  Observation of replica bands in the ARPES spectra of the single-layer FeSe on
strontium titanate substrate revealed a phonon component among mechanisms
behind high Tc superconductivity there. We study the interaction of the
in-layer FeSe electrons with the electric potential of the longitudinal (LO)
modes at surface of bulk SrTiO3. A two-dimensional system of charges at the
FeSe/SrTiO3 interface includes both the itinerant and immobile electrons. The
latter significantly change the interface characteristics, increasing screening
at the substrate surface and reducing thereby the strength of the electron-LO
phonon interactions. In that follows the dielectric constant serves as a free
parameter and is found from the ARPES data for the replicas. The
two-dimensional Coulomb screening is accounted for in the random phase
approximation. It is shown that the model is applicable within the whole range
of the parameters typical for current experiments. The estimates allow
concluding that the LO-phonon mediated pairing alone cannot account for the
temperatures of the superconducting transition Tc in the single-layer
FeSe/SrTiO3 reported in these experiments. This does not exclude that the
LO-phonons mechanism can become more significant in the differently and better
prepared single layer FeSe films. Available experiments are briefly discussed.
Thus far no data exist on the dependence of Tc on the concentration of
electrons doped into the in-layer FeSe band.
