Induced current densities js 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 Tirr in magnetic fields up to 7 T by means of sensitive capacitance torquemeters. The dynamical relaxation rate Q ≡ d ln js/d ln(dBe/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 js, the dynamical relaxation rate and the conventional relaxation rate R ≡ -d ln js/d ln t monitoring the time decay of js 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 js is analysed by means of a collective pinning theory. This analysis gives a good description of the observed time dependence of js, even for extremely fast relaxation processes leading to js(t)/js(0) < 0.01 in times as small as 10 s. The characteristic pinning energy Uc, obtained by fitting the observed time decay of js with a collective-creep model scales approximately with the c-axis component Be cos Θ of the magnetic field. This scaling behaviour is also observed in the angular dependence of Q and js. For the scaling of js one has to take into account that the current is induced by only the c-axis component of the sweep rate.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics
- Energy Engineering and Power Technology
- Electrical and Electronic Engineering