Suppression of the first order vortex melting transition by intrinsic pinning in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

The first order vortex melting transition in single crystal ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ is investigated for magnetic field direction close to H∥ab. For exact field alignment H∥ab, the ‘‘kink’’ in ρ(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 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 H∥c in the presence of only six twin boundaries.