Electron correlation tuned superconductivity in iron chalcogenide superconductors

The iron chalcogenide superconductors, A$_{\mathrm{x}}$Fe$_{\mathrm{2-y}}$Se$_{\mathrm{2}}$ (A$=$K, Rb, Cs), is an interesting system where superconductivity occurs without the existence of hole Fermi pockets, hence lacking the nesting conditions needed under a spin fluctuation mediated pairing scenario. It is then important to understand the ingredients needed for superconductivity in these materials. It has been shown that sulfur substitution for selenium in this system can continually reduce the T$_{\mathrm{C}}$ from 30K to zero, providing an opportunity for understanding the occurrence of superconductivity in these materials. In this talk, I will present angle-resolved photoemission spectroscopy data on the Rb$_{\mathrm{x}}$Fe$_{\mathrm{2}}$(Se$_{\mathrm{1-y}}$S$_{\mathrm{y}})_{\mathrm{2}}$ series, where we show that electron correlation strength is the crucial parameter that tunes superconductivity in this family.