Strong enhancement of magnetic order from bulk to stretched monolayer FeSe as Hund\u2019s metals

Chang-Youn Moon

doi:10.1038/s41524-020-00414-3

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 SrTiO3, 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 interorbital 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.

Highlights

Magnetism is one of universal features found in iron-based superconductors (IBS) as superconductivity generally appears in the vicinity of antiferromagnetic (AFM) phase with a specific stripe-type ordering pattern, from which electron pairing mechanisms of the magnetic origin were introduced[1,2,3,4,5,6,7]
Three different materials are considered in this work; namely, LaFeAsO as an archetypal IBS, bulk FeSe, and freestanding ML FeSe tensile-strained to the lattice constant of ML FeSe/SrTiO3, 3.90 Å17
A recent DFT + DMFT study demonstrated that the main effect of defect-free SrTiO3 substrate on the electronic structure of ML FeSe is to increase the Se–Fe–Se angle through increasing the lattice constant of ML FeSe28, and an earlier DFT study suggested a similar conclusion[29]

Summary

Introduction

Magnetism is one of universal features found in iron-based superconductors (IBS) as superconductivity generally appears in the vicinity of antiferromagnetic (AFM) phase with a specific stripe-type ordering pattern, from which electron pairing mechanisms of the magnetic origin were introduced[1,2,3,4,5,6,7]. Nematicity (spontaneous breaking of four-fold rotational symmetry of tetragonal phase), magnetism, and superconductivity in IBS are thought to be closely related[8,9,10,11]. In this context, understanding magnetism can be a starting point to unravel the complex interdependence of these properties. Whether or not ML FeSe/SrTiO3 is close to a magnetic instability is an intriguing question

Methods

Results

Conclusion