The electron-phonon interaction with forward scattering peak is a relevant approach to high Tc superconductivity in FeSe films on SrTiO3 and TiO2

TL;DR

This paper applies the electron-phonon interaction with a forward scattering peak theory to high-temperature superconductivity in FeSe films on SrTiO3 and TiO2, providing new insights and correcting previous misconceptions.

Contribution

It demonstrates the relevance of EPI with FSP in explaining high Tc in FeSe films and clarifies the role of isotope effects, impurities, and pairing states within this framework.

Findings

Tc0 is an interplay between pairing potential and FSP width

Gaps around 16 meV match ARPES data

Small oxygen isotope effect supports EPI-FSP relevance

Abstract

The theory of EPI with strong forward scattering peak (FSP)[1-3] is recently applied in [4-5] in studying high Tc(~100 K) in a FeSe grown on SrTiO3 [4] and TiO2 [6]. The EPI is due to a long-range dipolar electric field created by the high-energy oxygen vibrations (E0~90 meV) at the interface [4-5]. Besides obtaining new results we also correct some misleading results from the recent literature. We show that the mean-field critical temperature Tc0 is an interplay between the maximal pairing potential and the FSP-width qc. For Tc0~100 K the gap (G) is G~16 meV in agreement with ARPES experiments. We find that in leading order Tc0 is mass-independent and there is small oxygen isotope effect in next to leading order. In clean systems Tc0 for s-wave and d-wave pairing is degenerate but both are affected by non-magnetic impurities. The non-magnetic impurities are pair-weakening in the s-channel and pair-breaking in the d-channel. The normal state self-energy at the Fermi surface gives rise to the ARPES quasiparticle band at E=0 and to a replica band at Er=-E0(1+l(m))^1/2, respectively. The EPI coupling l(m), which enters the self-energy, is mass-dependent - the fact overlooked in the literature, makes at low energies the slope of the self-energy mass-dependent. The smallness of the oxygen isotope effect in Tc0 and its presence in the self-energy in FeSe films on SrTiO3 and TiO2 is a smoking-gun experiment for the application of the EPI-FSP theory to these systems. The EPI-FSP theory predicts a large number of low-laying pairing states (above the ground state) thus causing internal pair fluctuations. The latter reduce Tc0 additionally, by creating a pseudogap state for Tc<T<Tc0. Possibilities to increase Tc0 by designing novel structures are discussed in the framework of the EPI-FSP theory.