Magnetic penetration depth and flux melting in κ-(BEDT-TTF)2Cu[N(CN)2]Br

We have performed muon spin relaxation studies in the quasi-2 dimensional organic superconductor κ-(ET)2Cu[N(CN)2]Br, applying an external field of 3 kG perpendicular to the conducting planes of single crystal specimens. The relaxation rate in field-cooled measurements shows a linear variation for T ≤ 0.4Tc. This behavior is consistent with an anisotropic pairing state with line nodes in the superconducting energy gap. Zero-field-cooling measurements reveal that magnetic flux is not well-pinned until T ≤ 5 K, well below Tc = 12 K.