Order parameter anisotropy caused by the symmetry properties of coupling matrix element two-band model of a superconductor

A two band model of a superconductor with attraction in one band and effective interaction of arbitrary sign in another one is investigated. It was found that if the bands wave functions are of different symmetries, then the order parameters of such model are strongly anisotropic due to the properties of the matrix element of one-particle inter band coupling. In the case of a repulsion in the second band order parameters necessarily have nods, so the excitation spectrum is gapless. No suppositions about the strong anisotropy of the initial spectrum or interactions in the bands are needed. Copyright (C) 1996 Elsevier Science Ltd