Fermiology and Origin of Tc Enhancement in a Kagome Superconductor Cs(V1-xNbx)3Sb5

Kagome metals AV(3)Sb(5) (A = K, Rb, and Cs) exhibit a characteristic superconducting ground state coexisting with a charge density wave (CDW), whereas the mechanisms of the superconductivity and CDW have yet to be clarified. Here we report a systematic angle-resolved photoemission spectroscopy (ARPES) study of Cs(V1-xNbx)(3)Sb-5 as a function of Nb content x, where isovalent Nb substitution causes an enhancement of superconducting transition temperature (T-c) and the reduction of CDW temperature (T-CDW). We found that the Nb substitution shifts the Sb-derived electron band at the Gamma point downward and simultaneously moves the V-derived band around the M point upward to lift up the saddle point (SP) away from the Fermi level, leading to the reduction of the CDW-gap magnitude and T-CDW. This indicates a primary role of the SP density of states to stabilize the CDW. The present result also suggests that the enhancement of superconductivity by Nb substitution is caused by the cooperation between the expansion of the Sb-derived electron pocket and the recovery of the V-derived density of states at the Fermi level.

Automated summary

The study found that Nb substitution can increase the superconducting transition temperature and reduce the CDW temperature in Cs(V1-xNbx)(3)Sb-5. Nb substitution adjusts the position of the bands in the band structure, resulting in a decrease in the CDW gap and T-CDW. This study reveals the important role of the SP density of states in stabilizing the CDW, and suggests that the enhancement of superconductivity by Nb substitution is achieved through the expansion of the electron pocket and the recovery of the density of states at the Fermi level.