Correlation of flux lines in single-crystal ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ with columnar defects

A detailed study has been made of the irreversibility line in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$, both for unirradiated crystals and also for samples containing irradiation-induced columnar defects aligned at either 0° or 45° to the c axis. For the unirradiated reference sample the proposed scaling rules for the case of isotropic pinning provide an excellent description of the data. In contrast, for the case of the irradiated samples the breakdown of the scaling due to the presence of anisotropic pinning provides a powerful method with which to trace the development of anisotropic pinning due to correlated disorder. For fields below the matching field ${\mathit{B}}_{\mathrm{\Phi }}$ the existence of strong anisotropic pinning is demonstrated, while for B≫${\mathit{B}}_{\mathrm{\Phi }}$ pinning is shown to be isotropic. A characteristic temperature ${\mathit{T}}_1$ is identified, above which the effect of anisotropic pinning is strongly reduced due to increased thermally induced delocalization of the vortices away from the columnar defects.