Magnetoresistance and plastic dissipation in twinned YBa2Cu3O7-delta samples

Using electrical transport measurements, we studied the Ohmic plastic dissi pation of twinned YBa2Cu3O7-delta samples with the magnetic field aligned a long the c axis. The extracted activation energies U(T,B) were analyzed in the framework of a magnetically induced pinning by twin planes. Vortex-anti vortex interactions were derived from the magnetic image effect induced by the deformation of the surrounding supercurrents. The presence of twin plan es leads to a crossover from a field-independent activation energy (low fie lds) to a B-0.5 ln(B) dependence (high fields). The observation of this two -step response is compared with similar observations on heavy-ion-irradiate d samples. [S0163-1829(99)14425-4].