Tuning orbital-selective correlation effects in superconducting Rb$_{0.75}$Fe$_{1.6}$Se$_{2-z}$S$_z$

We report on terahertz time-domain spectroscopy on superconducting and metallic iron chalcogenides Rb$_{0.75}$Fe$_{1.6}$Se$_{2-z}$S$_z$. The superconducting transition is reduced from $T_c=$ 32 K ($z=0$) to 22 K ($z=1.0$), and finally suppressed ($z=1.4$) by isoelectronic substitution of Se with S. Dielectric constant and optical conductivity exhibit a metal-to-insulator transition associated with an orbital-selective Mott phase. This orbital-selective Mott transition appears at higher temperature $T_{met}$ with increasing sulfur content, identifying sulfur substitution as an efficient parameter to tune orbital-dependent correlation effects in iron-chalcogenide superconductors. The reduced correlations of the $d_{xy}$ charge carriers can account for the suppression of the superconductivity and the pseudogap-like feature between $T_c$ and $T_{met}$ that was observed for $z=0$.