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

We map a pinning boundary in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ 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 ${\mathit{B}}^{\mathrm{?}}$(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 ${\mathit{T}}_{\mathrm{dp}}$\ensuremath{\sim}41 K (below 0.5${\mathit{T}}_{\mathit{c}}$). \textcopyright{} 1996 The American Physical Society.