ORDER-PARAMETER ANISOTROPY CAUSED BY THE SYMMETRY PROPERTIES OF COUPLING MATRIX ELEMENT 2-BAND MODEL OF A SUPERCONDUCTOR

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