  Here we establish a more complete phase diagram for FeSe system, based on
experimental results of nonstoichiometric Fe1-xSe single crystals that we have
developed recently, as well as nearly stoichiometric FeSe single crystals. The
electronic correlation is found to be strongly enhanced in hole-dominated
Fe1-xSe, as compared with electron-dominated FeSe, from the magnetic
susceptibility and electrical transport measurements in the normal state. A
superconducting dome is found to emerge starting from the strongly correlated
hole-dominated regime with electron doping, while the tetragonal-orthorhombic
phase transition at ~90 K is observed only at higher electron-doping levels in
the electron-dominated regime.
