Properties of quasi-two-dimensional superconductors near a SDW instability: application to ET-salts

We describe some organic superconductors, such as the ET salts, by a model of quasi-2D Fermi liquid, in which the Fermi surface exhibits a quasi-1D electron pocket, with good nesting properties, and a quasi-2D hole pocket, possibly close to a Van Hove singularity. Such properties are responsible for large magnetic fluctuations, which influence the electron self energy and, therefore, all the properties of the normal phase. In particular, a pseudo-gap in the quasiparticle spectrum is opened. We propose to interpret in this way the anomaly recently observed in the NMR relaxation time of κ-(BEDT-TTF)2CuN(CN)2Br. Given the vicinity of a magnetic instability, a moderate magnetic field perpendicular to the most conducting plane can induce a SDW phase, which might explain the peculiar phase diagram recently observed in κ-(BEDT-TTF)2CuN(CN)2Cl.