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Gap Symmetry and Structure of Superconductors

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A special issue of Symmetry (ISSN 2073-8994). This special issue belongs to the section "Chemistry: Symmetry/Asymmetry".

Deadline for manuscript submissions: closed (29 February 2020) | Viewed by 32869

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Special Issue Editor

Prof. Dr. Maxim M. Korshunov

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Guest Editor

Kirensky Institute of Physics, Federal Research Center KSC SB RAS, 660036 Krasnoyarsk, Russia
Interests: unconventional and high-Tc superconductivity; spin fluctuation theory of pairing; impurity scattering; theory of strongly correlated electron systems

Special Issue Information

Dear Colleagues,

Symmetry of the gap is the fundamental property of the superconducting state in a material. It is tightly related to the underlying mechanism of Cooper pairing, and, therefore, knowledge of the symmetry put severe constraints on the theories of superconductivity. Even more information can be gained from the particular structure of the gap. The latter term is used to designate the momentum-dependent variation of an order parameter within a given symmetry class. That is, gaps with the same symmetry may have very different structures, such as s+- and s++ states belonging to the same A1g representation in iron pnictides.

The presence of several electronic orbitals in bands near the Fermi level provides both a rich set of properties and complications in revealing the pairing mechanism. Bright examples are the Fe-based pnictides and chalcogenides, sodium cobaltates, and strontium ruthenates. High-Tc cuprates, while sometimes described within single-band models, are also essentially multiband systems allowing to explore a variety of competing ground states. Important feature of these systems is the 'unconventional', non-s-wave, symmetry of the gap.

This Special Issue of Symmetry is devoted to theories and experiments that predict or reveal the gap symmetry and structure of superconductors. Special emphasis is put on the multiband systems with the unconventional order parameter. The scope includes theories of conventional and exotic mechanisms of pairing, and experimental techniques sensitive to the gap symmetry and structure, e.g., penetration depth, thermal conductivity, ARPES, Andreev spectroscopy, inelastic neutron scattering, quasiparticle interference, and Josephson junctions. Contributions can report both a new research and an overview of recent developments.

Prof. Maxim M. Korshunov
Guest Editor

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Keywords

superconducting gap symmetry and structure
unconventional superconductors
iron pnictides and chalcogenides
high-Tc cuprates
sodium cobaltates
strontium ruthenates
band structure of superconductors
ARPES
STM/STS
neutron scattering
NMR/NQR
Andreev spectroscopy
penetration depth

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Published Papers (5 papers)

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Research

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9 pages, 363 KiB

Open AccessArticle

Self-Consistent Two-Gap Description of MgB₂ Superconductor

by Hyunsoo Kim, Kyuil Cho, Makariy A. Tanatar, Valentin Taufour, Stella K. Kim, Sergey L. Bud’ko, Paul C. Canfield, Vladimir G. Kogan and Ruslan Prozorov

Symmetry 2019, 11(8), 1012; https://doi.org/10.3390/sym11081012 - 6 Aug 2019

Cited by 12 | Viewed by 5126

Abstract

A self-consistent two-gap

γ

-model is used to quantitatively describe several thermodynamic properties of MgB

_{2}

superconductor. The superconducting coupling matrix,

ν_{i j}

, was obtained from the fitting of the superfluid density in the entire superconducting temperature range. Using this input, [...] Read more.

A self-consistent two-gap

γ

-model is used to quantitatively describe several thermodynamic properties of MgB

_{2}

superconductor. The superconducting coupling matrix,

ν_{i j}

, was obtained from the fitting of the superfluid density in the entire superconducting temperature range. Using this input, temperature-dependent superconducting gaps, specific heat, and upper critical fields were calculated with no adjustable parameters and compared with the experimental data as well as with the first-principles calculations. The observed agreement between fit and data shows that

γ

-model provides adequate quantitative description of the two-gap superconductivity in MgB

_{2}

and may serve as a relatively simple and versatile self-consistent description of the thermodynamic quantities in multi-gap superconductors. Full article

(This article belongs to the Special Issue Gap Symmetry and Structure of Superconductors)

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12 pages, 297 KiB

Open AccessArticle

London Penetration Depth as a Test of Order Parameter Symmetry in Sodium Cobaltate Superconductors

by Dmitry M. Dzebisashvili and Andrey B. Klyuchantsev

Symmetry 2019, 11(5), 633; https://doi.org/10.3390/sym11050633 - 5 May 2019

Viewed by 2719

Abstract

Temperature dependence of the magnetic field penetration depth

λ

was calculated for water intercalated sodium cobaltate superconductor Na

_{x}

CoO

_{2} \cdot y

_{2}

O. Assuming that the system is in the chiral d+id–wave superconducting state, it was shown that the shifting [...] Read more.

Temperature dependence of the magnetic field penetration depth

λ

was calculated for water intercalated sodium cobaltate superconductor Na

_{x}

CoO

_{2} \cdot y

_{2}

O. Assuming that the system is in the chiral d+id–wave superconducting state, it was shown that the shifting of the excitation spectrum nodal points off the normal phase Fermi surface due to variation of the sodium content x changes the functional form of the temperature dependence of

λ^{- 2}

from exponential to linear in the low temperatures region. It is argued that this change in the functional form of T–dependence of the

λ^{- 2}

can serve as a proof for the chiral symmetry of the superconducting order parameter in the sodium cobaltate. Full article

(This article belongs to the Special Issue Gap Symmetry and Structure of Superconductors)

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12 pages, 437 KiB

Open AccessArticle

Particle–Hole Transformation in Strongly-Doped Iron-Based Superconductors

by Jose P. Rodriguez

Symmetry 2019, 11(3), 396; https://doi.org/10.3390/sym11030396 - 19 Mar 2019

Cited by 2 | Viewed by 3825

Abstract

An exact particle–hole transformation is discovered in a local-moment model for a single layer of heavily electron-doped FeSe. The model harbors hidden magnetic order between the iron

d_{x z}

and

d_{y z}

orbitals at the wavenumber [...] Read more.

An exact particle–hole transformation is discovered in a local-moment model for a single layer of heavily electron-doped FeSe. The model harbors hidden magnetic order between the iron

d_{x z}

and

d_{y z}

orbitals at the wavenumber

(π, π)

. It potentially is tied to the magnetic resonances about the very same Néel ordering vector that have been recently discovered in intercalated FeSe. Upon electron doping, the local-moment model successfully accounts for the electron-pocket Fermi surfaces observed experimentally at the corner of the two-iron Brillouin zone in electron-doped FeSe, as well as for isotropic Cooper pairs. Application of the particle–hole transformation predicts a surface-layer iron-based superconductor at strong hole doping that exhibits high

T_{c}

, and that shows hole-type Fermi-surface pockets at the center of the two-iron Brillouin zone. Full article

(This article belongs to the Special Issue Gap Symmetry and Structure of Superconductors)

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11 pages, 1221 KiB

Open AccessArticle

Temperature-Dependent s_± ↔ s₊₊ Transitions in the Multiband Model for Fe-Based Superconductors with Impurities

by V. A. Shestakov, M. M. Korshunov and O. V. Dolgov

Symmetry 2018, 10(8), 323; https://doi.org/10.3390/sym10080323 - 6 Aug 2018

Cited by 8 | Viewed by 3173

Abstract

We study the dependence of the superconducting gaps on both the disorder and the temperature within the two-band model for iron-based materials. In the clean limit, the system is in the

s_{\pm}

state with sign-changing gaps. Scattering by nonmagnetic impurities leads to [...] Read more.

We study the dependence of the superconducting gaps on both the disorder and the temperature within the two-band model for iron-based materials. In the clean limit, the system is in the

s_{\pm}

state with sign-changing gaps. Scattering by nonmagnetic impurities leads to the change of the sign of the smaller gap, resulting in a transition from the

s_{\pm}

to the

s_{+ +}

state with the sign-preserving gaps. We show here that the transition is temperature-dependent. Thus, there is a line of

s_{\pm} \to s_{+ +}

transition in the temperature–disorder phase diagram. There is a narrow range of impurity scattering rates, where the disorder-induced

s_{\pm} \to s_{+ +}

transition occurs at low temperatures, but then the low-temperature

s_{+ +}

state transforms back to the

s_{\pm}

state at higher temperatures. With increasing impurity scattering rate, the temperature of such

s_{+ +} \to s_{\pm}

transition shifts to the critical temperature

T_{c}

, and only the

s_{+ +}

state is left for higher amounts of disorder. Full article

(This article belongs to the Special Issue Gap Symmetry and Structure of Superconductors)

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Review

Jump to: Research

73 pages, 8657 KiB

Open AccessFeature PaperEditor’s ChoiceReview

On the Remarkable Superconductivity of FeSe and Its Close Cousins

by Andreas Kreisel, Peter J. Hirschfeld and Brian M. Andersen

Symmetry 2020, 12(9), 1402; https://doi.org/10.3390/sym12091402 - 23 Aug 2020

Cited by 108 | Viewed by 16301

Abstract

Emergent electronic phenomena in iron-based superconductors have been at the forefront of condensed matter physics for more than a decade. Much has been learned about the origins and intertwined roles of ordered phases, including nematicity, magnetism, and superconductivity, in this fascinating class of materials. In recent years, focus has been centered on the peculiar and highly unusual properties of FeSe and its close cousins. This family of materials has attracted considerable attention due to the discovery of unexpected superconducting gap structures, a wide range of superconducting critical temperatures, and evidence for nontrivial band topology, including associated spin-helical surface states and vortex-induced Majorana bound states. Here, we review superconductivity in iron chalcogenide superconductors, including bulk FeSe, doped bulk FeSe, FeTe

_{1 - x}

_{x}

, intercalated FeSe materials, and monolayer FeSe and FeTe

_{1 - x}

_{x}

on SrTiO

_{3}

. We focus on the superconducting properties, including a survey of the relevant experimental studies, and a discussion of the different proposed theoretical pairing scenarios. In the last part of the paper, we review the growing recent evidence for nontrivial topological effects in FeSe-related materials, focusing again on interesting implications for superconductivity. Full article

(This article belongs to the Special Issue Gap Symmetry and Structure of Superconductors)

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