Thickness dependence of band structure of FeSe

Thin film FeSe attracts a great deal of attention thanks to its unusually high-temperature (T_c) superconducting properties. We report results of our density functional theory based calculations of the electronic structures of FeSe consisting of 1 to 5 layers. For the case of monolayer, there is a gap of about 0.5 eV at Γ, the Brillouin zone center. We did not find significant difference between the band structures of higher layered FeSe, rather the characteristics slowly converge to that of bulk FeSe, in which the occupied bands cross the Fermi level overlapping with the conducting bands. We discuss the details of the electronic structure and comparison of the Fermi surfaces and phonon spectra of the thin films to shed light on the high T_c behavior of monolayer FeSe.