  Charge transfer and electron-phonon coupling (EPC) are proposed to be two
important constituents associated with enhanced superconductivity in the single
unit cell FeSe films on oxide surfaces. Using high-resolution electron energy
loss spectroscopy combined with first-principles calculations, we have explored
the lattice dynamics of ultrathin FeSe films grown on SrTiO3. We show that,
despite the significant effect from the substrate on the electronic structure
and superconductivity of the system, the FeSe phonons in the films are
unaffected. The energy dispersion and linewidth associated with the Fe- and
Se-derived vibrational modes are thickness- and temperature-independent.
Theoretical calculations indicate the crucial role of antiferromagnetic
correlation in FeSe to reproduce the experimental phonon dispersion.
Importantly, the only detectable change due to the growth of FeSe films is the
broadening of the Fuchs-Kliewer (F-K) phonons associated with the lattice
vibrations of SrTiO$_3$(001) substrate. If EPC plays any role in the
enhancement of film superconductivity, it must be the interfacial coupling
between the electrons in FeSe film and the F-K phonons from substrate rather
than the phonons of FeSe.
