  The mechanism of enhanced superconductivity in the one unit-cell (1UC) FeSe
film on a SrTiO3 (STO) substrate has stimulated significant research interest
but remains elusive. Using low-temperature, voltage-gated Raman spectroscopy
and low-temperature valence electron energy loss spectroscopy (VEELS), we
characterize the phonon behavior and interfacial charge transfer in single- and
few-layer FeSe films on STO. Raman measurements reveal ambipolar softening of
the FeSe vibrational modes, mimicking that of the underlying STO substrate. We
attribute this behavior to an interfacial coupling effect of STO on FeSe
lattice dynamics. This interfacial coupling effect is further supported by
local electron effective mass enhancement, which is determined from the
red-shift in the FeSe VEELS spectrum near the FeSe/STO interface. Our work
sheds light on the possible interfacial mechanisms contributing to the enhanced
superconductivity across the FeSe/STO interface and further unveils the
potential of low-temperature gated Raman spectroscopy and VEELS in clarifying a
broad category of quantum materials.
