Pairing due to Spin Fluctuations in Layered Organic Superconductors

I show that for a \kappa-type organic (BEDT-TTF)_2-X molecular crystal, a superconducting state with T_c ~ 10 K and gap nodes on the Fermi surface can be caused by short-ranged antiferromagnetic spin fluctuations. Using a two-band description for the anti-bonding orbitals on a BEDT-TTF dimer of the \kappa-type salt, and an intermediate local Coulomb repulsion between two holes on one dimer, the magnetic interaction and the superconducting gap-function are determined self consistently within the fluctuation exchange approximation. The pairing interaction is predominantly caused by inter-band coupling and additionally affected by spin excitations of the quasi one-dimensional band.