  Differential magneto-optical imaging of the vortex-lattice melting process in
Bi_2Sr_2CaCu_2O_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,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.
