The Collapse or Melting of the Charge Order in Organic Conductors

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The collapse or melting of the charge order in organic conductors have been studied by the transport and NMR measurements of (DI-DCNQI)_2Ag and θ-(ET)_2RbZn(SCN)_4.The (DI-DCNQI)_2Ag with a quasi-one-dimensional quarter-filled-band shows charge ordering, of which the evidences were provided by ^<13>C NMR, ^1H NMR and XRD measurements. This system has been metallized by pressure. It was found that the charge order melts into a curious metallic state with cubic temperature dependence of resistivity. We constructed the pressure-temperature phase diagram, where the melting line has a tricritical point dividing the second-order line at low pressures and the first-order line at high pressures just before it vanishes.The 8-(ET)_2RbZn(SCN)_4, with a quasi-two-dimensional quarter-filled band has triangular ET lattice, which imposes frustration on the charge location as on the spin alignment in the half-filled band system. It undergoes a charge ordering strongly coupled with the lattice modulation when cooled slowly. In case it is cooled rapidly, however, the NMR spectrum, which was sharp at room temperature, becomes broadened and the relaxation rate gets inhomogeneous. We have performed T_2 measurements on the two states, paying particular attention to the nature of the latter electronic state. It was found that the inhomogeneous NMR features are manifestations of the frustration-driven inhomogeneous charge, which can be called the charge glass.