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

We have obtained the parallel penetration depth of two representative compounds of layered organic superconductors, κ-(BEDT-TTF)2Cu[N(CN)2]Br and α-(BEDT-TTF)2NH4(SCN)4 by ac susceptibility measurements. The former salt gives the depth of 190 μm, while that for the latter is 1.4 mm. These values demonstrate that the NH4(SCN)4 salt is more highly-two-dimensional than the Cu[N(CN)2]Br salt. The ac susceptibility under dc magnetic field probes peculiarities of the vortex dynamics.