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

Abstract

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...

Paired electron crystal: Order from frustration in the quarter-filled band

“…It may be relevant in this context that 13 C-NMR experiments on both κ-CN 84 and dmitSb 91 find unusual line broadenings at low T that cannot be ascribed to disorder 92 , which might also indicate charge disproportionation. Similar line broadening at low T in EtMe 3 P[(dmit) 2 ] 2 occurs at the transition to the PEC 77 , which in turn gives way to superconductivity under pressure 82 .…”

“…The insulating ground state in κ-CN is close to SC, transition to superconductivity occurring under moderate pressure 81 . SC is found in the dmit family as well 82 .…”

“…What is interesting though is that the proposed scenario can give a unified approach to SC in all organic CTS, irrespective of whether the insulating state proximate to SC is AFM 102 , CO 67,103 or VBS 82 . Recall that within existing mean field theories the AFM-to-SC transition is driven by spin fluctuations [13][14][15][16][17][18][19][20][21][22][23] , while the CO-to-SC transition is driven by charge fluctuations 65 .…”

Variety of valence bond states formed of frustrated spins on triangular lattices based on a two-level system Pd(dmit)2

“…The stability is diminished by application of high magnetic field as to break the valence bond pairs. The field-induced insulator-metal transition is in fact observed by the resistivity measurements in the 4-8 T range at the pressure just near the insulator-metal boundary [74]. As expected for the valence bond related superconductivity, the highest T c 5.5 K is attained at the insulator-metal boundary.…”

“…It is the first report of bulk Meissner signals (figure 7) detected in the Z [Pd(dmit) 2 ] 2 series. Taking account of the resistivity results [16,74], the P-T phase diagram is inferred from the magnetic experiments [73] ( figure 7). An outstanding point is that the superconducting phase adjoins the spin-gapped insulating phase.…”