Simultaneous resistivity and a.c.-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 were observed around the pressure coinciding with a maximum in the superconducting critical temperature, Tcmax. These anomalies can be linked with an abrupt change of the Ce valence, and suggest a second quantum critical point at a pressure Pv≃4.5 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 Pv in this picture, while other anomalies found around Pv can be qualitatively understood using the same model.
@article{arxiv.cond-mat/0306054,
title = {Signatures of valence fluctuations in CeCu2Si2 under high pressure},
author = {A. T. Holmes and D. Jaccard and K. Miyake},
journal= {arXiv preprint arXiv:cond-mat/0306054},
year = {2007}
}