Topology of the Fermi surface and band structure near the Fermi level in the Pb-doped Bi₂Sr₂CuO_6+δ superconductor

We report the doping level dependence of the band dispersion and the shape of the Fermi surface for Pb-doped Bi₂Sr₂CuO_6+δ superconductor by use of angle resolved photoemission spectroscopy with extremely high angular and energy resolutions. It is found that the band dispersion of Bi₂Sr₂CuO_6+δ is fairly rigid regardless of its doping level in the over-doped range, and that it shifts toward higher binding energy with decreasing doping. We also found that one of the heavily over-doped samples possesses an electron-like Fermi surface centered at (0,0) point, while less doped samples show a hole-like Fermi surface. This is the first observation of the transformation between electron-like and hole-like Fermi surfaces with varying the doping level in Bi₂Sr₂CuO_6+δ.