Differential magneto-optical imaging of the vortex-lattice-melting process in $\mathrm{Bi}{}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$ crystals reveals unexpected effects of quenched disorder on the broadening of the first-order phase transition. The melting patterns show that the disorder-induced melting landscape $T_m(H,\mathbf{r})$ is not fixed, but rather changes dramatically with varying field and temperature along the melting line. The changes in both the scale and shape of the landscape are found to result from the competing contributions of different types of quenched disorder which have opposite effects on the local melting transition.

• Received 9 May 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.167001