How correlated is the FeSe/SrTiO$_3$ system ?

TL;DR

This study investigates the electronic correlations and structural effects in monolayer FeSe on SrTiO$_3$, revealing how correlations influence geometry and electronic properties, and how doping affects these correlations, which may explain the high critical temperature.

Contribution

The paper combines density functional theory and dynamical mean field theory to analyze the correlation effects and structural sensitivities in monolayer FeSe on SrTiO$_3$, highlighting the role of Hund's coupling and doping.

Findings

Electronic correlations significantly affect atomic geometry and electronic structure.

Correlation strength depends more on Hund's J than Hubbard U.

Electron doping enhances correlations, especially in the $d_{xy}$ orbital.

Abstract

Recent observation of $\sim$ 10 times higher critical temperature in FeSe monolayer compared with its bulk phase has drawn a great deal of attention because the electronic structure in the monolayer phase appears to be different than bulk FeSe. Using a combination of density functional theory and dynamical mean field theory, we find electronic correlations have important effects on the predicted atomic-scale geometry and the electronic structure of the monolayer FeSe on SrTiO$_3$. The electronic correlations are dominantly controlled by the Se-Fe-Se angle either in the bulk phase or the monolayer phase. But the angle sensitivity increases and the orbital differentiation decreases in the monolayer phase compared to the bulk phase. The correlations are more dependent on Hund's J than Hubbard U. The observed orbital selective incoherence to coherence crossover with temperature confirms the Hund's metallic nature of the monolayer FeSe. We also find electron doping by oxygen vacancies in SrTiO$_3$ increases the correlation strength, especially in the $d_{xy}$ orbital by reducing the Se-Fe-Se angle.