Superconducting gap structure of AV₃Sb₅ (A=K, Rb) probed by magnetic penetration depth measurements

Kagome materials have attracted much attention due to their unique electronic structure including Dirac dispersions, flat bands, and van Hove singularities. In AV₃Sb₅ materials, V atoms form the kagome lattice, and Fermi energy is located near the van Hove singularity of the V d-orbital bands. What is characteristic of these materials is that they show an exotic charge density wave (CDW) order and superconductivity. Although theoretical studies have proposed a close relationship between the CDW and superconducting orders, their superconducting gap structures are not established experimentally. We have already reported a penetration depth study on CsV₃Sb₅, evidencing that anisotropic but fully-gapped superconductivity is realized [1]. In this study, we measured the magnetic penetration depth in KV₃Sb₅ and RbV₃Sb₅. Our results suggest that the gap structures in KV₃Sb₅ and RbV₃Sb₅ are more isotropic than in CsV₃Sb₅. Furthermore, we confirmed the s-wave superconductivity from impurity effects on the superconducting transition temperature and the gap structure. We will discuss the relationship between the anisotropy of the superconducting gap and the CDW order.

[1] M. Roppongi, K. Ishihara, et al., Nat. Commun. 14, 667 (2023).