Signatures of valence fluctuations in CeCu2Si2 under high pressure

Simultaneous resistivity and ac specific heat measurements have been performed under pressure on single-crystalline CeCu2Si2 to over 6 GPa in a hydrostatic helium pressure medium. A series of anomalies was observed around the pressure coinciding with a maximum in the superconducting critical temperature, T-c(max). These anomalies can be linked with an abrupt change of the Ce valence and suggest a second quantum critical point at a pressure P(v)similar or equal to4.5 GPa, where critical valence fluctuations provide the superconducting pairing mechanism, as opposed to spin fluctuations at ambient pressure. Such a valence instability-and associated superconductivity-is predicted by an extended Anderson lattice model with Coulomb repulsion between the conduction and f electrons. We explain the T-linear resistivity found at P-v in this picture, while other anomalies found around P-v can be qualitatively understood using the same model.