Dissipation in high temperature superconductors in a magnetic field

The absence of a Lorentz-force dependence on dissipation in the highly anisotropic high-temperature superconductor Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub x/ has been measured over a wide range of current densities in broadened resistive transitions, current-voltage characteristics, magnetoresistances, and critical current densities, J/sub c/. The magnetoresistances are very useful for determining the correct temperature and field dependences of the activation energy. As an alternative to flux motion, the authors consider a Josephson-coupling model which is consistent with the broadened resistive transitions and the lack of Lorentz-force dependence. It is found that the Josephson-coupling model agrees with the temperature dependences of the activation energy and J/sub c/ and is better matched to the weak-field dependence of J/sub c/ than the flux creep model. Possible origins of Josephson junctions in high-quality films and single crystals are discussed.