Carrier doping into the molecular insulators and exploration of novel phases

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In the present study, we aim at doping carriers into the correlation-induced molecular insulators based on a) BEDT-TTF and b) DCNQI molecules, and explore possible novel phases.a) (BEDT-TTF)_4X [X=Hg_<2.89>Br_8Hg_<2.78>Cl_8]A family of two-dimensional organic systems, κ-(BEDT-TTF)_2X, with half-filled bands span the phase diagram of the 1 st-order Mott transition between the superconducting and antiferromagnetic phases as a function of band-width. In the present sudy, we first measured the ^<13>C-NMR spectra and relaxation rate, 1/T, for a doped organic Mott insulator, κ-(BEDT-TTF)_4Hg_<2.78>Cl_8, down to 30 mK.The l/(T_1T) continues to increase in proportion to 1/T down to 100 mK and then forms a broad peak. The spectra show low-temperature broadening indicative of the short-range AF ordering, but not the three-dimensional one. This is the first observation of the quantum critical spin fluctuations in organics and indicates a surprising contrast to the widely studied case of κ-(BEDT-TTF)_2X with fixed band-filling.b) (DI-DCNOl)_2M(DI-DCNQI)_2Ag, which has a quasi-one-dimensional quarter-filled π band, is a paramagnetic insulator. By ^<13>C-NMR study, this insulating phase is found to be in a charge-rodered state, into which the carrier doping was attempted by Cu doping into the Ag sites. As the Cu content is increased, the charge-gapped insulator changes to the nongapped insulator with the variable range hopping behaviour, eventually followed by the metallic phase. This result shows a manner of the collapse of the charge ordering and a route to the metallic phase with introduction of carriers and disorder.