Abstract
The metallic state of the molecular conductor β-(meso-DMBEDT-TTF)2X (DMBEDT-TTF = 2-(5,6-dihydro-1,3-dithiolo[4,5-b][1,4]dithiin-2-ylidene)-5,6-dihydro-5,6-dimethyl-1,3-dithiolo[4,5-b][1,4]dithiin, X = PF6, AsF6) is transformed into the checkerboard-type charge-ordered state at around 75–80 K with accompanying metal-insulator (MI) transition on the anisotropic triangular lattice. With lowering temperatures, the magnetic susceptibility decreases gradually and reveals a sudden drop at the MI transition. By applying pressure, the charge-ordered state is suppressed and superconductivity appears in β-(meso-DMBEDT-TTF)2AsF6 as well as in the reported β-(meso-DMBEDT-TTF)2PF6. The charge-ordered spin-gapped state and the pressure-induced superconducting state are discussed through the paired-electron crystal (PEC) model, where the spin-bonded electron pairs stay and become mobile in the crystal, namely the valence-bond solid (VBS) and the resonant valence bonded (RVB) state in the quarter-filled band structure.
Highlights
IntroductionMolecular conductors have attracted much interest as highly correlated electron systems
Molecular conductors have attracted much interest as highly correlated electron systems.Most molecular charge transfer (CT) complexes have [(donor)2]1+:1− = 2:1 stoichiometry and a3/4-filled band structure
The complex β-(meso-DMBEDT-TTF)2AsF6 is isostructural to the complex β-(meso-DMBEDTTTF)2PF6 both at 298 K and 30 K
Summary
Molecular conductors have attracted much interest as highly correlated electron systems. 1/2-filled band for κ and β'-type BEDT-TTF complexes, whereas the non-dimerized one reveals the charge-ordered state for θ-type complexes [1,2]. Among the CT salts based on BEDT-TTF derivatives, the complex β-(meso-DMBEDT-TTF)2PF6 with 3/4-filled band structure shows relatively weak metallic behavior from room temperature to 75 K and at an ambient pressure (Figure 1, [4,7]). In order to clarify the origin of the checkerboard-type CO and its relation to the superconductivity, the preparation, the crystal structure analysis at 30 K, and the electrical and the magnetic properties of β-(meso-DMBEDT-TTF)2AsF6 were investigated in addition to the PF6 complex [3,4,5,6,7,8,9]
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