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 temperature from T = 1.6 K up to the irreversibility temperature T_irr in magnetic fields up to 7 T by means of sensitive capacitance torquemeters. The dynamical relaxation rate Q ≡ d ln j_s/d ln(dB_e/dt) does not extrapolate to zero at T = 0 K, demonstrating the presence of quantum creep. The quantum creep rate Q(0) ≈ 0.03 at T = 0 is similar to values found in YBa₂Cu₃O₇ films, although Bi₂Sr₂CaCu₂O₈ is much more anisotropic than YBa₂Cu₃O₇. The weak field dependence of Q(0) is consistent with tunneling of 2D vortex pancakes. The induced current density j_s, the dynamical relaxation rate and the conventional relaxation rate R ≡ -d ln j_s/d ln t monitoring the time decay of j_s at fixed external field, are measured as a function of the field strength and its orientation with respect to the sample in detail at a fixed temperature T = 20 K. The observed non-logarithmic time dependence of j_s is analysed by means of a collective pinning theory. This analysis gives 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_e cos Θ of the magnetic field. This scaling behaviour is also observed in the angular dependence of Q and j_s. For the scaling of j_s one has to take into account that the current is induced by only the c-axis component of the sweep rate.