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 polarized absorption spectra over the range, 5000–37000 cm−1, were observed on the single crystals of the highly-conductive, mixed-valence tetrathiafulvalenium salts, (TTF)Br0.71, (TTF)I0.71, and (TTF)(SCN)0.57, by using a microspectrophotometer. The spectra of these crystals were very similar to each other. The observed spectra were successfully interpreted in terms of the localized model, considering the charge transfer transitions between TTF species and the local excitations associated with the intramolecular transitions of the molecule and ion of TTF. The spectrum of the (TTF) (ClO4) crystal were also studied to aid the interpretations of the spectra of the mixed-valence TTF salts.

We use zero-and longitudinal-field muon spin rotation and relaxation to study the muon states in the organic molecular ferromagnet: j3-phase para-nitrophenyl nitronyl nitroxide (p-NPNN). Below Tc = 0.65 K, oscillations in the relaxation function are observed in zero applied field which follow the magnetic order parameter, implying that the muon is bonded in an electronic spin-singlet state. We also study the behaviour of the oscillations in an applied longitudinal field and interpret the results by considering the magnetisation process and the demagnetising field. A significant field-induced repolarisation is also observed which is ascribed to the presence of spin-triplet states formed by a fraction of muons.Recently, there has been much interest in the search for a purely organic molecular ferromagnet which contains only light elements (like carbon, nitrogen, hydrogen, and oxygen) [l]. The frst such material to be found, the j3 crystal phase of para-nitrophenyl nitronyl nitroxide (p-NPNN, see fig. lb)), was reported to have a Curie temperature (T,) of 0.60K, a discovery which was made using heat capacity and magnetisation measurements [2,3]. The unpaired spin in this nitronyl nitroxide is associated with the N-0 groups. The role of the rest of the molecule is to ensure the appropriate overlap of the correct orbitals on neighbouring molecules to produce 3D ferromagnetism. A whole series of materials which incorporate this nitronyl nitroxide group are currently being synthesized in order to provide a systematic study of the effects of the variation of the rest of the molecule on the molecular packing and overlap and hence on the magnetic properties.Zero-field (ZF) muon spin rotation/relaxation (pSR) studies on aligned single crystals [4]and polycrystalline [51 samples have also demonstrated magnetic order with Tc -0.65 K; in these experiments, the muon spin precession rate directly yields the value of the internal magnetic field at the muon site. In a pSR experiment, a beam of spin-polarized muons is stopped in a target specimen. Each muon subsequently decays (with a mean lifetime of z p = = 2.2 ps), emitting a positron preferentially along the muon spin direction because of the parity violation of the weak interaction [6]. The time histograms N * (t) of positrons counted in the 8 Les Editions de Physique

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Polarized reflectance spectra of the organic superconductors protonated and deuterated~-(BEDT-TTF)2[Cu(NCS)2] (H and D salts) [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene] were measured over the range from 500 to 28000 cm ' at room temperature with light polarizations parallel to the crystallographic b and c axes which lie on the two-dimensional conducting plane. Polarized reflectance spectra of the organic superconductor P-(BEDT-TTF)2I3 and the organic metal P"-(BEDT-TTF)2AuBr2 were also measured in order to discuss the influence of different molecular arrangements and hydrogen-anion contacts on the electronic and vibrational properties of these salts. Frequency-dependent conductivities were calculated by a Kramers-Kronig transforrnation. By comparison of the infrared conductivity spectra of the H and D salts, the vibrational transitions induced by electronmolecular-vibration {EMV) coupling were clearly distinguished from the carbon-hydrogen bending modes of the BEDT-TTF moiety. A Drude-Lorentz dielectric function was used to evaluate the optical transport parameters and an excitation frequency of the charge-transfer {CT) band superimposed on a plasma-edge-like dispersion which was observed for each compound. The EMV-coupling energies are semiquantitatively estimated to be ca. 70 meV for both the H and D salts from the frequencies of the EMV coupling transition and the CT band in terms of the dimer charge-oscillation model. By use of the coupling energy, various parameters describing the superconducting state were evaluated and discussed on the basis of the BCS theory in a weak-coupling limit. Finally, the magnitudes of hydrogen-anion interaction were estimated from the frequency shifts of the C -H bending modes of the BEDT-TTF moiety.