PEAK EFFECT AS A PRECURSOR TO VORTEX LATTICE MELTING IN SINGLE-CRYSTAL YBA2CU3O7-DELTA

We present direct evidence of a ''peak effect'' as a precursor to vort ex lattice melting in a single crystal of YBa2CU3O7-delta containing o nly two twin planes. The peak effect occurs just below the vortex liqu id to solid phase transition in a narrow field region between 0.3 and 1.5 T for H parallel to c and is clearly observed as a function of bot h magnetic field and temperature. We demonstrate that the peak effect is due to enhanced vortex pinning by the twin boundaries, indicative o f lattice softening prior to melting. In addition, we show evidence fo r a non-Ohmic behavior in the vortex liquid state due to depinning of the vortices from the two twin boundaries with increasing Lorentz forc e.