Competing collinear magnetic structures in superconducting FeSe by first principles quantum Monte Carlo calculations

TL;DR

This study uses advanced quantum Monte Carlo methods to accurately analyze magnetic and structural properties of FeSe under pressure, revealing magnetic configurations linked to superconductivity enhancement.

Contribution

It provides the first high-precision quantum Monte Carlo analysis of FeSe, identifying magnetic configurations and their pressure-dependent behavior related to superconductivity.

Findings

Collinear magnetic configurations are most energetically favorable.

Near 7 GPa, collinear and bicollinear states become nearly degenerate.

Pressure influences magnetic order and charge/orbital coupling.

Abstract

Resolving the interplay between magnetic interactions and structural properties in strongly correlated materials through a quantitatively accurate approach has been a major challenge in condensed matter physics. Here we apply highly accurate first principles quantum Monte Carlo (QMC) techniques to obtain structural and magnetic properties of the iron selenide (FeSe) superconductor under pressure. Where comparable, the computed properties are very close to the experimental values. Of potential ordered magnetic configurations, collinear spin configurations are the most energetically favorable over the explored pressure range. They become nearly degenerate in energy with bicollinear spin orderings at around 7 GPa, when the experimental critical temperature $T_c$ is the highest. On the other hand, ferromagnetic, checkerboard, and staggered dimer configurations become relatively higher in energy as the pressure increases. The behavior under pressure is explained by an accurate analysis of the charge compressibility and the orbital occupation as described by the QMC many-body wave function, which reveals how spin, charge and orbital degrees of freedom are strongly coupled in this compound. This remarkable pressure evolution suggests that stripe-like magnetic fluctuations may be responsible for the enhanced $T_c$ in FeSe and that higher T$_c$ is associated with nearness to a crossover between collinear and bicollinear ordering.