High Tc via spin fluctuations from incipient bands: application to monolayers and intercalates of FeSe

TL;DR

This paper explores how spin fluctuations from incipient bands can significantly enhance superconductivity, potentially leading to high critical temperatures in FeSe monolayers and intercalates, with a focus on the interplay of magnetic and phononic effects.

Contribution

It demonstrates that incipient bands contribute to spin-fluctuation pairing and can produce high Tc, highlighting the importance of band positioning and electron-phonon interactions.

Findings

Incipient bands significantly boost spin-fluctuation pairing.

Tc exhibits a dome-like dependence on incipient band position.

Electron-phonon coupling further enhances Tc.

Abstract

We investigate superconductivity in a two-band system with an electron- and hole-like band, where one of the bands is away from the Fermi level (or incipient). We argue that the incipient band contributes significantly to spin-fluctuation pairing in the strong coupling limit where the system is close to a magnetic instability, and can lead to a large Tc. In this case, Tc is limited by a competition between the frequency range of the coupling (set by an isolated paramagnon) and the coupling strength itself, such that a dome-like Tc dependence on the incipient band position is obtained. The coupling of electrons to phonons is found to further enhance Tc. The results are discussed in the context of experiments on monolayers and intercalates of FeSe.