VORTEX PHASE-TRANSITION WITH DECOUPLING OF THE ADJACENT LAYERS IN THEORGANIC SUPERCONDUCTOR KAPPA-(BEDT-TTF)(2)CU[N(CN)(2)]BR

The temperature dependence of the local magnetic induction measured by using a miniature Hall probe shows a tiny step at a field-dependent t emperature in the titled compound. This result implies that there exis ts a first-order phase transition in the vortex system in this organic superconductor as reported in high-T-c cuprates. The height of the st ep is of the order of 10 mG, from which the entropy change at the tran sition is estimated to be about 0.1 (k(B)/vortex)/layer. The measureme nts of the Josephson plasma resonance reveal that the interlayer phase coherence abruptly changes across the transition line, indicating the decoupling of the adjacent layers.