NONLINEAR U(J) DEPENDENCE DETERMINED DIRECTLY FROM LOW-ELECTRIC-FIELDE-J(S) CURVES IN YBA2CU3O7-DELTA THIN-FILMS

Low-electric-field E=j(s) curves, normalized dynamical relaxation rate Q(T)=d ln(j(s))/d In(E), and experimental critical current density j( s)(E = E(c)) were measured from 10 to 85 K with the magnetic sweeping method. By taking a general assumption for the activation energy U(j(s ), T) = U-c( T)f [j(s)(T)/j(c)(T)], with t = T/T-c, we obtained the U( j, T=O K) relation by scaling all the E-j(s) curves of different tempe ratures to one smooth curve with U-c(T) and j(c)(T) determined by the so-called generalized inversion scheme. It is shown that the important parameter c = In(upsilon(o)B), which was actually taken as a fitting parameter in, the scaling process in previous work, can be predetermin ed. It is also found that the resulting U(j, T=O K) relation based on the E=j(s) curves for T less than or equal to 79 K can be well interpr eted with the collective-pinning model. However, the data for T>79 K s tart to deviate from this interpretation, which may be attributed to t he reverse hopping of vortices.