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

Abstract

We study superconducting FeSe $({T}_{\mathrm{c}}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}9\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ exhibiting the tetragonal-orthorhombic structural transition $({T}_{\mathrm{s}}\phantom{\rule{0.16em}{0ex}}\ensuremath{\sim}\phantom{\rule{0.16em}{0ex}}90\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ 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 $50\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ higher than that of ${d}_{xz}$, indicating the orbital order similar to the NaFeAs and ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ 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 (${T}_{\mathrm{o}}$) occurs very close to ${T}_{\mathrm{s}}$, thus suggesting that the electronic nematicity above ${T}_{\mathrm{s}}$ is considerably weaker in FeSe compared to ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ family.