Theory of Antiferromagnetic Superconductors

Abstract

It is useful to summarize the basic theoretical picture that we now have of antiferromagnetic (AF) superconductors. For definiteness we’ll focus on the two types of ternary compounds RERh₄B₄ and REMo₆S₈ where RE denotes a rare earth element. There are a number of factors which aid the coexistence of antiferromagnetism and superconductivity. In the first place there are believed to be three distinct classes of electrons which play an important role in these systems. The RE f electrons undergo magnetic order. The d electrons associated with Rh or Mo are superconducting. Finally the electrons which mediate the RKKY interactions between the RE localized electrons have probably little to do with the superconducting electrons and constitute a distinct (third) class of electrons. It is important to recognize that in the ternary superconductors the electrons which mediate or undergo magnetic order are distinct from those which are superconducting. The second factor which aids coexistence has to do with the nature of the AF order parameter, called <S_Q>. The fact that the wave vector Q of the AF order is considerably larger than ξ⁻¹ means that on the size scale of a Cooper pair the average molecular field vanishes. By contrast,in ferromagnets the fact that Q<ξ⁻¹ is at the heart of the very strong incompatibility between ferromagnetism and superconductivity.