Evidence for s-wave Pairing with Atomic Scale Disorder in the van der Waals Superconductor NaSn2As2

TL;DR

This study demonstrates that NaSn$_2$As$_2$ is a disordered superconductor with s-wave pairing, showing a long penetration depth and fully gapped behavior despite atomic scale disorder, making it ideal for studying quantum phase fluctuations.

Contribution

It provides the first detailed measurement of magnetic penetration depth in NaSn$_2$As$_2$, revealing s-wave pairing in a highly disordered, van der Waals superconductor.

Findings

Long penetration depth of 960 nm indicating strong disorder.

Superfluid density consistent with fully gapped s-wave superconductivity.

Atomic scale disorder linked to valence-skipping states of Sn.

Abstract

The recent discovery of superconductivity in NaSn$_2$As$_2$ with a van der Waals layered structure raises immediate questions on its pairing mechanism and underlying electronic structure. Here, we present measurements of the temperature-dependent magnetic penetration depth $\lambda(T)$ in single crystals of NaSn$_2$As$_2$ down to $\sim40$ mK. We find a very long penetration depth $\lambda (0) = 960$ nm, which is strongly enhanced from the estimate of first-principles calculations. This enhancement comes from a short mean free path $\ell \approx 1.7$ nm, indicating atomic scale disorder possibly associated with the valence-skipping states of Sn. The temperature dependence of superfluid density is fully consistent with the conventional fully gapped s-wave state in the dirty limit. These results suggest that NaSn$_2$As$_2$ is an ideal material to study quantum phase fluctuations in strongly disordered superconductors with its controllable dimensionality.