Lifshitz Transition and Chemical Instabilities in Ba sub(1-x)K sub(x)Fe sub(2)As sub(2) Superconductors

For solid-solution Ba sub(1-x)K sub(x)Fe sub(2)As sub(2) Fermi surface evolution is mapped via Bloch spectral functions calculated using density functional theory implemented in Korringa-Kohn-Rostoker multiple scattering theory with the coherent-potential approximation. Spectral functions reveal electronic dispersion, topology, orbital character, and broadening (electron-lifetime effects) due to chemical disorder. Dissolution of electron cylinders occurs near x ~ 0.9 with a nonuniform, topological (Lifshitz) transition, reducing the interband interactions; yet the dispersion maintains its d sub(xz) or d sub(yz) character. Formation energies indicate alloying at x = 0.35, as observed, and a tendency for segregation on the K-rich (x > 0.6) side, explaining the difficulty of controlling sample quality and the conflicting results between characterized electronic structures. Our results reveal Fermi surface transitions in alloyed samples that influence s super(+ or -) to nodal superconductivity and suggest the origin for deviations of common trends in Fe-based superconductors, such as Bud'ko-Ni-Canfield scaling.