Magnetic relaxation and quantum tunneling of vortices in a polycrystalline Hg0.8Tl0.2Ba2Ca2Cu3O8 superconductor

Magnetic relaxation in a polycrystalline Hg0.8Tl0.2Ba2Ca2Cu3O8 [(Hg,Tl)-1223] superconductor was measured in the temperature range 1.85 20 K with different magnetic fields applied during the cooling process. The relaxation curves show a nearly perfect linear ln(t) behavior. The temperature dependence of the normalized relaxation rate, Rd(M/M0)/dln(t), was studied for two applied magnetic fields. With the lower applied field (Ha=3 kOe), a temperature-independent R is found below 2.1 K and explained in terms of quantum tunneling of vortices. As the applied field increases (Ha=10 kOe), the transition from the thermal to the quantum regime is not found in the experimentally 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.