Observation of two distinct dxz/dyz band splittings in FeSe

We report the temperature evolution of the detailed electronic band structure in FeSe single crystals measured by angle-resolved photoemission spectroscopy (ARPES), including the degeneracy removal of the dxz and dyz orbitals at the \textunderscore \textunderscore Gamma/Z and M points, and the orbital-selective hybridization between the dxy and dxz/yz orbitals. The temperature dependences of the splittings at the Gamma/Z and M points are different, indicating that they are controlled by different order parameters. The splitting at the M point is closely related to the structural transition and is attributed to orbital ordering defined on Fe-Fe bonds with a d-wave form in the reciprocal space that breaks the rotational symmetry. In contrast, the band splitting at the \textunderscore \textunderscore Gamma/Z points remains at temperature far above the structural transition. Although the origin of this latter splitting remains unclear, our experimental results exclude the previously proposed ferro-orbital ordering scenario.