Effect of Disorder on the Vortex-Lattice Melting Transition

We use a three-dimensional stacked triangular network of Josephson junctions as a model for the study of vortex structure in the mixed state of high $T_c$ superconductors. We show that the addition of disorder destroys the first order melting transition occurring for clean samples. The melting transition splits in two different (continuous) transitions, occurring at temperatures $T_i$ and $T_p$ $(>T_i)$. At $T_i$ the perpendicular-to-field superconductivity is lost, and at $T_p$ the parallel-to-field superconductivity is lost. These results agree well with recent experiments in YBaCuO.