# Full-gap superconductivity robust against disorder in heavy-fermion CeCu2Si2

## TL;DR

This study shows that the superconductor CeCu2Si2 maintains a fully-gapped state despite disorder, providing evidence for an s++-wave pairing mechanism without sign reversal, challenging previous assumptions about its unconventional pairing.

## Contribution

The paper provides bulk evidence that CeCu2Si2 exhibits s++-wave superconductivity, demonstrating robustness against disorder and absence of impurity-induced bound states.

## Key findings

- Superconducting gap in CeCu2Si2 is robust against disorder.
- No low-energy bound states are induced by nonmagnetic impurities.
- Results support s++-wave pairing without sign reversal.

## Abstract

A key aspect of unconventional pairing by the antiferromagnetic spin-fluctuation mechanism is that the superconducting energy gap must have opposite sign on different parts of the Fermi surface. Recent observations of non-nodal gap structure in the heavy-fermion superconductor CeCu$_2$Si$_2$ were then very surprising, given that this material has long been considered a prototypical example of a superconductor where the Cooper pairing is magnetically mediated. Here we present a study of the effect of controlled point defects, introduced by electron irradiation, on the temperature-dependent magnetic penetration depth $\lambda(T)$ in CeCu$_2$Si$_2$. We find that the fully-gapped state is robust against disorder, demonstrating that low-energy bound states, expected for sign-changing gap structures, are not induced by nonmagnetic impurities. This provides bulk evidence for $s_{++}$-wave superconductivity without sign reversal.