Superfluid density and pairing in planar organic superconductors

We calculate self-consistently in-plane superfluid density ρparallels in planar organic superconductors for three types of pairing symmetry: dx2-y2, dxy-like and extended s-wave pairing. Assuming strong dimerization, we consider a single band model with elliptical Fermi surface to evaluate the temperature dependence ρparallels(T) in κ-(BEDT-TTF)2X compounds for those types of pairing. The obtained results, compared with measurements of the superfluid density in κ-(BEDT-TTF)2Cu[N(CN)2]Br, strongly suggest that in these compounds the gap symmetry, influenced by cooling history, in the 'ground state' with the least disorder is of the extended s-wave type. Possible physical reasons for agreement of dx2-y2, and/or isotropic s-wave pairing with experimental results in more disordered 'intermediate state' are discussed