Impurity Effect on the In-plane Penetration Depth of the Organic Superconductors κ-(BEDT-TTF)_2X (X = Cu(NCS)2 and Cu[N(CN)2]Br)

We report the in-plane penetration depth λ_\| of single crystals κ-(BEDT-TTF)_2X (X = Cu(NCS)₂ and Cu[N(CN)₂]Br) by means of the reversible magnetization measurements under the control of cooling-rate. In X = Cu(NCS)₂, λ_\|(0) obtained by an extrapolation toward T = 0 K does not change by cooling-rate within the experimental accuracy, while T_c is slightly reduced. On the other hand, in X = Cu[N(CN)_2]Br, λ_\|(0) indicates a distinct increase by cooling faster. The different behavior of λ_\|(0) on cooling-rate between the two salts is quantitatively explained in terms of the local-clean approximation (the London model), considering that the former salt belongs to the very clean system and the latter the moderate clean one. The good agreement with this model demonstrates that disorders of the ethylene-group in BEDT-TTF introduced by cooling faster increase the electron(quasiparticle)-scattering, resulting in shorter mean free path.