Peak Effect in Untwinned YBa ₂ Cu ₃ O 7-δ Single Crystals

For YBa 2 Cu 3 O 7-δ untwinned single crystals of differing purity a peak effect of the critical current density as a function of the temperature is observed by transverse a.c. susceptometry at very low frequency. Increasing purity shifts the peak effect towards T c and globally reduces J c to lower values. We prove the 3D regime by testing whether or not cutting and reconnecting the flux lines in magnetically induced "force-free configurations" is possible. The peak effect appears to result from the pre-melt softening of the flux line lattice.