  We have performed systematic angle-resolved photoemission spectroscopy
(ARPES) of iron-chalcogenide superconductor FeTe1-xSex to elucidate the
electronic states relevant to the superconductivity. While the Fermi-surface
shape is nearly independent of x, we found that the ARPES spectral line shape
shows prominent x dependence. A broad ARPES spectrum characterized by a small
quasiparticle weight at x = 0, indicative of incoherent electronic states,
becomes progressively sharper with increasing x, and a well-defined
quasiparticle peak appears around x = 0.45 where bulk superconductivity is
realized. The present result suggests the evolution from incoherent to coherent
electronic states and its close relationship to the emergence of
superconductivity.
