# Fully gapped superconductivity with no sign change in the prototypical heavy-fermion CeCu2Si2

## TL;DR

This study demonstrates that CeCu2Si2, a prototypical heavy-fermion superconductor, exhibits a fully gapped s-wave state without sign change, challenging the conventional view of its pairing mechanism being purely magnetic and unconventional.

## Contribution

It provides experimental evidence that heavy-fermion superconductivity in CeCu2Si2 can be fully gapped and s-wave, even with strong Coulomb repulsions, contradicting previous assumptions of unconventional pairing.

## Key findings

- No gap nodes detected on the Fermi surface.
- Superconductivity persists despite impurity scattering.
- Evidence for a fully gapped s-wave state in CeCu2Si2.

## Abstract

In exotic superconductors including high-$T_c$ copper-oxides, the interactions mediating electron Cooper-pairing are widely considered to have a magnetic rather than the conventional electron-phonon origin. Interest in such exotic pairing was initiated by the 1979 discovery of heavy-fermion superconductivity in CeCu$_2$Si$_2$, which exhibits strong antiferromagnetic fluctuations. A hallmark of unconventional pairing by anisotropic repulsive interactions is that the superconducting energy gap changes sign as a function of the electron momentum, often leading to nodes where the gap goes to zero. Here, we report low-temperature specific heat, thermal conductivity and magnetic penetration depth measurements in CeCu$_2$Si$_2$, demonstrating the absence of gap nodes at any point on the Fermi surface. Moreover, electron-irradiation experiments reveal that the superconductivity survives even when the electron mean free path becomes substantially shorter than the superconducting coherence length. This indicates that superconductivity is robust against impurities, implying that there is no sign change in the gap function. These results show that, contrary to long-standing belief, heavy electrons with extremely strong Coulomb repulsions can condense into a fully-gapped s-wave superconducting state, which has an on-site attractive pairing interaction.