  A high-frequency optical phonon mode of SrTiO3 (STO) was found to assist the
high-temperature superconductivity observed recently at the interface between
monolayer FeSe and STO substrate. However, the origin of this mode is not
clear. Through first-principles calculations, we find that there is a novel
polar phonon mode on the surface layers of the STO substrate, which does not
exist in the STO crystals. The oxygen vacancies near the FeSe/STO interface
drives the dispersion of this phonon mode to be flat and lowers its energy,
whereas the charge transfer between STO substrate and FeSe monolayer further
reduces its energy to 81 meV. This energy is in good agreement with the
experimental value fitted by Lee et al. for the phonon mode responsible for the
observed replica band separations and the increased superconducting gap. The
oxygen-vacancy-induced flat and polar phonon mode provides clues for
understanding the origin of high Tc superconductivity at the FeSe/STO
interface.
