Complex susceptibility and penetration depth of BEDT-TTF based layered superconductors

Summary form only given. Organic superconductors based on the BEDT-TTF donor have layered structure and therefore two-dimensional nature in superconductivity. The degree of the interlayer superconducting coupling can be examined by measurements of the ac susceptibility at a field parallel to the layer and be evaluated quantitatively as parallel penetration depth. We have measured the parallel penetration depth of the two representative compounds, /spl kappa/-(BET-TF)/sub 2/Cu[N(CN)/sub 2/]Br and /spl alpha/-(BEDT-TTF)/sub 2/NH/sub 4/(SCN)/sub 4/. The former belongs to the /spl kappa/-phase family with transition temperature above 10 K while the latter has a much lower transition temperature but is known to have extremely two dimensional nature. The former salt gives the depth of 190 /spl mu/m which was reproducible for several crystals. The preliminary value for the latter is 1.4 mm. These values demonstrate that the NH/sub 4/(SCN)/sub 4/ salt is more highly-two-dimensinal than the Cu[N(CN) ]Br salt. We also present the susceptibility behavior under magnetic field and discuss the vortex state in these compounds.