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.

33 pages, 15 figures, 2 tables

References in corpus (48)

• Fermi-liquid instabilities at magnetic quantum phase transitions
• Quantum Criticality in Heavy Fermion Metals
• Graphene Bilayers with a Twist
• Quantum criticality
• Heavy Fermions and Quantum Phase Transitions
• Superconductivity on the border of weak itinerant ferromagnetism in UCoGe
• Weak magnetism and non-Fermi liquids near heavy-fermion critical points
• Determination of the phase diagram of the electron doped superconductor Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$
• Hidden Magnetism and Quantum Criticality in the Heavy Fermion Superconductor CeRhIn5
• Structure determination and coexistence of superconductivity and antiferromagnetic order in (Li0.8Fe0.2)OHFeSe
• Signatures of valence fluctuations in CeCu2Si2 under high pressure
• Unconventional Superconductivity in Heavy Fermion UTe2
• Unconventional Superconductivity
• Observation of broken time-reversal symmetry in the heavy fermion superconductor UPt${}_3$
• Magnetically driven superconductivity in CeCu2Si2
• Multiple energy scales at a quantum critical point
• Heavy Fermion Superconductivity in the Quadrupole Ordered State of PrV2Al20
• Field-induced transition from even to odd parity superconductivity in CeRh${}_2$As${}_2$
• Fermi-surface collapse and dynamical scaling near a quantum critical point
• Strong nodeless pairing on separate electron Fermi surface sheets in (Tl, K)Fe${}_{1.78}$Se${}_2$ probed by ARPES
• Andreev Reflection in Heavy-Fermion Superconductors and Order Parameter Symmetry in CeCoIn_5
• Evidence for broken time-reversal symmetry in the superconducting phase of URu${}_2$Si${}_2$
• Emergence of superconductivity in the canonical heavy-electron metal YbRh2Si2
• Incommensurate Spin Fluctuations in the Spin-triplet Superconductor Candidate UTe${}_2$
• Feedback spin resonance in superconducting CeCu${}_2$Si${}_2$ and CeCoIn${}_5$
• Angle-resolved photoemission evidence of s-wave superconducting gap in KxFe2-ySe2 superconductor
• Low-dimensional antiferromagnetic fluctuations in the heavy-fermion paramagnetic ladder UTe${}_2$
• Resonance from antiferromagnetic spin fluctuations for superconductivity in UTe${}_2$
• Hidden order and beyond: an experimental-theoretical overview of the multifaceted behavior of URu${}_2$Si${}_2$
• Superconducting Properties of the $s^{\pm }$-wave state: Fe-based superconductors
• Unconventional superconductors under rotating magnetic field I: density of states and specific heat
• Emergent loop-nodal s${}_{\pm }$-wave superconductivity in CeCu${}_2$Si${}_2$: similarities to the iron-based superconductors
• Superconductivity in an extreme strange metal
• Superconductivity in Ce- and U-based "122" heavy-fermion compounds
• Anisotropic c-f hybridization in the ferromagnetic quantum critical metal CeRh${}_6$Ge${}_4$
• Thermal conductivity evidence for d_{x^2-y^2} pairing symmetry in the heavy-fermion CeIrIn5 superconductor
• Robust Upward Dispersion of the Neutron Spin Resonance in the Heavy Fermion Superconductor Ce${}_{1-x}$Yb${}_x$CoIn${}_5$
• Observation of an isotropic superconducting gap at the Brillouin zone center of Tl${}_{0.63}$K${}_{0.37}$Fe${}_{1.78}$Se${}_2$
• Thermodynamic study of gap structure and pair-breaking effect by magnetic field in the heavy-fermion superconductor CeCu2Si2
• Revealing the Heavy Quasiparticles in the Heavy-Fermion Superconductor CeCu2Si2
• Pressure Tuning of the Interplay of Magnetism and Superconductivity in CeCu2Si2
• Nature of the spin resonance mode in CeCoIn${}_5$
• Hexadecapole fluctuation mechanism for s-wave heavy fermion superconductor CeCu2Si2: Interplay between intra- and inter-orbital Cooper pairs
• Possible multi-orbital ground state in CeCu${}_2$Si${}_2$
• Magnetic and superconducting properties on S-type single-crystal CeCu${}_2$Si${}_2$ probed by ${}^{63}$Cu nuclear magnetic resonance and nuclear quadrupole resonance
• Inversion of specific heat oscillations with in-plane magnetic field angle in 2D d-wave superconductors
• High-energy magnetic excitations from heavy quasiparticles in CeCu${}_2$Si${}_2$
• Effective pairing theory for strongly correlated d-wave superconductors

Cited by in corpus (10)

• Fermi Surface Nesting with Heavy Quasiparticles in the Locally Noncentrosymmetric Superconductor CeRh${}_2$As${}_2$
• Pressure-tuned quantum criticality in the locally non-centrosymmetric superconductor CeRh${}_2$As${}_2$
• Investigation of spin excitations and charge order in bulk crystals of the infinite-layer nickelate LaNiO${}_2$
• Emergent ferromagnetic ladder excitations in heavy fermion superconductor CeSb${}_2$
• 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