ARPES Study of the Phase Diagram of Fe$_{1+y}$Te$_{1-x}$Se$_x$

Iron chalcogenide Fe$_{1+y}$Te$_{1-x}$Se$_x$ is a unique member among the iron-based superconductor family due to its simplicity in structure and richness in physics. The end member Fe$_{1+y}$Te has an antiferromagnetic order with Neel temperature $\sim$72K. Substitution of Se for Te suppresses this long-range magnetic order and enters a ``spin-glass'' phase where ($\pi$,0) short-range magnetic order contributes to weak charge carrier localization. Superconductivity emerges by further substitution of Se and suppression of the short-range magnetic order. Here we present ARPES study on this system, providing evidence of the underlying physics in the phase diagram by analyzing electronic structure information. The comparison of iron chalcogenide and other iron-based systems help us identify the governing physics in this new family of superconductors.