We report a high-resolution angle-resolved photo-emission 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 87K to 58K for x=0.15. With increasing S substitution, we find
smaller splitting between bands with dyz and dxz 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.Comment: Latex, 5 pages, 4 figure
