Doping Dependencies of Onset Temperatures for the Pseudogap and Superconductive Fluctuation in Bi2Sr2CaCu2O8+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 (ρab) and out-of-plane (ρc) resistivity for oxygen-controlled Bi2Sr2CaCu2O8+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 Tc, whereas the pseudogap opening temperature T* is determined by the semiconductive upturn of the zero-field ρc. Tscf was found to scale roughly as Tc, 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 overdo...