Effects of disorder in FeSe: an ab initio study.

Using the coherent-potential approximation, we have studied the effects of excess Fe, Se deficiency and substitutions of S and Te on the Se sub-lattice and Co, Ni and Cu on the Fe sub-lattice in FeSe. Our results show that (i) a small amount of excess Fe substantially disorders the Fe-derived bands while an Se deficiency affects mainly the Se-derived bands, (ii) the substitution of S or Te enhances the possibility of Fermi surface nesting, especially in FeSe(0.5)Te(0.5), in spite of disordering the Se-derived bands and (iii) the electron doping through Co, Ni or Cu disorders the system and pushes down the Fe-derived bands, thereby destroying the possibility of Fermi surface nesting. A comparison of these results with the rigid-band, virtual-crystal and supercell approximations reveals the importance of describing disorder with the coherent-potential approximation.