Vortex solid-liquid transition in Bi₂Sr₂CaCu ₂O_8+δ with a high density of strong pins

The introduction of a large density of columnar defects in Bi ₂Sr₂CaCu₂O_8+δ crystals does not, at sufficiently low vortex densities, increase the irreversibility line beyond the first order transition (FOT) field of pristine crystals. At such low fields, the flux line wandering length r_w behaves as in pristine crystals. Next, vortex positional correlations along the c axis in the vortex Bose glass at fields above the FOT are smaller than in the low-field vortex solid. Third, the Bose-glass-to-vortex liquid transition is signaled by a rapid decrease in c-axis phase correlations. These observations are understood in terms of the "discrete superconductor" model.