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.