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 ∼ 60 mK. We find that λ(T) is consistent with a fully-gapped state but with significant gap anisotropy. The magnitude of the gap minima obtained are 0.16k_BT_cand 0.21k_BT_cfor 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 λ(T=0) which we suggest is caused by spatially suppressed superconductivity.