  Iron-based chalcogenides are complex superconducting systems in which
orbitally-dependent electronic correlations play an important role. Here, using
high-resolution angle-resolved photoemission spectroscopy, we investigate the
effect of these electronic correlations outside the nematic phase in the
tetragonal phase of superconducting FeSe1-xSx (x = 0; 0:18; 1). With increasing
sulfur substitution, the Fermi velocities increase significantly and the band
renormalizations are suppressed towards a factor of 1.5-2 for FeS. Furthermore,
the chemical pressure leads to an increase in the size of the quasi-two
dimensional Fermi surface, compared with that of FeSe, however, it remains
smaller than the predicted one from first principle calculations for FeS. Our
results show that the isoelectronic substitution is an effective way to tune
electronic correlations in FeSe1-xSx, being weakened for FeS with a lower
superconducting transition temperature. This suggests indirectly that
electronic correlations could help to promote higher-Tc superconductivity in
FeSe.
