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

Abstract

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.