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

Abstract

We performed dc magnetization measurements under the control of cooling-rate in the organic superconductors κ-(BEDT-TTF) 2 X (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.