Unconventional fully-gapped superconductivity in the heavy-fermion metal CeCu${}_2$Si${}_2$

Abstract

The heavy-fermion metal CeCu${}_2$Si${}_2$ was the first discovered unconventional, non-phonon-mediated superconductor, and for a long time was believed to exhibit single-band $d$-wave superconductivity, as inferred from various measurements hinting at a nodal gap structure. More recently however, measurements using a range of techniques at very low temperatures ($T⪅0.1$ K) provided evidence for a fully-gapped superconducting order parameter. In this Colloquium, after a brief historical overview we survey the apparently conflicting results of numerous experimental studies on this compound. We then address the different theoretical scenarios which have been applied to understand the particular gap structure, including both isotropic (sign-preserving) and anisotropic two-band $s$-wave superconductivity, as well as an effective two-band $d$-wave model, where the latter can explain the currently available experimental data on CeCu${}_2$Si${}_2$. The lessons from CeCu${}_2$Si${}_2$ are expected to help uncover the Cooper-pair states in other unconventional, fully-gapped superconductors with strongly correlated carriers, and in particular highlight the rich variety of such states enabled by orbital degrees of freedom.

arXiv comment33 pages, 15 figures, 2 tables

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• Pressure-tuned quantum criticality in the locally non-centrosymmetric superconductor CeRh${}_2$As${}_2$
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• Anomalous impurity effect in the heavy-fermion superconductor CeCu2Si2
• Removal of excess iron by annealing processes and emergence of bulk superconductivity in sulfur-substituted FeTe
• Magnetic properties of $R$Rh${}_6$Ge${}_4$ ($R$ = Pr, Nd, Sm, Gd-Er) single crystals
• Gap Anisotropy in Layered Superconductors Due to Rashba and Dresselhaus Spin-Orbit Interactions
• Orientation-dependent superconductivity and electronic structure of the rare-earth metal/KTaO3 interfaces
• Dimensionality tuning of heavy-fermion states in ultrathin CeSi2 films