Flat-Band-Induced Itinerant Ferromagnetism In Rbco2se2

ACo(2)Se(2) (A=K, Rb, Cs) is a homologue of the iron-based superconductor AFe(2)Se(2). From a comprehensive study of RbCo2Se2 via measurements of magnetization, transport, neutron diffraction, angle-resolved photoemission spectroscopy, and first-principles 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(x2-y2) flat band 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(x2-y2) flat band to be a common feature in the band structures of both iron chalcogenides and iron pnictides, accessible via heavy electron doping.