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

Abstract

Recent studies on the physical properties of the triangular system based on the Pd(dmit) 2 salts (dmit = 1,3-dithiole-2-thione-4,5-dithiolate) are reviewed. Quantum chemical architectures of the Pd(dmit) 2 molecule and its dimer are introduced with emphasis on the strong dimerization of a two-level system, which provides unique physical properties of the salts. The magnetic properties are outlined in view of the magneto-structural correlation specific to the frustrated spin systems. Some newly discovered ground states and their origins are discussed, for which the valence bond formation plays a key role. Among them, the two-level structure is crucial for the novel charge-separated state found in two salts. The valence bond ordering, similar to the spin-Peierls transition, has been found in a two-dimensional frustrated spin system. The physical aspects and possible relation to the pressure-induced superconductivity are discussed.

Cation Dependence of Crystal Structure and Band Parameters in a Series of Molecular Conductors, β'-(Cation)[Pd(dmit)2]2 (dmit = 1,3-dithiole-2-thione-4,5-dithiolate)

“…The metal dithiolene complex Pd(dmit) 2 (dmit = 1,3-dithiole-2-thione-4,5-dithiolate; Figure 1) provides an isostructural series of anion radical salts, β'-(Et x Me 4−x Z)[Pd(dmit) 2 ] 2 (x = 0-2, Z = P, As, Sb) [2][3][4][5]. Here, the monovalent counter cations, Et x Me 4−x Z + , form the insulating part while electrical conduction is attributed to the anion radical of Pd(dmit) 2 .…”

“…In the Mott insulating state, the localized spins exhibit various ground states (antiferromagnetic longrange order [6], quantum spin liquid state [7][8][9][10][11], and non-magnetic charge-ordered state [12][13][14]), depending on the counter cation. In the background of all these transport and magnetic properties, electron correlation and geometrical frustration operate [3,6]. The strength of the electron correlation and degree of frustration are characterized by the intra-and interdimer transfer integrals between the frontier molecular orbitals [2,15].…”

Triangular-lattice anisotropic dimerized Heisenberg antiferromagnet: Stability and excitations of the quantum paramagnetic phase

“…20) and X[Pd(dmit) 2 ] 2 (Refs. [21][22][23][24][25], with X = EtMe 3 Sb or EtMe 3 P), all showing some degree of spatial anisotropy. In the latter two, the crystal lattice is rather soft: pressure can be used to induce an insulator-to-metal transition, with superconductivity appearing at low temperature.…”

Photoinduced Phase Transition in Strongly Electron-Lattice and Electron–Electron Correlated Molecular Crystals

“…, which exhibits a unique charge-ordered insulating phase due to HOMO-LUMO interplay in the Pd(dmit) 2 dimer [30][31][32], crystallizes in a layered structure, in which quasi-two-dimensional conducting anion layers made of Pd(dmit) 2 ( Figure 2a) and insulating cation layers of Et 2 Me 2 Sb are alternately stacked. The crystal structures of the conducting anion layers of the high-and low-temperature phases are schematically shown in Figures 2b,c.…”

“…The valence of the dimers in this low temperature phase is not uniform, and the two types of dimers, which have different intermolecular spacing, are ordered as schematically shown in Figure 2c. The microscopic mechanism involved in this low-temperature phase is based on the unique nature of the multi-level electronic structure of the dimer of Pd(dmit) 2 , HOMO-LUMO interplay in the dimer and the energetically stable neutral dimer (Figure 2d), instead of inter-site Coulombic repulsion which is the origin of the usual charge-ordering phase [32]. Thus, this low temperature phase is referred to as the "charge-separated" (CS) phase to distinguish it from the conventional charge-ordering phase.…”