  Using angle-resolved photoemission spectroscopy (ARPES), we studied the
evolution of the electronic structure of Fe1-xCuxSe from x = 0 to 0.10. We
found that the Cu dopant introduces extra electron carriers. The hole bands
near the gamma point are observed to steadily shift downward with increasing
doping and completely sink down below the Fermi level (EF) for x > 0.05.
Meanwhile, the electron pocket near the M point becomes larger but loses the
spectral weight near EF. We also observed that effective mass of the electron
band near the M point increases with doping. Our result explains why
superconductivity disappears and metal insulator transition (MIT) like behavior
occurs upon Cu doping in terms of electronic structure, and provide insight
into emergent magnetic fluctuation in Fe1-xCuxSe.
