We have studied the vortex phase diagram of the organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2Br], which has layer structure like high temperature superconductors, by means of in- and out-of plane resistivity, ρ∥ and ρ⊥, and DC magnetization measurements as a function of temperature in magnetic fields H perpendicular to the conducting plane. A sharp drop of resistance is observed for the first time at a characteristic temperature Tm both in ρ∥ and ρ⊥. Around this temperature, the DC magnetization shows a discontinuous step. These results strongly suggest the existence of the first-order phase transition of vortex system, which can be ascribed to either the vortex lattice melting, or the simultaneous melting and decoupling.
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