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

摘要

We study superconducting FeSe (T_c = 9 K) exhibiting the tetragonal-orthorhombic structural transition (T_s ～ 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 50meV higher than that of d_(xz), indicating the orbital order similar to the NaFeAs and BaFe_2As_2 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 (T_o) occurs very close to J_s, thus suggesting that the electronic nematicity above T_s is considerably weaker in FeSe compared to BaFe_2As_2 family.