Routes to heavy-fermion superconductivity

TL;DR

This paper reviews various microscopic mechanisms proposed for heavy-fermion superconductivity, highlighting experimental findings and the conditions under which different types of quantum critical points relate to superconductivity.

Contribution

It provides a comparative analysis of different heavy-fermion compounds and their quantum critical points, emphasizing the diversity of superconducting mechanisms.

Findings

Superconductivity can originate from valence, quadrupole, or spin fluctuations.

CeCu2Si2 exhibits spin-fluctuation mediated superconductivity near a spin-density-wave quantum critical point.

CeRhIn5 shows signs of superconductivity near an antiferromagnetic quantum critical point.

Abstract

Superconductivity in lanthanide- and actinide-based heavy-fermion metals can have different microscopic origins. Among others, Cooper pair formation based on fluctuations of the valence, of the quadrupole moment or of the spin of the localized 4f/5f shell have been proposed. Spin-fluctuation mediated superconductivity in CeCu2Si2 was demonstrated by inelastic neutron scattering to exist in the vicinity of a spin-density-wave quantum critical point. The isostructural HF compound YbRh2Si2 which is prototypical for a Kondo-breakdown quantum critical point has so far not shown any sign of superconductivity down to approximately 10mK. In contrast, results of de-Haas-van-Alphen experiments by Shishido et al. (J. Phys. Soc. Jpn. 74, 1103 (2005)) suggest superconductivity in CeRhIn5 close to an antiferromagnetic quantum critical point beyond the spin-density-wave type, at which the Kondo effect breaks down. For the related compound CeCoIn5 however, a field-induced quantum critical point of spin-density-wave type is extrapolated to exist inside the superconducting phase.