Vortex dynamic properties of vortex-liquid phase in YBa2Cu3O7-delta epitaxial films

The current-voltage characteristics (CVC) of YBa2Cu3O7-delta epitaxial films were determined at temperatures and magnetic fields covering the vortex-liquid region. Using functional fitting and scaling method, we derived the temperature and magnetic field dependences of the pinning potential U-0(T,H) and critical current density J(c)(T,H), and current, temperature and magnetic field dependences of the effective pinning potential U-eff(H,T,J) from the CVC. It is shown that the nonlinear resistance region of the vortex-liquid phase displays a process from vortex plastic motion to free flow with the dynamic exponent changing from -0.5 to 0.