Superconductivity and Spin Fluctuations

The organizers of the Memorial Session for Herman Rietschel asked that I review some of the history of the interplay of superconductivity and spin fluctuations. Initially, Berk and Schrieffer showed how paramagnon spin fluctuations could suppress superconductivity in nearly-ferromagnetic materials. Following this, Rietschel and various co-workers wrote a number of papers in which they investigated the role of spin fluctuations in reducing the Tc of various electron-phonon superconductors. Paramagnon spin fluctuations are also believed to provide the p-wave pairing mechanism responsible for the superfluid phases of He. More recently, antiferromagnetic spin fluctuations have been proposed as the mechanism for d-wave pairing in the heavy-fermion superconductors and in some organic materials as well as possibly the high Tc cuprates. Here I will review some of this early history and discuss some of the things we have learned more recently from numerical simulations. The interplay of magnetic spin fluctuations and superconductivity has an interesting history, beginning as a way to understand the suppression of Tc in some of the traditional metals, then providing a mechanism for p-wave pairing in He, and finally as a suggested d-wave pairing mechanism for some organic superconductors, heavy-fermion systems, and possibly the high Tc cuprates. Here I will review some of the early history and discuss what has been learned in recent years from numerical studies. In 1966 Berk and Schrieffer [1] and Doniach and Engelsberg [2] discussed the behavior of nearly-ferromagnetic systems in terms of paramagnon spin fluctuations. Berk and Schrieffer Presented at MOS99, International Conference on Physics and Chemistry of Molecular and Oxide Superconductors, Stockholm, Sweden, 28 July–2 Aug, 1999. email: [email protected]