Band-Saddle-Point Engineering in Mixed $A$-site $A$V${}_3$Sb${}_5$ Kagome Metals

Abstract

$A$V${}_3$Sb${}_5$ compounds ($A$ = K, Rb, Cs) offer an experimental platform for probing the physics of kagome metals near ideal Van Hove filling. Here, we use optical spectroscopy on $A$V${}_3$Sb${}_5$ with mixed $A$-site compositions to reveal the effect of the alkali metal on the low-energy electronic structure. Supported by density-functional-theory calculations, we identify a band-saddle-point inversion as a chemical consequence of Cs substitution, revealing that $A$-site composition controls the orbital character of Van Hove singularities, Fermi surface topology, and electron-phonon interactions while leaving the kagome layer intact.