Superconducting energy gap structure of CsV 3 Sb 5 from magnetic penetration depth measurements

Experimental determination of the structure of the superconducting order parameter in thekagome lattice compound CsV3Sb5 is an essential step towards understanding the nature of thesuperconducting pairing in this material. Here we report measurements of the temperaturedependence of the in-plane magnetic penetration depth, λ(T), in crystals of CsV3Sb5 down to∼60mK. We find that λ(T) is consistent with a fully-gapped state but with significant gapanisotropy. The magnitude of the gap minima are in the range ∼0.2–0.3 Tc for the measuredsamples, markedly smaller than previous estimates. We discuss different forms of potentialanisotropy and how these can be linked to the V and Sb Fermi surface sheets. We highlight asignificant discrepancy between the calculated and measured values of λ(T = 0) which wesuggest is caused by spatially suppressed superconductivity.