  The observation of substantially enhanced superconductivity of single-layer
FeSe films on SrTiO3 has stimulated intensive research interest. At present,
conclusive experimental data on the corresponding electron-boson interaction is
still missing. Here we use inelastic electron scattering spectroscopy and angle
resolved photoemission spectroscopy to show that the electrons in these systems
are dressed by the strongly polarized lattice distortions of the SrTiO3, and
the indispensable non-adiabatic nature of such a coupling leads to the
formation of dynamic interfacial polarons. Furthermore, the collective motion
of the polarons results in a polaronic plasmon mode, which is unambiguously
correlated with the surface phonons of SrTiO3 in the presence of the FeSe
films. A microscopic model is developed showing that the interfacial
polaron-polaron interaction leads to the superconductivity enhancement.
