NONLOCAL CONDUCTIVITY IN HIGH-TEMPERATURE SUPERCONDUCTORS

We examine nonlocal conductivity in high-temperature superconductors f rom a phenomenological paint of view. One wants to deduce the properti es of the conductivity, especially its inherent length scales, from th e transport data. Although this is a challenging inverse problem, comp licated further by the experimental data not being completely self-con sistent, we have made some progress. We find that if a certain form fo r the conductivity is postulated then one requires positive ''viscosit y'' coefficients to reproduce some of the experimental results. We are able to show that the effects of surfaces on the conductivity are lik ely to be important and draw comparisons with the treatment of the sur face within the hydrodynamic approach put forth by Huse and Majumdar. We also develop an approximation scheme for the conductivity which is more robust than the hydrodynamic one, since it is stable for both pos itive and negative viscosity coefficients, and discuss the results obt ained using it.