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...
Measurements of the dc resistivity of surface-state electrons on liquid helium exposed to microwave radiation are reported. It is shown that the resonant microwave excitation of surface-state electrons is accompanied by a strong increase in their resistivity, which is opposite to the result expected from the previously used two-level model. We show that even a very small fraction of electrons excited to the first excited state and decaying back due to vapor-atom scattering strongly heat the electron system, causing a population of higher subbands. The calculated resistivity change is in good agreement with the observed data.
Nonlinear transport of electron crystal floating on superfluid 4He is investigated in channels 8 and 15 mum in width, where the electron velocity and driving electric field are uniform. At a high excitation, we observe a jump in the velocity caused by the decoupling of the electrons from the underlying surface deformation. The obtained driving field at the jump indicates that the decoupling occurs from the dynamically deepened surface deformation as a result of the Bragg-Cherenkov scattering of surface waves. Our results also account for the unusual nonlinear transport reported by Glasson et al. [Phys. Rev. Lett. 87, 176802 (2001)10.1103/PhysRevLett.87.176802] considering the electrode geometry.
We demonstrate, for the first time, that a quasi-one-dimensional Wigner crystal formed on superfluid (4)He with only a few electrons in the confined direction shows reentrant melting. By transport measurements, we find oscillations in current as a function of linear density measured at a fixed driving voltage at high temperatures, and detailed analyses of transport data reveal that the oscillations originate from the periodic reduction of the melting temperature as a function of linear density. Comparison with the structural phase diagram suggests that the reduction of the melting temperature occurs at the boundaries between the different structures as the structure evolves from a single, double, followed by a triple chain.
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