Suppression of the first order vortex melting transition by intrinsic pinning in YBa2Cu3O7- delta.

The first order vortex melting transition in single crystal YBa[sub 2]Cu[sub 3]O[sub 7[minus][delta]] is investigated for magnetic field direction close to [bold H][parallel][ital a][bold b]. For exact field alignment [bold H][parallel][ital a][bold b], the kink'' in [rho]([ital T]) associated with the first order melting transition is replaced with a continuous resistive transition in the presence of intrinsic pinning, indicative of a second order transition with dynamic scaling exponent [ital s]=1.4. In addition, we demonstrate the abatement of the kink'' in the temperature dependence of the magnetoresistivity, associated with the first order melting transition for [bold H][parallel][bold c] in the presence of only six twin boundaries.