We present direct evidence of a "peak effect" as a precursor to vortex lattice melting in a single crystal of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ containing only two twin planes. The peak effect occurs just below the vortex liquid to solid phase transition in a narrow field region between 0.3 and 1.5 T for $H\parallel \mathbf{c}$ and is clearly observed as a function of both magnetic field and temperature. We demonstrate that the peak effect is due to enhanced vortex pinning by the twin boundaries, indicative of lattice softening prior to melting. In addition, we show evidence for a non-Ohmic behavior in the vortex liquid state due to depinning of the vortices from the two twin boundaries with increasing Lorentz force.

• Received 16 May 1994

DOI:https://doi.org/10.1103/PhysRevLett.73.2614