^<13>C-NMR Study of the Organic Superconductor near the Superconductor-Insulator Phase Boundary

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^<13>C-NMR study on non-deulerated and partially deuterated κ-(BEDT-TTF)_2Cu[N(CN)_2]Br is reported, where BEDT-TTF is bis(ethylenedithio)tetra-thiafulvalene. By using the BEDT-TTF molecule having ^<13>C-enriched on only one side of the central carbon sites, we could estimate the density of states from the Knight-shift and the spin-lattice relaxation rate. Although the reduced mass effect predicted from the specific heat measurement was not observed in the ^<13>C-NMR, the melal-to-anliferromagnetic phase transition was observed with the phase separation around 18K on the half-deuterated κ-(BEDT-TTF)_2Cu[N(CN)_2]Br under a high magnetic field. Such a transition does not occur under a zero or low field on the **lf-deuterated sal. These results suggest a first order field-induced metal-insulator transition. These findings can explain the complex phenomena near the phase boundary under a high magnetic field. We also found that the phase separation is observed at around T^* with the applied field parallel to the conducting layer, unlike partially deuterated κ-(BEDT-TTF-d[n,n])_2Cu[N(CN)_2]Br. The former result suggests the field induces AF transition. the latter result suggests there is the field direction dependence of the M-1 phase separation behavior in κ-(BEDT-TTF-d[4,4])_2Cu[N(CN)_2]Br.