  Modifications of the electronic bands of thin FeSe films due to oxygen
vacancies in the supporting SrTiO 3 (001) substrate - and the interplay with
spin-orbit coupling, magnetism, and epitaxy - are investigated by
first-principles supercell calculations. Unfolded (k-projected) bands show that
the oxygen vacancies both provide electron doping to the interface FeSe layer
and also have notable effects on the details of the bands around the Fermi
level, including renormalizing the width of the Fe-3d band near the Fermi level
by a factor of about 0.6, and causing a splitting of ~40 meV at the M point for
the checkerboard antiferromagnetic configuration. For an FeSe bilayer, the
modifications to the bands are mainly limited to the interface FeSe layer.
While spin-orbit-coupling induced band splittings of ~30 meV at M for the ideal
FeSe/SrTiO3 (001) interfaces are comparable to the splitting due to oxygen
vacancies, the effects are not simply additive. Calculations and comparison to
our scanning tunneling microscopy images of MBE-grown FeSe films on SrTiO3
(001) suggest that a common defect may be Se bound to an oxygen vacancy at the
interface
