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Crystals 2012, 2(4), 1502-1513; doi:10.3390/cryst2041502
Article

Magnetism and Pressure-Induced Superconductivity of Checkerboard-Type Charge-Ordered Molecular Conductor β-(meso-DMBEDT-TTF)2X (X = PF6 and AsF6)

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1 The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277–8581, Japan 2 Department of Chemistry, Graduate School of Science, Kobe University, Kobe, Hyogo 657–8501, Japan 3 Research Center for Neutron Science and Technology, CROSS, Tokai–mura 319–1106, Japan 4 Condensed Matter Research Center and Photon Factory, Institute of Materials Structure Science, KEK, Tsukuba 305–0801, Japan 5 Department of Physics, Toho University, Chiba 274-8510, Japan
* Author to whom correspondence should be addressed.
Received: 12 September 2012 / Revised: 5 November 2012 / Accepted: 14 November 2012 / Published: 29 November 2012
(This article belongs to the Special Issue Molecular Conductors)
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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.
Keywords: superconductor; dimer state; checkerboard-type charge ordering; singlet state; valence bond solid; paired-electron crystal superconductor; dimer state; checkerboard-type charge ordering; singlet state; valence bond solid; paired-electron crystal
This is an open access article distributed under the Creative Commons Attribution License (CC BY 3.0).
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Shikama, T.; Shimokawa, T.; Lee, S.; Isono, T.; Ueda, A.; Takahashi, K.; Nakao, A.; Kumai, R.; Nakao, H.; Kobayashi, K.; Murakami, Y.; Kimata, M.; Tajima, H.; Matsubayashi, K.; Uwatoko, Y.; Nishio, Y.; Kajita, K.; Mori, H. Magnetism and Pressure-Induced Superconductivity of Checkerboard-Type Charge-Ordered Molecular Conductor β-(meso-DMBEDT-TTF)2X (X = PF6 and AsF6). Crystals 2012, 2, 1502-1513.

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