Flatband-Induced Itinerant Ferromagnetism in RbCo$_2$Se$_2$

TL;DR

This study reveals that RbCo$_2$Se$_2$ exhibits flatband-induced itinerant ferromagnetism, with a dominant $d_{x^2-y^2}$ flatband near the Fermi level, highlighting a common feature in iron-based superconductors.

Contribution

It demonstrates the role of a flatband in inducing ferromagnetism in RbCo$_2$Se$_2$, combining experimental and theoretical approaches to uncover its electronic origin.

Findings

Identification of ferromagnetic order in RbCo$_2$Se$_2$

Observation of a $d_{x^2-y^2}$ flatband near the Fermi level

Flatband exhibits the largest spin splitting, indicating itinerant ferromagnetism

Abstract

$A$Co$_2$Se$_2$ ($A$=K,Rb,Cs) is a homologue of the iron-based superconductor, $A$Fe$_2$Se$_2$. From a comprehensive study of RbCo$_2$Se$_2$ via measurements of magnetization, transport, neutron diffraction, angle-resolved photoemission spectroscopy, and first-principle calculations, we identify a ferromagnetic order accompanied by an orbital-dependent spin-splitting of the electronic dispersions. Furthermore, we identify the ordered moment to be dominated by a $d_{x^2-y^2}$ flatband near the Fermi level, which exhibits the largest spin splitting across the ferromagnetic transition, suggesting an itinerant origin of the ferromagnetism. In the broader context of the iron-based superconductors, we find this $d_{x^2-y^2}$ flatband to be a common feature in the band structures of both iron-chalcogenides and iron-pnictides, accessible via heavy electron doping.