Possible superconductivity in nearly antiferromagnetic itinerant fermion systems

Strong spin fluctuations arising in itinerant fermion systems close to a magnetic instability may induce or inhibit superconductivity depending on the nesting wave vector ${\mathrm{q}}_0$ for which the instability occurs. If ‖${\mathrm{q}}_0$‖ is small but finite, triplet pairing is favored and singlet pairing is suppressed as efficiently as in nearly ferromagnetic systems ($q_0$=0). If ‖${\mathrm{q}}_0$‖ is large, there is a repulsive contribution from backward scattering by which triplet as well as singlet pairings are strongly depressed. The cases of ${\mathrm{UPt}}_3$, ${\mathrm{CePb}}_3$, and some organic compounds are considered.