A NEW METHOD TO OBTAIN U(J) RELATIONSHIP IN HIGH-TEMPERATURE SUPERCONDUCTOR

An implicit expression for the time dependence of the current density J(t) is derived without the constant assumption of dU/dJ or [dU/dJ], b ased on which a new method to extract U(J) relationship from magnetic relaxation data in high temperature superconductors is described. This method is applied to a melt textured growth YBa2Cu3O6+x sample and we obtain U(J) similar to J(-mu) with mu = 0.90 for H parallel to c, whi ch is in accordance with that derived by the usual method suggested by Malery et al. (mu = 0.87). Moreover, both methods give the same value of the constant C, which implies that the determination of C by the r equirement of continuity of the U(J) curve at different temperatures i s reliable. And the results U(J) similar to J(-mu) by both methods are evidence for the collective pinning or the vortex glass theory.