Momentum-dependent sign inversion of orbital order in superconducting FeSe

We investigate the electronic reconstruction across the tetragonal-orthorhombic structural transition in FeSe by employing polarization-dependent angle-resolved photoemission spectroscopy on detwinned single crystals. Across the structural transition, the electronic structures around the Γ and M points are modified from fourfold to twofold symmetry due to the lifting of degeneracy in dxz/dyz orbitals. The dxz band shifts upward at the Γ point, while it moves downward at the M point, suggesting that the electronic structure of orthorhombic FeSe is characterized by a momentum-dependent sign-changing orbital polarization. Due to this sign-changing orbital order, the elongated directions of the elliptical Fermi surfaces at the Γ and M points are rotated by 90° with respect to each other, which makes the nesting condition between these FSs imperfect. The present result, supported by calculations, indicates the possible suppression of the spin-fluctuation mediated superconductivity in the orthorhombic FeSe, as compared to the orbital-ordered state without sign change.