We report a high-resolution angle-resolved photoemission spectroscopy study of the evolution of the electronic structure of FeSe1-xSx single crystals. Isovalent S substitution onto the Se site constitutes a chemical pressure which subtly modifies the electronic structure of FeSe at high temperatures and induces a suppression of the tetragonal-symmetry-breaking structural transition temperature from 87 to 58 K for x = 0.15. With increasing S substitution, we find smaller splitting between bands with d(yz) and d(xz) orbital character and weaker anisotropic distortions of the low-temperature Fermi surfaces. These effects evolve systematically as a function of both S substitution and temperature, providing strong evidence that an orbital ordering is the underlying order parameter of the structural transition in FeSe1-xSx. Finally, we detect the small inner hole pocket for x = 0.12, which is pushed below the Fermi level in the orbitally ordered low-temperature Fermi surface of FeSe.
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