Sign-Reversing Orbital Polarization in the Nematic Phase of FeSe due to the C₂ Symmetry Breaking in the Self-Energy

To understand the nematicity in Fe-based superconductors, nontrivial k dependence of the orbital polarization [ΔExz(k), ΔEyz(k)] in the nematic phase, such as the sign reversal of the orbital splitting between Γ and X, Y points in FeSe, provides significant information. To solve this problem, we study the spontaneous symmetry breaking with respect to the orbital polarization and spin susceptibility self-consistently. In FeSe, due to the sign-reversing orbital order, the hole and electron pockets are elongated along the ky and kx axes, respectively, consistently with experiments. In addition, an electron pocket splits into two Dirac cone Fermi pockets while increasing the orbital polarization. The orbital order in Fe-based superconductors originates from the strong positive feedback between the nematic orbital order and spin susceptibility.