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

$A$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.