By means of high-sensitivity capacitance torque magnetometers we have measured the superconducting current js and the dynamic magnetic-moment relaxation of YBa2Cu3O7 and YBa2Cu4O8 films of typically 100 nm thickness at temperatures between 2 K and Tc in magnetic fields up to 6 T. For the measurements of the dynamic relaxation rate Q≡d ln js/d ln (dBe/dt) magnetic-field sweep rates were varied between 0.5 and 40 mT/s. At low fields (typically 0.5 T) the dynamical relaxation rate exhibits a plateau at Q≈0.06 in YBa2Cu3O7 and 0.04 in YBa2Cu4O8. At high fields (Be=μ0He≈ 6 T) the plateaus have completely disappeared and Q increases almost linearly with increasing temperature. At all fields a sharp increase up to Q≊1 is observed when the irreversibility line is approached.

By means of the generalized inversion scheme (GIS), the js(T, Be) and Q (T, Be) data are used to determined the current dependent activation energy U (j, T, Be) for thermally activted flux creep. Although the GIS does not make any a priori assumptions about the explicit functional dependences on T and j, the U(j, T=0, Be) function derived from the experimental data by means of the GIS can remarkably well be described with the collective-creep interpolation formula U(j)=(Uc/μ)[(jc/j)μ-1] with μ≈0.6 for currents j>0.15jc (T=0, Be) where Jc(T=0, Be) is the critical current at T=0, and wi Uc depending on Be. At lower current densities U(j, T=0, Be) does not diverge as j-0.6 but shifts gradually to a weaker ln(jc/j) dependence. At low temperatures the current and relaxation data cannot be explained in terms of a thermally activated flux-motion model. Quantum creep has an influence up to ∼13 K. ... Read more

The supercurrent js and the dynamical relaxation rate Q ≡ dlnjs/dln(dBe/dt) of YBa2Cu3O7 and YBa2Cu4O8 films of about 100 nm thickness have been measured by means of a torque magnetometer for temperatures up to Tc and fields up to Be = 7 T. From these data the true critical current jc(T, Be) and the activation energy function U(j, T, Be) for thermally activated flux motion have been calculated. For fields smaller than about 2 T, both samples are in the single vortex regime. The obtained U(j, T = 0, Be) dependence is rather well described by the collective pinning interpolation formula. ... Read more

The critical current jc and the pinning energy Uc have been determined for three types of yttrium-based superconducting films from current js and dynamic relaxation Q=d lnjs/d ln(dB/dt) data by means of the generalized inversion scheme. For B<2 T and T<80 K the temperature dependence of jc and Uc for all films is found to be in excellent agreement with a model of single vortices pinned by randomly distributed weak pinning centers via spatial fluctuations of the charge carrier mean free path. Pinning due to spatial fluctuations of Tc is not observed. ... Read more

The dynamical relaxation rate Q dln js/dln(dB/dt) where js is the current induced in a superconductor when the external field is swept at a rate dB/dt, has been measured in YBa2Cu3O7 and Y2Ba4Cu8O16 films of 100 nm and 180 nm thickness as a function of magnetic field at temperatures between 2 K and Tc. The data show i) a non zero relaxation at low T due to quantum creep, ii) a plateau between 15 and ∼ 50 K and iii) a divergence when T approaches the irreversibility line. The data are analyzed within the collective pinning model. At low temperatures it is shown that both js and Q are influenced by quantum creep. ... Read more