Valence fluctuation mediated superconductivity in CeCu2Si2

TL;DR

This paper discusses the unique superconducting state in CeCu2Si2, mediated by valence fluctuations near a charge instability, highlighting unusual features such as coexistence with high residual resistivity and pressure-dependent behaviors.

Contribution

It provides a detailed analysis of valence fluctuation mediated superconductivity and its distinct experimental signatures in CeCu2Si2.

Findings

Superconductivity coexists with high residual resistivity near a charge instability.

Resistive transition widths are large and pressure-dependent.

Specific heat jump shape varies asymmetrically with pressure.

Abstract

It has been proposed that there are two types of superconductivity in CeCu2Si2, mediated by spin fluctuations at ambient pressure, and by critical valence fluctuations around a charge instability at a pressure P_v \simeq 4.5 GPa. We present in detail some of the unusual features of this novel type of superconducting state, including the coexistence of superconductivity and huge residual resistivity of the order of the Ioffe-Regel limit, large and pressure dependent resistive transition widths in a single crystal measured under hydrostatic conditions, asymmetric pressure dependence of the specific heat jump shape, unrelated to the resistivity width, and negative temperature dependence of the normal state resistivity below 10 K at very high pressure.