Normal-state properties of BEDT-TTF compounds and the superconductivity pairing mechanism

The BEDT-TTF salt family is described by a two parameter two-dimensional (2D) Fermi surface model, in which bandwidth and departure from perfect nesting can be varied. We have studied the spin fluctuation effects on the normal-state properties in a random phase approximation approach. The calculated NMR relaxation rate exhibits a peak in ${1/T}_{1}T,$ which strongly decreases when the departure from perfect nesting of the Fermi surface and the bandwidth increase. These results are in perfect agreement with NMR experiments done in $(\mathrm{BEDT}-\mathrm{TTF})X$ $(X=\mathrm{Cu}[\mathrm{N}\left(\mathrm{CN}\right){]}_2\mathrm{Br},$ $\mathrm{Cu}\left(\mathrm{NCS}{)}_2\right).$ We investigate the superconductivity pairing due to spin fluctuations and the same theoretical model of quasi-2D electrons close to the spin density wave instability. The coupling constants of various channels are calculated. They strongly depend on the nesting properties of the Fermi surface and on the bandwidth.