Muon-spin-relaxation measurements of magnetic penetration depth in organic superconductors (BEDT-TTF)2-X: X=Cu(NCS)2 and Cu[N(CN)2]Br

The magnetic-field penetration depth λ in the organic superconductors κ-(BEDT-TTF)2 Cu(NCS)2 and κ-(BEDT-TTF)2 Cu[N(CN)2]Br has been measured over a wide temperature region 20 mK≤T≤15 K via the muon-spin-relaxation technique. Linear variation of λ with T at low temperatures, found in both systems, is consistent with anisotropic superconducting pairings with line nodes in the energy gap. In the latter compound, we observe flux depinning at T∼5 K, well below Tc=12 K.