Abstract
Low-temperature magnetic relaxation experiments were performed in different Tl- and Hg-based high-temperature superconducting cuprates (HTSCs). The temperature dependence of the time-logarithmic, normalized relaxation rate R = \d(M/M(o))/dln(t)\ gives evidence of a crossover from thermally activated to quantum vortex motion as temperature decreases. In addition, the field dependence of R reflects a crossover in the dimensionality of the tunneling object, from 3D flux lines to 2D vortices as field increases. Both results are consistent one with each other and with theories of quantum vortex motion in HTSCs.