Superconductivity in a magnetically ordered background

Borocarbide compounds with the formula RN2B2C show interesting superconduct ing and magnetic properties and the coexistence of the two phenomena. BCS t heory is extended to systems with underlying commensurate magnetic order. I n the case of helical phases the technique may be extended to any e-vector and there exists a well-defined limit for incommensurate values. The way ma gnetic order influences superconductivity depends crucially on the details of both the magnetic structure and the electron bands, but some qualitative criteria may be given. As an example, we give a brief analysis of the comp ound HoNi2B2C. (C) 1999 Elsevier Science B.V. All rights reserved.