Study of organic conductors by NMR at selectively enriched nuclear

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A series of organic systems, (BEDT-TTF)_2 X,have been investigated by ^<13>C-NMR.Comparison of the nuclear spin-lattice relaxation rate, conducting property and molecular arrangement indicates a close relationship between the molecular arrangement and manifestation of electron correlation. The metal/nonmetal phases in the isostructural kappa- (BEDT-TTF)_2 X systems, which are in the strong dimeric regime, are understood as highly correlated metals and insulators crossing the Mott transition. The deuterated Cu [N(CN)_2]Br salt is found to be situated in the critical region of the transition between the superconducting and antiferromagnetic phases.For the superconducting phase (Cu[N(CN)_2]Br salt) situated near the Mott transition, the absence of coherence peak and a low-temperature T^3 -dependence of the nuclear spin-lattice relaxation rate suggest unconventional superconductivity with line nodes in gap parameter or highly anisotropic one. The low-temperature specific heat is consistent … More with the unconventional one in both temperatura and field dependence.In order to investigate the electronic state of the metallic (DMe-DCNQI)_2 Cu, we prepared isotopeenriched DMe-DCNQI molecules where each site of=N-CN was substituted by ^<13>C or ^<15>N and performed ^<13>C and ^<15>N-NMR measurements for the Cu salt and the Li salt as a reference material of pi-electron system without hybridization. The shift and relaxation rate analysis demonstarated that the Cu salt is a multiband system due to the pi-d hybridization ; the spin susceptibility of the nonbonding pi band and the bonding pi-d band has opposite temperature dependence. On the other hand, (DI-DCNQI)_2 Ag, a new member in the DCNQI-Metal complexes, has been investigated by electrical resistance, magnetic susceptibility and ^1H-NMR measurements and found to be non-metallic with a charge gap of -490K but without spin gap. An antiferromagnetic ordering is evidenced at a low temperature. By ^<13>C-NMR,this system was found to show charge ordering below -200K,which indicates important role of the intersite Coulomb interaction, V. Less