Lifting of xz/yz orbital degeneracy at the structural transition in detwinned FeSe

We study superconducting FeSe (T-c = 9K) exhibiting the tetragonal-orthorhombic structural transition (T-s similar to 90 K) without any antiferromagnetic ordering, by utilizing angle-resolved photoemission spectroscopy. In the detwinned orthorhombic state, the energy position of the d(yz) orbital band at the Brillouin zone corner is 50 meV higher than that of d(xz), indicating the orbital order similar to the NaFeAs and BaFe2As2 families. Evidence of orbital order also appears in the hole bands at the Brillouin zone center. Precisely measured temperature dependence using strain-free samples shows that the onset of the orbital ordering (To) occurs very close to T-s, thus suggesting that the electronic nematicity above T-s is considerably weaker in FeSe compared to BaFe2As2 family.