NEGATIVE MAGNETORESISTANCE IN THE C-AXIS RESISTIVITY OF BI2SR2CACU2O8-DELTA AND YBA2CU3O6+X

The c-axis resistivity rho(c) in Bi2Sr2CaCu2O8+delta and oxygen-reduced YBa2Cu3O6+x is quite sensitive to a magnetic field H. The magnetoresistance (MR) is large (1% at 14 T and 100 K), weakly anisotropic, and negative over a wide range of temperatures T above the critical temperature T-c. The magnitude of the MR is activated in temperature with a characteristic energy U-L that varies with the oxygen content delta. We interpret the activated form of rho(c) and the negative-MR in terms of a pseudogap Delta that is slightly reduced by H.