Valence Instability and Superconductivity in Heavy Fermion Systems

TL;DR

This paper explores how valence instability and charge transfer fluctuations in cerium-based heavy fermion compounds influence their superconducting properties, suggesting a common weakly first order valence transition as a key factor.

Contribution

It proposes that weakly first order valence transitions are a general feature in Ce-based heavy fermion systems and links critical valence fluctuations to superconductivity.

Findings

Superconductivity coincides with valence changes in CeCu2Si2 and CeCu2Ge2.

Valence instability may be a common feature in Ce heavy fermion compounds.

Both magnetic and valence fluctuations could mediate pairing.

Abstract

Many cerium-based heavy fermion (HF) compounds have pressure-temperature phase diagrams in which a superconducting region extends far from a magnetic quantum critical point. In at least two compounds, CeCu2Si2 and CeCu2Ge2, an enhancement of the superconducting transition temperature was found to coincide with an abrupt valence change, with strong circumstantial evidence for pairing mediated by critical valence, or charge transfer, fluctuations. This pairing mechanism, and the valence instability, is a consequence of a f-c Coulomb repulsion term U_fc in the hamiltonian. While some non-superconducting Ce compounds show a clear first order valence instability, analogous to the Ce alpha-gamma transition, we argue that a weakly first order valence transition may be a general feature of Ce-based HF systems, and both magnetic and critical valence fluctuations may be responsible for the superconductivity in these systems.