Mott Transition in a Valence-Bond Solid Insulator with a Triangular Lattice

We have investigated the Mott transition in a quasi-two-dimensional Mott insulator EtMe₃P[Pd(dmit)₂]₂ with a spin-frustrated triangular-lattice in hydrostatic pressure and magnetic-field [Et and Me denote C₂H₅ and CH₃, respectively, and Pd(dmit)₂ (dmit=1,3-dithiole-0ex0ex2-thione-4,5-dithiolate,dithiolate) is an electron-acceptor molecule]. In the pressure-temperature (P-T) phase diagram, a valence-bond solid phase is found to neighbor the superconductor and metal phases at low temperatures. The profile of the phase diagram is common to those of Mott insulators with antiferromagnetic order. In contrast to the antiferromagnetic Mott insulators, the resistivity in the metallic phase exhibits anomalous temperature dependence, ρ=ρ₀+AT2.5.