  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.
