# Effect of electron correlations on the electronic structure and phase stability of FeSe upon lattice expansion

## TL;DR

This study uses advanced theoretical methods to show how lattice expansion in FeSe causes significant electronic and magnetic changes, including a Lifshitz transition, orbital-selective localization, and altered magnetic correlations, revealing the role of electron correlations.

## Contribution

It provides a detailed theoretical analysis of FeSe's electronic structure and phase stability changes upon lattice expansion using DFT+DMFT, highlighting a Lifshitz transition and orbital-selective effects.

## Key findings

- Lifshitz transition with Fermi surface reconstruction upon lattice expansion
- Orbital-selective mass renormalization, especially in the Fe xy orbital
- Change in magnetic nesting vector and magnetic correlations

## Abstract

We present results of a detailed theoretical study of the electronic, magnetic, and structural properties of the chalcogenide parent system FeSe using a fully charge self-consistent implementation of the density functional theory plus dynamical mean-field theory (DFT+DMFT) method. In particular, we predict a remarkable change of the electronic structure of FeSe which is accompanied by a complete reconstruction of the Fermi surface topology (Lifshitz transition) upon a moderate expansion of the lattice volume. The phase transition results in a change of the in-plane magnetic nesting wave vector from $(\pi,\pi)$ to $(\pi,0)$ and is associated with a transition from itinerant to orbital-selective localized magnetic moments. We attribute this behavior to a correlation-induced shift of the van Hove singularity of the Fe $t_{2}$ bands at the M-point across the Fermi level. Our results reveal a strong orbital-selective renormalization of the effective mass $m^*/m$ of the Fe $3d$ electrons upon expansion. The largest effect occurs in the Fe $xy$ orbital, which gives rise to a non-Fermi-liquid-like behavior above the transition. The behavior of the momentum-resolved magnetic susceptibility $\chi({\bf q})$ demonstrates that magnetic correlations are also characterized by a pronounced orbital selectivity, suggesting a spin-fluctuation origin of the nematic phase of paramagnetic FeSe. We conjecture that the anomalous behavior of FeSe upon expansion is associated with the proximity of the Fe $t_{2}$ van Hove singularity to the Fermi level and the sensitive dependence of its position on external conditions.