Cooling rate dependence of the in-plane magnetic penetration depth in κ-type BEDT-TTF superconductors

We performed dc magnetization measurements under the control of cooling-rate in the organic superconductors κ-(BEDT-TTF)₂X (X=Cu(NCS)2 and Cu[N(CN)₂]Br). A distinct different cooling-rate dependence of the in-plane magnetic penetration depth (λ//(0)) was observed between two salts, where λ//(0) was estimated from a linear extrapolation to 0 K. In the Cu(NCS)₂ salt, λ//(0) remains unchanged regardless of cooling-rate, while T_c decreases slightly in faster-cooling. On the other hand,λ//(0) in the Cu[N(CN)₂]Br salt becomes long significantly in faster-cooling. We quantitatively explain the change of λ//(0) in the cooling-rate as an impurity effect by terminal ethylene disorder in BEDT-TTF molecules in terms of the local and clean limit approximation.