Abstract
The results of low-temperature magnetic-relaxation experiments in TlBaCaCuO superconductors (2212 and 2223 phase) are reported. They give evidence of a crossover from a thermally activated regime of flux motion to a nonthermally activated one. The magnetic-relaxation rate is found to become temperature independent below congruent to 7 K and congruent to 3 K, for the 2212 and 2223 phase, respectively. Its value compares well with that predicted by the theories of flux motion by quantum tunnelling.