Nonlinear U(j) dependence determined directly from low-electric-field E-js curves in YBa2Cu3O7- thin films

Low-electric-field E-js curves, normalized dynamical relaxation rate Q(T)=d ln(js)/d ln(E), and experimental critical current density js(E=Ec) were measured from 10 to 85 K with the magnetic sweeping method. By taking a general assumption for the activation energy U(js,T)=Uc(T)f[js(T)/jc(T)], with t=T/Tc, we obtained the U(j,T=0 K) relation by scaling all the E-js curves of different temperatures to one smooth curve with Uc(T) and jc(T) determined by the so-called generalized inversion scheme. It is shown that the important parameter c=ln(v0B), which was actually taken as a fitting parameter in the scaling process in previous work, can be predetermined. It is also found that the resulting U(j,T=0 K) relation based on the E-js curves for T 79 K can be well interpreted with the collective-pinning model. However, the data for T>79 K start to deviate from this interpretation, which may be attributed to the reverse hopping of vortices.