Vertex dominated superconductivity in intercalated FeSe

TL;DR

Intercalation of alkali elements in FeSe significantly enhances its superconducting critical temperature by modifying electronic screening and magnetic instabilities, with implications for understanding unconventional superconductivity.

Contribution

This study reveals the link between magnetic instabilities, electronic screening, and superconductivity in intercalated FeSe, highlighting the limited role of single-particle electronic structure alone.

Findings

Superconducting T$_{c}$ increases five-fold with intercalation.

Enhancement in magnetic instabilities correlates with increased pairing vertex.

Intercalation softens charge excitations, indicating possible electron-phonon involvement.

Abstract

Bulk FeSe becomes superconducting below 9\,K, but the critical temperature (T$_{c}$) is enhanced almost universally by a factor of $\sim$4-5 when it is intercalated with alkali elements. How intercalation modifies the structure is known from in-situ X-ray and neutron scattering techniques, but why T$_{c}$ changes so dramatically is not known. Here we show that there is one-to-one correspondence between the enhancement in magnetic instabilities at certain $\bf q$ vectors and superconducting pairing vertex, even while the nuclear spin relaxation rate ${1}/{(T_{1}T)}$ may not reflect this enhancement. Intercalation modifies electronic screening both in the plane and also between layers. We disentangle quantitatively how superconducting pairing vertex gains from each such changes in electronic screening. Intercalated FeSe provides an archetypal example of superconductivity where information derived from the single-particle electronic structure appears to be insufficient to account for the origins of superconductivity, even when they are computed including correlation effects. We show that the five-fold enhancement in T$_{c}$ on intercalation is not sensitive to the exact position of the d$_{xy}$ at $\Gamma$ point, as long as it stays close to E$_{F}$. Finally we show that intercalation also significantly softens the collective charge excitations, suggesting the electron-phonon interaction could play some role in intercalated FeSe.