Strong enhancement of magnetic order from bulk to stretched monolayer FeSe as Hund's metals

TL;DR

This study uses advanced theoretical methods to compare magnetic properties of bulk and monolayer FeSe, revealing how Hund's coupling and doping influence magnetic order and its relation to superconductivity.

Contribution

It provides a comprehensive understanding of magnetic order in FeSe, highlighting the role of Hund's coupling, crystal field effects, and doping in different dimensionalities.

Findings

Bulk FeSe's suppressed magnetism is linked to Hund's coupling and crystal field effects.

Expanded monolayer FeSe exhibits strong magnetic order that can be suppressed by electron doping.

Magnetic properties are crucial for understanding superconductivity in FeSe systems.

Abstract

Despite of the importance of magnetism in possible relation to other key properties in iron-based superconductors, its understanding is still far from complete especially for FeSe systems. On one hand, the origin of the absence of magnetic orders in bulk FeSe is yet to be clarified. On the other hand, it is still not clear how close monolayer FeSe on SrTiO$_3$, with the highest transition temperature among iron-based superconductors, is to a magnetic instability. Here we investigate magnetic properties of bulk and monolayer FeSe using dynamical mean-field theory combined with density-functional theory. We find that suppressed magnetic order in bulk FeSe is associated with the reduction of inter-orbital charge fluctuations, an effect of Hund's coupling, enhanced by a larger crystal field splitting. Meanwhile, spatial isolation of Fe atoms in expanded monolayer FeSe leads into a strong magnetic order, which is completely destroyed by a small electron doping. Our work provides a comprehensive understanding of the magnetic order in iron-based superconductors and other general multi-orbital correlated systems as Hund's metals.