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.
The Shubnikov-de Haas (SdH) eA'ect has been observed in the recently discovered ambientpressure organic superconductor [bis(ethylenedithiolo)tetrathiafulvalenel&Cu(NCS)z. This is the first unambiguous report on the SdH effect in organic compounds. The SdH signal is observed below 1 K, above 8 T, and at angles 8 between the crystal a -axis and a magnetic field of less than 30'. The period 0.0015cose (T ') corresponds to the cylindrical closed Fermi surface obtained by the tight-binding method based on the extended Huckel approximation, which is critically dependent on the crystal symmetry. The temperature dependence of the oscillation amplitude gives the effective mass at the Fermi surface to be about 3.5mo.Since the discovery of an organic superconductor in 1980 (Ref. 1), many studies have been devoted to magnetoresistance measurements.The first report on the Shubnikov-de Haas (SdH) effect in an organic compound was given by Kwak, Schirber, Greene, and Engler, about (TMTSF)2PFs under pressure (TMTSF is tetramethyltetraselenafulvalene).The observed frequency was 0.76 MG (period =0.013 T '). Afterwards, Bando et a/. , reported the SdH effect in (TMTSF)2C104. They observed two types of oscillations with periods of 0.015 and 0.0036 T '. The longer one nearly corresponds to that of (TMTSF)2PFs. But later studies revealed the stepwise variation of the Hall field, indicating an abrupt change of carrier density at the fields corresponding to the longerperiod oscillation. s Therefore, this branch of the oscillation is thought to occur due to a series of phase transitions among different spin-density-wave states rather than the SdH eff'ect.Meanwhile, the shorter period was considered as SdH for a while. But the oscillation amplitude decreases at sufftciently low temperature, which is different from the ordinary SdH behavior. A similar behavior has also been reported on the shorter-period branch found later in (TMTSF)2PFs. ' Therefore, different origins are also proposed for this branch, and now the consensus is that this branch is not SdH, though the origin has not been identified. "'2 Therefore the experimental results so far reported as SdH were apparently due to the SdH effect.
The crystal structures of an organic superconductor (BEDT-TTF)2Cu(NCS)2 (Tc=10.4 K) at 298 K and 104 K are determined by X-ray analysis. The packing pattern of donors is nearly analogous to κ-(BEDT-TTF)2I3. The counter anion, Cu(NCS)2−, has no positional disorder and constructs a peculiar sheet where a copper ion is coordinated with a sulfur and two nitrogen atoms to form a one-dimensional polymer with a permanent dipole moment.
Studies of the IR vibrational spectroscopy of single crystals of the H-bonded quinhydrone chargetransfer complexes show that a new phase transition occurs upon application of hydrostatic pressure, associated with the proton transfer in the 2D proton lattice. The temperature-and pressure-induced changes in the O-H vibrational spectra are accounted for in terms of the melting of the proton lattice due to the proton tunneling effect.
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