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, acid at T-p the parallel-to-field superconductivity is lost. These results agree well with recent experiments in YBaCuO.