Accommodation of vortices to columnar defects: Evidence for large entropic reduction of vortex localization

We map a pinning boundary in YBa₂{Cu}₃O_7-δ crystals with columnar defects via (i) a finite large drop of the persistent current J(T), (ii) a maximum in the thermal relaxation rate S(T), and (iii) the onset of 1/H field dependence of J(H). This boundary is consistent with the ``accommodation'' field B?(T) proposed by Nelson and Vinokur, separating the regime of vortices well localized on columnar pins from the interaction-dominated collective regime. The strong-pinning regime is limited by an unexpectedly low vortex depinning temperature Tdp{~}41 K (below 0.5Tc). {} 1996 The American Physical Society.