Superconducting energy gap structure of CsV₃Sb₅from magnetic penetration depth measurements

Experimental determination of the structure of the superconducting order parameter in the kagome lattice compound CsV₃Sb₅is 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,λ(T), in crystals of CsV₃Sb₅down to∼60mK. We find thatλ(T)is consistent with a fully-gapped state but with significant gap anisotropy. The magnitude of the gap minima are in the range∼0.2-0.3 Tcfor 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 ofλ(T=0)which we suggest is caused by spatially suppressed superconductivity.