  We present a systematic angle-resolved photoemission spectroscopy study of
the substitution-dependence of the electronic structure of
Rb$_{0.8}$Fe$_{2}$(Se$_{1-z}$S$_z$)$_2$ (z = 0, 0.5, 1), where
superconductivity is continuously suppressed into a metallic phase. Going from
the non-superconducting Rb$_{0.8}$Fe$_{2}$(Se$_{1-z}$S$_z$)$_2$ to
superconducting Rb$_{0.8}$Fe$_{2}$Se$_2$, we observe little change of the Fermi
surface topology, but a reduction of the overall bandwidth by a factor of 2 as
well as an increase of the orbital-dependent renormalization in the $d_{xy}$
orbital. Hence for these heavily electron-doped iron chalcogenides, we have
identified electron correlation as explicitly manifested in the quasiparticle
bandwidth to be the important tuning parameter for superconductivity, and that
moderate correlation is essential to achieving high $T_C$.
