An ambient pressure superconductivity of (BEDT-TTF)2Cu(SCN)2 was observed by d.c. magnetic susceptibility and electrical conductivity measurements. The superconducting critical temperature is the highest (TC=10.4 K) among the organic superconductors so far obtained, even though the anion has a positional disorder in the crystal.

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The magnetism of an insulator, tr-(BEDT-TTF) &Cu[N(CN) 2]C1, situated near the 10 K superconducting phase in~-type BEDT-TTF family, has been studied by 'H NMR for the first time. A commensurate antiferromagnetic ordering with a moment of (0.4 -1.0)ILe/dimer was found below 26 -27 K. The features, quite different from the conventional spin density wave, suggest that the magnetic order is driven by strong electron correlation rather than by nesting of the Fermi surface, and that the present salt is a Mott insulator. Magnetization measurements show spin canting parallel to the conducting layer below 23 K at low fields. PACS numbers: 74.70.Kn, 74.25.Nf The metal-insulator (M-I) transition has been always a center of interest in the physics of organic conductors [1]. The well known material, TTF-TCNQ (tetrathiafulvalenetetracyanoquinodimethane), which is the first generation of organic conductors, undergoes a transition into a charge-density-wave (CDW) insulator from a metallic state around 53 K. The second generation, TMTSF (tetramethyltetraselenafulvalene), compounds also show a spin-density-wave (SDW) transition around 10 K. Although the type of insulators is different in these epochmaking compounds, the mechanism of the transition has a common nature; that is, nesting of an open Fermi surface in one-dimensional electronic systems. The third generation, BEDT-TTF [bis(ethylenedithio)tetrathiafulvalene], compounds are novel in that many salts have quasi-twodimensional nature in the electronic state [2], where the Fermi surface is closed and is against nesting. Thus, the metallic state in many BEDT-TTF salts is kept stabilized down to lower temperatures, being free from the M-I transitions encountered in quasi-one-dimensional systems. Representative of them is the~-phase family of BEDT-TTF compounds, which include superconductors, tr-(BEDT-TTF)2X [X = Cu(NCS)2 [3] and Cu[N(CN)2]Br [4]], with transition temperatures in excess of 10 K. In spite of structural similarity, however, the salt with X = Cu[N(CN) 2]C1 is an insulator [5]. Considering the two-dimensionality of the electronic state in the~-phase family, one cannot take for granted the fact that the same scenario as in the above one-dimensional conductors will work for the M-I transition in this case. There is a speculation on the origin of the insulating state based on lattice disorder due to conformational disorder of the ethylene groups of BEDT-TTF [6]. Characterization of the insulating phase is important for an understanding of the origin of the M-I transition in the~-phase family in terms of two-dimensional systems of correlated electrons. In the present work, the magnetism of tr-(BEDT-TTF) 2Cu[N(CN) 2]C1 at ambient pressure has been investigated by NMR for the first time, along with magnetization measurements. This letter reports on unambiguous evidence for antiferromagnetic ordering at 27 K and spin canting below 23 K. A spin structure different from conventional SDW is also uncovered. Single crystals of tc-(BEDT-TTF) 2Cu[N(CN)2]C1 were grown by the conven...

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Charge distribution in the insulating phase of -(BEDT-TTF) 2 RbZn(SCN) 4 ͓BEDT-TTF denotes bis͑ethyl-enedithiolo͒tetrathiafulvalene͔ is investigated using the polarized Raman and infrared ͑IR͒ spectroscopy. Both of the Raman and IR spectra showed a multiple-peaks pattern below metal-insulator transition temperature. We assigned the carbon-double-bond (CvC) stretching-modes based on the isotope shift in the 13 C-substituted sample, and performed a factor-group analysis for the polarized IR and Raman spectra. These results proved that charge disproportionation ͑CD͒ takes place in the insulating phase, and that the spatial pattern of the charge ordering agrees with the symmetry determined by the x-ray diffraction. The CD ratio was estimated from the frequency of a ring CvC stretching mode that is weakly coupled with electronic excited states. A systematic way to investigate the CD phenomenon based on vibrational spectroscopy is discussed.

We performed the single-crystal 13C NMR studies on a quasi-two-dimensional system, deuterated kappa-(BEDT-TTF)2Cu[N(CN)(2)]Br, which is just on the border of the Mott transition. The NMR spectra are separated into two parts coming from the metallic (superconducting) and insulating phases due to the phase separation at low temperature. The examination of the separated spectra revealed that the Mott transition in this system is characterized by the first-order transition between the pseudogapped superconductor and the simplest commensurate antiferromagnet with a moment of 0.26 mu(B)/dimer.

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