We study superconducting FeSe (Tc = 9 K) exhibiting thetetragonal-orthorhombic structural transition (Ts = 90 K) without anyantiferromagnetic ordering, by utilizing angle-resolved photoemissionspectroscopy. In the detwinned orthorhombic state, the energy position of thedyz orbital band at the Brillouin zone corner is 50 meV higher than that ofdxz, indicating the orbital order similar to NaFeAs and BaFe2As2 families.Evidence of orbital order also appears in the hole bands at the Brillouin zonecenter. Precisely measured temperature dependence using strain-free samplesshows that the onset of the orbital ordering (To) occurs very close to Ts, thussuggesting that the electronic nematicity above Ts is considerably weaker inFeSe compared to BaFe2As2 family.
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