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

Kagome metals AV3Sb5 (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)3Sb5 as a function of Nb content x, where isovalent Nb substitution causes an enhancement of superconducting transition temperature (Tc) and the reduction of CDW temperature (TCDW). We found that the Nb substitution shifts the Sb-derived electron band at the Γ 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 TCDW. 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.