LOW-TEMPERATURE MAGNETIC-RELAXATION IN HGBA2CA2CU3O8-CRYSTALS WITH COLUMNAR DEFECTS(DELTA SINGLE)

Micrometer-sized Hall probes are used to measure the low-temperature m agnetic properties of HgBa2Ca2Cu3O8+delta single crystals with and wit hout columnar defects. Introduction of columnar defects by heavy-ion i rradiation suppresses the quasiexponential temperature dependence of t he critical current, but only when the density of flux lines is less t han the density of defects. The current-dependent creep activation ene rgy U(J) is found to be best described by the power law: U(J)= U-0[(J( c)/J)(mu) - 1]. At high current densities J, and low fields, there is a crossover to a quantum vortex creep process with a temperature-indep endent magnetic relaxation rate. On the low-J side of this crossover i n the unirradiated sample, mu much greater than 1, indicating that the thermally activated vortex creep process is consistent with collectiv e creep. When the flux-line density exceeds the columnar defect densit y in the irradiated sample, the vortex creep process is characterized by mu similar to 1. For flux-line densities less than the defect densi ty, an anomalously large exponent is found (mu greater than or equal t o 2), which is inconsistent with vortex transport occurring either by the nucleation and expansion of half-loop excitations or by variable-r ange hopping of vortex lines as described by the Bose-glass theory.