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• Shikama, T
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• Lee, S
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• Kobayashi, K
• Murakami, Y
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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)

Takahisa Shikama ¹ , Tatsuya Shimokawa ¹ , Sanguchul Lee ¹ , Takayuki Isono ¹ , Akira Ueda ¹ , Kazuyuki Takahashi ² , Akiko Nakao ³ , Reiji Kumai ⁴ , Hironori Nakao ⁴ , Kensuke Kobayashi ⁴ , Youichi Murakami ⁴ , Motoi Kimata ¹ , Hiroyuki Tajima ¹ , Kazuyuki Matsubayashi ¹ , Yoshiya Uwatoko ¹ , Yutaka Nishio ⁵ , Koji Kajita ⁵  and Hatsumi Mori ^1,*

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; in revised form: 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

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