Enhanced superconductivity due to forward scattering in FeSe thin films on SrTiO3 substrates

We examine the consequences of an electron-phonon ($e$-$ph$) interaction that is strongly peaked in the forward scattering (${\bf q} = 0$) direction in a two-dimensional superconductor. We find that strong forward scattering results in an enhanced Tc that is linearly proportional to the strength of the dimensionless $e$-$ph$ coupling constant $\lambda_m$ in the weak coupling limit. This is in stack contrast to the exponential dependence commonly derived in conventional BCS theory. This interaction also produces distinct replica bands in the single-particle spectral function, similar to those observed in recent angle-resolved photoemission experiments on FeSe monolayers on SrTiO$_3$ and BaTiO$_3$ substrates. By comparing our model to photoemission experiments, we infer an $e$-$ph$ coupling strength that can provide a significant portion of the observed high $T_c$ in these systems. [Reference: arXiv:1507.03967]