MAGNETIC-RELAXATION AND QUANTUM TUNNELING OF VORTICES IN A POLYCRYSTALLINE HG0.8TL0.2BA2CA2CU3O8+DELTA SUPERCONDUCTOR

Magnetic relaxation in a polycrystalline Hg0.8Tl0.2Ba2Ca2Cu3O8+delta [ (Hg,Tl)-1223] superconductor was measured in the temperature range 1.8 5-20 K with different magnetic fields applied during the cooling proce ss. The relaxation curves show a nearly perfect linear ln(t) behavior. The temperature dependence of the normalized relaxation rate, R = \d( M/M(0))/dln(t)\, was studied for two applied magnetic fields. With the lower applied field (H-alpha = 3 kOe), a temperature-independent R is found below 2.1 K and explained in terms of quantum tunneling of vort ices. As the applied field increases (H-alpha = 10 kOe), the transitio n from the thermal to the quantum regime is not found in the experimen tally accessible temperature range above 1.85 K. The field dependence of R at 2.8 K is studied in order to analyze the dimensionality of the flux-line lattice.