Nonlinear current response of one- and two-band superconductors

We have calculated the nonlinear current of a number of single band s-wave electron-phonon superconductors. Among issues considered were those of dimensionality, strong electron-phonon coupling, impurities, and comparison with BCS. For the case of two bands, particular attention is paid to the role of anisotropy, the integration effects of the off-diagonal electron-phonon interaction, as well as inter- and intraband impurities. For the specific case of MgB2, we present results based on the known microscopic parameters of band theory.