Doping Dependencies of Onset Temperatures for the Pseudogap and Superconductive Fluctuation in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$, Studied from both In-Plane and Out-of-Plane Magnetoresistance Measurements

摘要

To investigate the relationship between the pseudogap and superconductivity,we measured both the in-plane ($\rho_{ab}$) and out-of-plane ($\rho_c$)resistivity for oxygen-controlled Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$single crystals subject to magnetic fields (parallel to the $c$ axis) of up to17.5 T. The onset temperature for the superconductive fluctuation, $T_{scf}$,is determined by the large positive in-plane magnetoresistance (MR) andnegative out-of-plane MR observed near $T_c$, whereas the pseudogap openingtemperature $T^*$ is determined by the semiconductive upturn of the zero-field$\rho_c$. $T_{scf}$ was found to scale roughly as $T_c$, with a decreasingtemperature interval between them upon doping. On the other hand, $T^*$ startsout much higher than $T_{scf}$ but decreases monotonically upon doping;finally, at a heavily overdoped state, it is not observed above $T_{scf}$.These results imply that the pseudogap is not a simple precursor ofsuperconductivity, but that further study is needed to determine whether or not$T^*$ exists below $T_{scf}$ in the heavily overdoped state.