Fully gapped $d$-wave superconductivity in CeCu$_2$Si$_2$

TL;DR

This paper investigates the pairing symmetry of CeCu$_2$Si$_2$, revealing fully gapped $d+d$ band-mixing superconductivity that reconciles previous conflicting experimental results.

Contribution

It proposes a novel fully gapped $d+d$ band-mixing pairing state for CeCu$_2$Si$_2$, explaining diverse experimental observations.

Findings

London penetration depth indicates fully gapped behavior

Proposed $d+d$ band-mixing state fits superfluid density and specific heat data

Accounts for sign change in the superconducting order parameter

Abstract

The nature of the pairing symmetry of the first heavy fermion superconductor CeCu$_2$Si$_2$ has recently become the subject of controversy. While CeCu$_2$Si$_2$ was generally believed to be a $d$-wave superconductor, recent low temperature specific heat measurements showed evidence for fully gapped superconductivity, contrary to the nodal behavior inferred from earlier results. Here we report London penetration depth measurements, which also reveal fully gapped behavior at very low temperatures. To explain these seemingly conflicting results, we propose a fully gapped $d+d$ band-mixing pairing state for CeCu$_2$Si$_2$, which yields very good fits to both the superfluid density and specific heat, as well as accounting for a sign change of the superconducting order parameter, as previously concluded from inelastic neutron scattering results.