VORTEX LOCALIZATION IN SINGLE-CRYSTALS OF TL2BA2CUO6+DELTA WITH COLUMNAR DEFECTS

We investigate the effect of columnar defects on the low-temperature m agnetic response of a highly anisotropic superconductor. Using a local magnetic field sensor, the vortex creep rate in single crystals of Tl 2Ba2CuO6+delta containing columnar defects is found to be a highly non monotonic function of vortex density. The creep rate is largest when t he vortex density is less than the defect density. A strong suppressio n of the creep rate is observed near the matching field, B-phi, corres ponding to equal numbers of vortices and defects. We tentatively assoc iate this finding with the Mott-insulator line phase proposed by Nelso n and Vinokur [Phys. Rev. B 48, 13 060 (1993)]. For higher vortex dens ities, an additional minimum is found near 3B(phi), emphasizing the im portance of vortex interactions.