Antiferromagnetic Interactions and the Superconducting Gap Function

Spin-fluctuation-mediated superconductivity is conventionally associated with d_{x^2-y^2} pairing. We show that a generalized model of antiferromagnetic spin fluctuations in three dimensions may also yield a state with formal ``s-wave'' (A_{1g}) symmetry but with line nodes at k_z \approx \pm \pi / 2c. We study this new state within both BCS and Eliashberg theories using a realistic band structure and find that it is more stable than the d_{x^2-y^2} (B_{1g}) state over a wide range of parameters. Thus, models of spin-fluctuation-mediated superconductivity must consider both possibilities on an equal footing.