For yB% Cu, O7e' 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, and globally reduces J, 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.

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