INFLUENCE OF TWINS ON THE PEAK EFFECT AND VORTEX PINNING IN YBA2CU3OYSINGLE-CRYSTALS

Investigating twinned and detwinned YBa2Cu3O7-delta Single crystals, w e have observed differences in the shape of the peak effect, in the fi eld dependence of the relaxation rate, and in the values of the irreve rsibility field and shielding current. It is shown that twins produce an additional peak or a plateaulike behavior in the magnetization hyst eresis at an intermediate temperature range. A significant smaller val ue of the irreversibility field was found in the detwinned samples. Th e influence of twins on the shielding current j(s) changes with the va riation of the bulk pinning force. Twin boundaries increase j(s) in th e region of small currents. However, for stronger bulk pinning they re duce the current and presumably become channels for easier flux penetr ation. The influence of twin boundaries on pinning is significant only within the trapping angle of about 15 degrees.