We report density functional calculations of the electronic structure, Fermi surface, phonon spectrum, magnetism, and electron-phonon coupling for the superconducting phase FeSe, as well as the related compounds FeS and FeTe. We find that the Fermi-surface structure of these compounds is very similar to that of the Fe-As based superconductors, with cylindrical electron sections at the zone corner, cylindrical hole surface sections, and depending on the compound, other small hole sections at the zone center. As in the Fe-As based materials, these surfaces are separated by a two-dimensional nesting vector at $(\ensuremath{\pi},\ensuremath{\pi})$. The density of states, nesting, and Fermi-surface size increase, going from FeSe to FeTe. Both FeSe and FeTe show spin-density wave (SDW) ground states, while FeS is close to instability. In a scenario where superconductivity is mediated by spin fluctuations at the SDW nesting vector, the strongest superconductor in this series would be doped FeTe.

Density functional study of FeS, FeSe, and FeTe: Electronic structure, magnetism, phonons, and superconductivity