Impurity scattering in highly anisotropic superconductors and interband sign reversal of the order parameter

We discuss various mechanisms that can lead to interband sign reversal of the order parameter in a multiband superconductor. In particular, we generalize Abrikosov-Gor'kov solution of the problem of weakly coupled superconductor with magnetic and nonmagnetic impurities on the case of arbitary order parameter anisotropy, including extreme cases as $d-$pairing or interband sign reversal of the order parameter, and show that interband scattering by magnetic impurities can stabilize an interband sign-reversal state. We discuss a possibility of such state in YBa$_2$Cu$_3$O$_7$ in the context of various experiments: Josephson tunneling, neutron scattering, isotope effect measurements.