title: Organic superconductors with an incommensurate anion structure.
abstract: Superconducting incommensurate organic composite crystals based on the methylenedithio-tetraselenafulvalene (MDT-TSF) series donors, where the energy band filling deviates from the usual 3/4-filled, are reviewed. The incommensurate anion potential reconstructs the Fermi surface for both (MDT-TSF)(AuI2)0.436and (MDT-ST)(I3)0.417neither by the fundamental anion periodicityqnor by 2q, but by 3q, where MDT-ST is 5H-2-(1,3-dithiol-2-ylidene)-1,3-diselena-4,6-dithiapentalene, andqis the reciprocal lattice vector of the anion lattice. Theselection ruleof the reconstructing vectors is associated with the magnitude of the incommensurate potential. The considerably large interlayer transfer integral and three-dimensional superconducting properties are due to the direct donor-donor interactions coming from the characteristic corrugated conducting sheet structure. The materials with high superconducting transition temperature,Tc, have large ratios of the observed cyclotron masses to the bare ones, which indicates that the strength of the many-body effect is the major determinant ofTc. (MDT-TS)(AuI2)0.441shows a metal-insulator transition atTMI=50 K, where MDT-TS is 5H-2-(1,3-diselenol-2-ylidene)-1,3,4,6-tetrathiapentalene, and the insulating phase is an antiferromagnet with a high Néel temperature (TN=50 K) and a high spin-flop field (Bsf=6.9 T). There is a possibility that this material is anincommensurate Mott insulator. Hydrostatic pressure suppresses the insulating state and induces superconductivity atTc=3.2 K above 1.05 GPa, whereTcrises to the maximum,Tcmax=4.9 K at 1.27 GPa. This compound shows a usual temperature-pressure phase diagram, in which the superconducting phase borders on the antiferromagnetic insulating phase, despite the unusual band filling.
authors.affiliation: Department of Organic and Polymeric Materials, Graduate School of Science and Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan.
authors.forename: Tadashi
authors.initials: T
authors.lastname: Kawamoto
authors.affiliation: Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527, Japan.
authors.forename: Kazuo
authors.initials: K
authors.lastname: Takimiya
keywords: Fermi surface; incommensurate composite crystal; metal-insulator transition; organic superconductor