Vortex liquid state in an electron irradiated untwinned YBa2Cu3O7-δ crystal

We report on the temperature and field dependencies of the resistivity of an untwinned YBa2Cu3O7-δ single crystal before and after 1 MeV electron irradiation in fields up to 8 T. We show, for the first time, direct suppression of the first order vortex liquid-to-solid phase transition due to the controlled introduction of point defects, and that transport in the vortex liquid is dominated by viscosity rather than by single vortex pinning. Furthermore, we demonstrate that the first order vortex transition can be recovered by reducing the density of point defects through subsequent annealing of the sample.