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. 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 + AT 2.5 .PACS numbers: 71.20. Rv, 71.30.+h, 74.25.Nf, 74.70.Kn The Mott transition has been one of the fundamental issues in condensed matter physics [1]. Much attention has been paid on the superconducting state near the Mott transition in transition metal compounds [2] and in organic materials [3]. Mott insulators so far studied usually exhibit antiferromagnetic long-range order, and the relation between the existence of the ordered state and the appearance of superconducting states has been widely discussed [2] [7], in which the metal-insulator transition has not been accessible. Thus the Mott transition and superconductivity appearing from the quantum disordered state have been longed for the last few decades.Here we show that an organic material is a good playground of the quantum spin physics on the frustrated lattice accessible to Mott physics. An anion radical salt EtMe 3 P[Pd(dmit) 2 ] 2 is the 2D VBS Mott insulator synthesized recently [8], where Et and Me denote C 2 H 5 and CH 3 respectively, and Pd(dmit) 2 (dmit = 1,3-dithiole-2-thione-4,5-dithiolate, C 3 S 2− 5 ) is an electron-acceptor molecule. Its crystal structure consists of two layers: the Pd(dmit) 2 layer that involves in conduction and magnetism, and the insulating closed-shell EtMe 3 P + layer. In the conduction layer, pairs of Pd(dmit) 2 molecules form dimers arranged in a triangular lattice in terms of transfer integrals, t and t ′ (t ′ /t = 1.05) [see Fig. 1 inset]. The conduction band is half filling, consisting of an anitibonding combination of the highest-occupied molecular orbital of Pd(dmit) 2 [9]. The large onsite Coulomb interaction in the dimer, compared with the bandwidth, produces a Mott-Hubbard insulating state with a spin-1/2 at each dimer site. The magnetic susceptibility behaves in accordance with the triangular-lattice antiferromagnetic Heisenberg model over a wide temperature range (60 K < T < 300 K) with J = 250 K [10], indicating the presence of spin frustration and hence highly degenerate ground states. The ground state is, however, settled into a spin-gapped phase by the lattice distortion via the spin-phonon coupling at 25 K, as shown in Fig. 1 inset [8,10], while most of the Pd(dmit) 2 salts having anisotropic triangular lattice (0.55 < t ′ /t < 0.85) show antiferromagnetic order [9]. Application of hydrostatic pressure, which increases the bandwidth, induces a superconducting transition at T c = 5 K [8]. However, the nature of the ins...

Pressure-induced superconducting behavior was found in two anion radical salts, EtMe3Z[Pd(dmit)2]2 (dmit = 1,3-dithiole-2-thione-4,5-dithiolate, Z = P, As), that are Mott insulators and exhibit different magnetic and structural transitions at ambient pressure.

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The pressure-induced Mott transition is investigated in a quasi-two-dimensional molecular conductor, EtMe 3 P[Pd(dmit) 2 ] 2 (P2 1 /m) with a triangular lattice, which shows a valence bond solid state at ambient pressure. Near the critical pressure of the Mott boundary, an insulator-to-metal transition occurs at 18 K, followed by a first-order reentrant metal-to-insulator transition below 12 K. Finally, a superconducting transition from the insulating state is observed at 5.2 K. The reentrant metal-insulator transition temperature is lowered by application of magnetic field. The result indicates the presence of a spin gap diminished by the Zeeman energy in the reentrant insulating phase neighbouring on the superconducting phase.

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