Unconventional Superconductivity

Abstract

Conventional superconductivity, as used in this review, refers to electron-phonon coupled superconducting electron-pairs described by BCS theory. Unconventional superconductivity refers to superconductors where the Cooper pairs are not bound together by phonon exchange but instead by exchange of some other kind, e. g. spin fluctuations in a superconductor with magnetic order either coexistent or nearby in the phase diagram. Such unconventional superconductivity has been known experimentally since heavy fermion CeCu2Si2, with its strongly correlated 4f electrons, was discovered to superconduct below 0.6 K in 1979. Since the discovery of unconventional superconductivity in the layered cuprates in 1986, the study of these materials saw Tc jump to 164 K by 1994. Further progress in high temperature superconductivity would be aided by understanding the cause of such unconventional pairing. This review compares the fundamental properties of 9 unconventional superconducting classes of materials - from 4f-electron heavy fermions to organic superconductors to classes where only three known members exist to the cuprates with over 200 examples, with the hope that common features will emerge to help theory explain (and predict!) these phenomena. In addition, three new emerging classes of superconductors (topological, interfacial [e. g. FeSe on SrTiO3], and H2S under high pressure) are briefly covered, even though their conventionality is not yet fully determined.

accepted for publication in Advances in Physics; comparison of 9 unconventional superconducting classes of materials, plus 3 emerging classes of superconductors whose conventionality is still under discussion; 30 figures, 13 tables, 712 references. This version has minor typographical errors fixed

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• Towards high-throughput superconductor discovery via machine learning
• Superconductivity emerging from density-wave-like order in a correlated kagome metal
• Incommensurate Spin Fluctuations in the Spin-triplet Superconductor Candidate UTe${}_2$
• Theory of Cross Phenomena and Their Coefficients Beyond Onsager Theorem
• Quantum geometry in condensed matter
• Comparison of two superconducting phases induced by a magnetic field in UTe2
• Superconductivity in an extreme strange metal
• Ab initio theory of plasmonic superconductivity within the Eliashberg and density-functional formalisms
• Observation of triplet superconductivity in CoSi${}_2$/TiSi${}_2$ heterostructures
• Exceeding the Landau superflow speed limit with topological Bogoliubov Fermi surfaces
• Magic angles and correlations in twisted nodal superconductors
• Observing dynamical phases of BCS superconductors in a cavity QED simulator
• Enhanced triplet superconductivity in next generation ultraclean UTe2
• Current- and field-induced topology in twisted nodal superconductors
• Strong in-plane magnetic field induced reemergent superconductivity in the van der Waals heterointerface of NbSe2 and CrCl3
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• Antiferromagnetism in two-dimensional materials: progress and computational challenges
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• Evidence for unconventional superconductivity in a spinel oxide
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• A Recipe for Flat Bands in Pyrochlore Materials: A Chemist's Perspective
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• Superconductor discovery in the emerging paradigm of Materials Informatics
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• Emergent Topological Semimetal
• A proposal for realizing Majorana fermions without external magnetic field in strongly correlated nanowires
• The Meissner effect in superconductors: emergence versus reductionism
• Theory of $(s+id)$ pairing in mixed-valent correlated metals
• A magnetic excitation linking quasi-1D Chevrel-type selenide and arsenide superconductors
• A Group-Equivariant Autoencoder for Identifying Spontaneously Broken Symmetries
• Algebraic Hastatic Order in One-Dimensional Two-Channel Kondo Lattice
• Twisted Nodal Superconductors
• Optical and Raman selection rules for odd-parity clean superconductors
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• Antiferromagnetism and Stripe Channel Order in the $\mathrm{SU}(N)$-Symmetric Two-Channel Kondo Lattice Model