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

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 $\sim 0.2-0.3T_{\mathrm{c}}$ 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.