Magnetism Driven by Anion Vacancies in Superconducting $\alpha$--FeSe$_{1-x}$

TL;DR

This study uses first principles calculations to reveal how anion vacancies in FeSe$_{1-x}$ induce magnetic clusters and influence its superconducting properties, highlighting the role of magnetism driven by vacancies.

Contribution

It demonstrates that Se vacancies create ferrimagnetic clusters leading to half metallic conduction in FeSe$_{1-x}$, a novel insight into vacancy-induced magnetism in superconductors.

Findings

Se vacancies form ferrimagnetic clusters of Fe atoms.

Ordered x=1/8 alloy exhibits half metallic conduction.

Low energy excitations are antiparamagnon-like with short correlation length.

Abstract

To study the microscopic electronic and magnetic interactions in the substoichiometric iron chalcogenide FeSe$_{1-x}$ which is observed to superconduct at x~1/8 up to $T_c$=27 K, we use first principles methods to study the Se vacancy in this nearly magnetic FeSe system. The vacancy forms a ferrimagnetic cluster of eight Fe atoms, which for the ordered x=1/8 alloy leads to half metallic conduction. Similar magnetic clusters are obtained for FeTe$_{1-x}$ and for BaFe$_2$As$_2$ with an As vacancy, although neither of these are half metallic. Based on fixed spin density results, we suggest the low energy excitations in FeSe$_{1-x}$ are antiparamagnon-like with short correlation length.