Superconducting Energy Gap Structure of CsV$_3$Sb$_5$ from Magnetic Penetration Depth Measurements

TL;DR

This study measures the magnetic penetration depth in CsV$_3$Sb$_5$, revealing a fully-gapped but anisotropic superconducting state with smaller gap minima than previously thought, highlighting complex gap structures.

Contribution

First direct measurement of the temperature dependence of the magnetic penetration depth in CsV$_3$Sb$_5$, revealing detailed gap anisotropy and discrepancies with theoretical predictions.

Findings

Superconducting gap is fully gapped but anisotropic.

Gap minima are approximately 0.2-0.3 T_c, smaller than previous estimates.

Discrepancy between calculated and measured penetration depth suggests suppressed superconductivity.

Abstract

Experimental determination of the structure of the superconducting order parameter in the kagome lattice compound CsV$_3$Sb$_5$ is an essential step towards understanding the nature of the superconducting pairing in this material. Here we report measurements of the temperature dependence of the in-plane magnetic penetration depth, $\lambda(T)$, in crystals of CsV$_3$Sb$_5$ down to $\sim 60\,\mathrm{mK}$. We find that $\lambda(T)$ is consistent with a fully-gapped state but with significant gap anisotropy. The magnitude of the gap minima are in the range $\sim 0.2 - 0.3 T_\mathrm{c}$ for the measured samples, markedly smaller than previous estimates. We discuss different forms of potential anisotropy and how these can be linked to the V and Sb Fermi surface sheets. We highlight a significant discrepancy between the calculated and measured values of $\lambda(T=0)$ which we suggest is caused by spatially suppressed superconductivity.