Abstract:The metallic state of the molecular conductor β-(meso-DMBEDT-TTF) 2 X (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 = PF 6 , AsF 6 ) is transformed into the checkerboard-type charge-ordered state at around 75-80 K with accompanying metalinsulator (MI) transition on the anisotropic triangular lattice. With lowering temperatures, the magnetic susceptibility decreases gradually and reveals a sudden drop at the MI OPEN ACCESSCrystals 2012, 2 1503 transition. By applying pressure, the charge-ordered state is suppressed and superconductivity appears in β-(meso-DMBEDT-TTF) 2 AsF 6 as well as in the reported β-(meso-DMBEDT-TTF) 2 PF 6 . 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.

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