| The layered organics, k-(ET)2X, are model systems for the study of strongly correlated half-filled-band electrons. Here we present two progresses in the Mott physics through the transport and NMR studies on this family of materials.
One is the Mott criticality in 2D. k-(ET)2Cu[N(CN)2]Cl is a Mott insulator with a quite low critical pressure to Mott transition. The resistance measurements of this material under precisely controlled pressure showed that the first-order Mott transition has a critical endpoint at 40 K, where the resistive jump vanishes and critical pressure derivative of resistance is divergent. Remarkably, the transport critical exponents obtained do not belong to any universality class known so far. The implication of this finding is discussed. A recent NMR study on the Mott criticality is also presented.
The other is the realization of the spin liquid and its Mott transition. The Mott insulator k-(ET)2Cu2(CN)3 has a nearly isotropic triangular lattice and is a model system of frustrated quantum spins. The 1H and 13C NMR experiments showed no indication of magnetic ordering down to 30 mK. The spins are likely in the quantum liquid state. Under pressure, it undergoes Mott transition to the Fermi liquid which shows superconductivity at low temperatures. We present the pressure-temperature phase diagram and the NMR/transport results on the nature of the spin liquid and superconductivity.
This work is a collaboration with F. Kagawa, Y. Shimizu, Y. Kurosaki, H. Kasahara, T. Kobashi, K. Miyagawa, M. Maesato and G. Saito.
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