Vortex localization in single crystals of Tl2Ba2CuO61d with columnar defects

We investigate the effect of columnar defects on the low-temperature magnetic response of a highly anisotropic superconductor. Using a local magnetic field sensor, the vortex creep rate in single crystals of Tl2Ba2CuO61d containing columnar defects is found to be a highly nonmonotonic function of vortex density. The creep rate is largest when the vortex density is less than the defect density. A strong suppression of the creep rate is observed near the matching field, Bf , corresponding to equal numbers of vortices and defects. We tentatively associate this finding with the Mott-insulator line phase proposed by Nelson and Vinokur @Phys. Rev. B 48, 13 060 ~1993!#. For higher vortex densities, an additional minimum is found near 3Bf , emphasizing the importance of vortex interactions. @S0163-1829~96!50542-4#