Quantum Chemical Research on the Raise of T_c for F-doped Bi-base Superconductors

The probable position of F replacing O has been discussed and both clusters for Bi-base superconductors of F-doped or not have been calculated using quantum chemical DV-X_α method. The differences of atomic charge and density of state have been studied. It is concluded that the raise of T_c with the doping F is attributed to the increase of O_p holes in CuO plane, the strengthening of Cu-O bond and the shifting of the local σ state to the mobile π state.