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.

10 pages, 6 figures

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• Unconventional incommensurate epitaxy of superconducting FeSe films on SrTiO${}_3$
• Temperature evolution of the Fermi surface of the FeSe monolayer on SrTiO${}_3$