QUANTUM CREEP AND FAST THERMALLY ACTIVATED VORTEX DYNAMICS IN A BI2SR2CACU2O8 SINGLE-CRYSTAL

Induced current densities j(s) and flux relaxation of a Bi2Sr2CaCu2O8 single crystal have been measured in detail as a function of temperatu re from T = 1.6 K up to the irreversibility temperature T-irr in magne tic fields up to 7 T by means of sensitive capacitance torquemeters. T he dynamical relaxation rate Q = d ln j(s)/d ln(dB(c)/dt) does not ext rapolate to zero at T = 0 K, demonstrating the presence of quantum cre ep. The quantum creep rate Q(0) approximate to 0.03 at T = 0 is simila r to values found in YBa2Cu3O7 films, although Bi2Sr2CaCu2O8 is much m ore anisotropic than YBa2Cu3O7. The weak field dependence of Q(0) is c onsistent with tunneling of 2D vortex pancakes. The induced current de nsity j(s), the dynamical relaxation rate and the conventional relaxat ion rate R = -d ln j(s)/d ln t monitoring the time decay of j(s) at fi xed external field, are measured as a function of the field strength a nd its orientation with respect to the sample in detail at a fixed tem perature T = 20 K. The observed non-logarithmic time dependence of j(s ) is analysed by means of a collective pinning theory. This analysis g ives a good description of the observed time dependence of j(s), even for extremely fast relaxation processes leading to j(s)(t)/j(s)(0) < 0 .01 in times as small as 10 s. The characteristic pinning energy U-c, obtained by fitting the observed time decay of j(s) with a collective- creep model scales approximately with the c-axis component B-c cos The ta of the magnetic field. This scaling behaviour is also observed in t he angular dependence of Q and j(s). For the scaling of j(s) one has t o take into account that the current is induced by only the c-axis com ponent of the sweep rate.