  Alkali-metal (potassium) adsorption on FeSe thin films with thickness from
two unit cells (UC) to 4-UC on SrTiO3 grown by molecular beam epitaxy is
investigated with a low-temperature scanning tunneling microscope. At
appropriate potassium coverage (0.2-0.3 monolayer), the tunneling spectra of
the films all exhibit a superconducting-like gap larger than 11 meV (five times
the gap value of bulk FeSe), and two distinct features of characteristic phonon
modes at 11 meV and 21 meV. The results reveal the critical role of the
interface enhanced electron-phonon coupling for possible high temperature
superconductivity in the system and is consistent with recent theories. Our
study provides compelling evidence for the conventional pairing mechanism for
this type of heterostructure superconducting systems.
