Ab initio Study of Cross-Interface Electron-Phonon Couplings in FeSe Thin Films on SrTiO\(_3\) and BaTiO\(_3\)

Abstract

We study the electron-phonon coupling strength near the interface of monolayer and bilayer FeSe thin films on SrTiO\(_3\), BaTiO\(_3\), and oxygen-vacant SrTiO\(_3\) substrates, using ab initio methods. The calculated total electron-phonon coupling strength \(\lambda=0.2\text{--}0.3\) cannot account for the high \(T_c\sim 70\) K observed in these systems through the conventional phonon-mediated pairing mechanism. In all of these systems, however, we find that the coupling constant of a polar oxygen branch peaks at \(\mathbf{q}=0\) with negligible coupling elsewhere, while the energy of this mode coincides with the offset energy of the replica bands measured recently by angle-resolved photoemission spectroscopy experiments. But the integrated coupling strength for this mode from our current calculations is still too small to produce the observed high \(T_c\), even through the more efficient pairing mechanism provided by the forward scattering. We arrive at the same qualitative conclusion when considering a checkerboard antiferromagnetic configuration in the Fe layer. In light of the experimental observations of the replica band feature and the relatively high \(T_c\) of FeSe monolayers on polar substrates, our results point towards a cooperative role for the electron-phonon interaction, where the cross-interface interaction acts in conjunction with a purely electronic interaction. We also discuss a few scenarios where the coupling strength obtained here may be enhanced.