Anisotropic magnetoresistance in optimally doped and overdoped Bi₂Sr₂CaCu₂O_8+x single crystals

We report on measurements of the in-plane and out-of-plane resistivity and the magnetoresistance (MR) of Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+x/ single crystals with different oxygen contents. The in-plane resistivity obeys a metallic-like temperature dependence with positive MR. The out-of-plane resistivity shows an activated behaviour above T/sub c/ with a metallic region at higher temperatures and negative MR. In-plane and out-of-plane measurements were analysed in the framework of a recent model for thermodynamic order parameter fluctuations. Within this model the positive in-plane MR above T/sub c/ is caused by the suppression of the fluctuation conductivity enhancement, whereas the negative out-of-plane MR arises from the reduction of the fluctuation-induced pseudogap in the one-electron density of states by the magnetic field. Using this theoretical background, and considering the MR of normal-state quasiparticles, our experimental data can be explained excellently over a wide temperature range.