpaired electrons. Here we obtain the entropy and heat capacity and confirm once again that GBEC contains as a special case the Bardeen–Cooper–Schrieffer (BCS) theory. The energy gap is first calculated and compared with that of BCS theory for different values of a new dimensionless coupling parameter n/n[Formula: see text] where n is the total electron number density and n[Formula: see text] that of unpaired electrons at zero absolute temperature. Then, from the entropy, the heat capacity is calculated. Results compare well with elemental-superconductor data suggesting that 2hCPs are indispensable to describe superconductors (SCs).

scholarly journals Smart meta-superconductor MgB2 constructed by the dopant phase of luminescent nanocomposite

2019 ◽  
Vol 9 (1) ◽  
Author(s):  
Yongbo Li ◽  
Honggang Chen ◽  
Mingzhong Wang ◽  
Longxuan Xu ◽  
Xiaopeng Zhao
Keyword(s):  
Transition Temperature ◽  
Superconducting Transition Temperature ◽  
Ex Situ ◽  
Cooper Pairs ◽  
Superconducting Transition ◽  
Luminescence Characteristic ◽  
Dopant Content ◽  
Strong Luminescence

Abstract On the basis of the idea that the injecting energy will improve the conditions for the formation of Cooper pairs, a smart meta-superconductor (SMSC) was prepared by doping luminescent nanocomposite Y2O3:Eu3+/Ag in MgB2. To improve the superconducting transition temperature (TC) of the MgB2-based superconductor, two types of Y2O3:Eu3+/Ag, which has the strong luminescence characteristic, with different sizes were prepared and marked as m-Y2O3:Eu3+/Ag and n-Y2O3:Eu3+/Ag. MgB2 SMSC was prepared through an ex situ process. Results show that when the dopant content was fixed at 2.0 wt.%, the TC of MgB2 SMSC increased initially then decreased with the increase in the Ag content in the dopant. When the Ag content is 5%, the TC of MgB2 SMSC was 37.2–38.0 K, which was similar to that of pure MgB2. Meanwhile, the TC of MgB2 SMSC doped with n-Y2O3:Eu3+/Ag increased initially then decreased basically with the increase in the content of n-Y2O3:Eu3+/Ag, in which the Ag content is fixed at 5%. The TC of MgB2 SMSC doped with 0.5 wt.% n-Y2O3:Eu3+/Ag was 37.6–38.4 K, which was 0.4 K higher than that of pure MgB2. It is thought that the doping luminescent nanocomposite into the superconductor is a new means to improve the TC of SMSC.

scholarly journals Finite-size effects in metallic superlattice systems

1995 ◽  
Vol 73 (9-10) ◽  
pp. 545-553
Author(s):  
J. Chen ◽  
R. Kobes ◽  
J. Wang
Keyword(s):  
Critical Temperature ◽  
Simple Model ◽  
Proximity Effect ◽  
Size Effects ◽  
Finite Size ◽  
Cooper Pairs ◽  
Superconducting Layer ◽  
Finite Size Effects ◽  
Realistic Form ◽  
Pair Amplitude

Clean metallic superlattice systems composed of alternating layers of superconducting and normal materials are considered, particularly aspects of the proximity effect as it affects the critical temperature. A simple model is used to address the question of when a finite–sized system theoretically approximates well a true infinite superlattice. The methods used in the analysis afford some tests of the approximation used that the pair amplitude of the Cooper pairs is constant over a superconducting region. We also use these methods to construct a model of a single superconducting layer which intends to incorporate a more realistic form of the pair amplitude than a simple constant.

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