Bandwidth-controlled metal-superconductor-insulator phase diagram in iron-chalcogenides

Using angle- resolved photoemission spectroscopy, we studied isovalently doped K\textunderscore {\$}x{\$}Fe\textunderscore {\$}\textbraceleft 2$-$y\textbraceright {\$}Se\textunderscore {\$}\textbraceleft 2$-$z\textbraceright {\$}S\textunderscore {\$}z{\$}, Rb\textunderscore {\$}x{\$}Fe\textunderscore {\$}\textbraceleft 2$-$y\textbraceright {\$}Se\textunderscore {\$}\textbraceleft 2$-$z\textbraceright {\$}Te\textunderscore {\$}z{\$} and (Tl,K)\textunderscore {\$}x{\$}Fe\textunderscore {\$}\textbraceleft 2$-$y\textbraceright {\$}Se\textunderscore {\$}\textbraceleft 2$-$z\textbraceright {\$}S\textunderscore {\$}z{\$}, in which the superconducting transition temperature decreases with either positive or negative chemical pressures. The bandwidths of Fe 3d bands in the energy window of [0, -0.5] eV in these materials change systematically with doping: with the decreasing of bandwidth, the ground state evolves from a metal to a superconductor, and eventually to an insulator. This systematic study of electronic structures discovered the correlation-driven insulator state by tuning the bandwidth, which is independent with carrier density. The results also indicate that moderate correlation strength is beneficial to enhance superconductivity.