Pinned Vortex Liquid above the Critical Point of the First-Order Melting Transition: A Consequence of Pointlike Disorder

We report on a strongly pinned entangled vortex liquid above the critical point (CP) of the first-order melting transition in untwinned YBa₂Cu₃O_7-δ single crystals. Above CP, the c axis dissipation is finite where the in-plane dissipation is undetectable, implying large in-plane vortex pinning in the presence of vortex cutting and recombination. We demonstrate that the behavior above CP is governed by pointlike disorder. The same behavior is reproduced in twinned samples with proton-induced point defects.