Multichannel superconductivity of monolayer FeSe on SrTiO$_3$: Interplay of spin fluctuations and electron-phonon interaction

TL;DR

This study models how electron-phonon interactions and spin fluctuations influence superconductivity in monolayer FeSe on SrTiO$_3$, revealing that electron-phonon coupling is crucial for high Tc, while spin fluctuations favor d-wave pairing.

Contribution

It presents a comprehensive multichannel Eliashberg theory that simultaneously considers electron-phonon and spin fluctuation effects on superconductivity in monolayer FeSe.

Findings

Electron-phonon interaction is essential for high critical temperature.

Spin fluctuations lead to d-wave pairing symmetry.

Absence of spin fluctuations results in s-wave symmetry.

Abstract

We investigate the effects of electron-phonon coupling, as well as of spin and charge fluctuations on the superconducting state in a single layer of FeSe on SrTiO$_3$ substrate. These three bosonic mediators of Cooper pairing are treated on equal footing in a multichannel, full-bandwidth, multiband, and anisotropic Eliashberg theory of the interacting state. Our self-consistent calculations show that an s-wave symmetry of the superconducting gap is compatible only with a complete absence of spin fluctuations. When spin fluctuations are present, the sign-changing nodeless d-wave pairing symmetry is always obtained, yet the essential ingredient for explaining the gap magnitude and critical temperature is still the interfacial electron-phonon interaction.