Signatures of valence fluctuations in CeCu2Si2 under high pressure

TL;DR

This study investigates how valence fluctuations in CeCu2Si2 under high pressure influence superconductivity, revealing a second quantum critical point linked to valence changes and providing insights into the pairing mechanism.

Contribution

It provides experimental evidence for a second quantum critical point driven by valence fluctuations in CeCu2Si2, supported by an extended Anderson lattice model.

Findings

Identification of anomalies near maximum T_c

Evidence for a valence-driven quantum critical point at ~4.5 GPa

Explanation of T-linear resistivity at P_v

Abstract

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, $T_c^{max}$. These anomalies can be linked with an abrupt change of the Ce valence, and suggest a second quantum critical point at a pressure $P_v \simeq 4.5$ 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 $P_v$ in this picture, while other anomalies found around $P_v$ can be qualitatively understood using the same model.