PEAK EFFECT IN UNTWINNED YBA2CU3O7-DELTA SINGLE-CRYSTALS

For YBa2Cu3O7-delta untwinned single crystals of differing purity a pe ak effect of the critical current density as a function of the tempera ture is observed by transverse a.c. susceptometry at very low frequenc y. Increasing purity shifts the peak effect towards T(c) and globally reduces J(c) to lower values. We prove the 3D regime by testing whethe r or not cutting and reconnecting the flux lines in magnetically induc ed <<force-free configurations>> is possible. The peak effect appears to result from the pre-melt softening of the flux line lattice.