Electronic Structure Properties in the Nematic Phases of FeSe

We investigate the electronic structures of FeSe in the presence of different possible orders and spin-orbit coupling (SOC). It is found that only the ferro-orbital order (FO) and the collinear antiferro-magnetism (C-AFM) can simultaneously induce splittings at Γ and M. Bicollinear antiferro-magnetism (B-AFM) and SOC have very similar band structures on Γ–M near the Fermi level. The temperature T insensitive splitting at Γ and the T-dependent splitting at M observed in recent experiments can be explained by the d-wave bond nematic (dBN) order together with SOC. The recent observed Dirac cones and their T-dependence in FeSe thin films can also be well explained by the dBN order together with the band renormalization. Their thickness- and cobalt-doping-dependent behaviors are the consequences of electron doping and reduction of Se height. All these suggest that the nematic order in the FeSe system is the dBN order.