EFFECT OF COLUMNAR DEFECTS ON THE VORTEX-SOLID MELTING TRANSITION IN BI2SR2CACU2O8

The first-order phase transition of the vortex-lattice in Bi2Sr2CaCu2O 8 crystals is shown to transform into a continuous transition when dis order is introduced via a very low concentration of columnar defects. We demonstrate fine tuning of the disorder strength and the recovery o f the first-order transition by varying the concentration of the colum ns, by changing the temperature along the melting line, or by tilting the magnetic field with respect to the columns.