Doping dependencies of onset temperatures for the pseudogap and superconductive fluctuation in Bi₂Sr₂CaCu₂O_8+δ, studied from both in-plane and out-of-plane magnetoresistance measurements

To investigate the relationship between the pseudogap and superconductivity, we measured both the in-plane (ρ_ab) and out-of-plane (ρ_c) resistivity for oxygen-controlled Bi₂Sr₂CaCu₂O_8+δ single crystals subject to magnetic fields (parallel to the c-axis) of up to 17.5 T. The onset temperature for the superconductive fluctuation, Tscf, is determined by the large positive in-plane magnetoresistance (MR) and negative out-of-plane MR observed near T_c, whereas the pseudogap opening temperature T is determined by the semiconductive upturn of the zero-field ρ_c. Tscf was found to scale roughly as T_c, with a decreasing temperature interval between them upon doping. On the other hand, T starts out much higher than Tscf but decreases monotonically upon doping; finally, at a heavily overdoped state, it is not observed above Tscf. These results imply that the pseudogap is not a simple precursor of superconductivity, but that further study is needed to determine whether or not T exists below Tscf in the heavily overdoped state.