C. Lenoir

Short range modulation in the 10 K organic superconductor (BEDT-TTF)2Cu(NCS)2

Abstract The 10 K ambient pressure organic superconductor (BEDT-TTF) 2 Cu(NCS) 2 shows a temperature independent short range modulation at the wave vector q =0.26(2) b ∗ . We analyze this modulation as a periodic succession of faults, both in the orientation of neighbouring dimers of BEDT-TTF and in the zig zag sequence of the Cu(NCS) 2 polymeric chains, which could be stabilized by the one dimensional open portions of the Fermi surface.

Dielectric constant and dielectric relaxation of the pinned spin-density wave in the alloys (TMTSF)2(AsF6)1−x(SbF6x

Abstract We present the first measurements of the low frequency dielectric constant for a spin-density wave (SDW) system for a wide range of pinning energies. The pinning is created by a controlled introduction of disorder by alloying in the system (TMTSF) 2 (AsF 6 ) 1− x (SbF 6 ) x , 0 ≤ x ≤ 1. The static dialectric constant is shown to be inversely proportional to the depinning threshold field. At high frequencies the dielectric constant falls off as ω − α with α = 0.7. This frequency dependence is attributed to a distribution of relaxation times of the pinned SDW. We show that for a wide range of pinning energies and temperatures the distribution of the relaxation times characterised by α does not change significantly. The mean dielectric relaxation time is a measure of the damping of the SDW excitations and it can therefore be compared to the dc nonlinear SDW conductivity. At temperatures below 2.5 K the SDW conductivity has a stronger decrease than the increase of damping calculated from the dielectric relaxation. We suggest that this is due to a transition to a commensurate phase in a small fraction of the sample volume.

Far infrared photoconductivity of (TMTSF)2ClO4 in magnetic field

Abstract We report the first far infrared low temperature photoconductivity measurements performed on the Bechgaard salt (TMTSF) 2 ClO 4 in the relaxed state and for magnetic field up to 14 T. Measurements at 2.1 and 3.1 K show the presence of photoconductivity peaks associated with the cascade of field induced spin density wave transitions. The field position of the features appears to be independent of the far infrared wavelength. We show that these structures result from the variation of the SDW gap due to the presence of photoexcited carriers.

Isotopic shifts and Raman line shapes of the organic superconductor β-(BEDT-TTF)2I3

Abstract The behavior vs. temperature of the vibrational modes of normal and 13C enriched β-(BEDT-TTF)2I3 single crystals is reported. The Raman isotopic shift evidences that the inter-ring CC stretching mainly participates to the lowest component of the CC band. Comparison of this line with spectra of ET crystals evinces that the charge transfer comes from the central CC bond. The line shape was fitted at low temperatures using Fano functions with negative asymmetry parameters q. This observation is to be linked to the occurrence of a strong electron-phonon coupling. We thus assume, near Tc, that the phonons couple with a continuum of electronic origin. We conclude that internal CC vibrations may contribute for superconductivity in these systems.

Far-infrared photoconductivity resonance in the FISDW phase of (TMTSF)2ClO4

Abstract The FISDW phase of (TMTSF) 2 ClO 4 is studied by far-infrared photoconductivity. The spectra present a resonance showing up as a peak in the photoconductivity signal. This peak moves to lower magnetic field as the excitation energy is decreased. We report a series of measurements performed between 8 and 16 Tesla for photon energies ranging between 16.7 and 24.4 cm −1 . The results are discussed in connection with the ⪡ Standard ⪢ model for the cascade of FISDW phases.

Cooling rate dependence of the lattice parameters of (TMTSF)/sub 2/ClO/sub 4/

Summary form only given. The quasi-one-dimensional conductor (TMTSF)/sub 2/CIO/sub 4/ exhibits a ground state whose nature depends upon the cooling rate of the sample in the vinicity of the 24 K anion ordering transition. Relaxed (R) samples are superconductor with a T/sub c/ /spl ap/ 1.2 K, while quenched (Q) samples exhibit a spin density wave (SDW) ground state below about 6 K. Although it has been previously established that the fraction and size of ordered domains of C10/sub 4//sup -/ depend strongly on the cooling rate [1], the interplay between structural and electronic degrees of freedom is not clearly understood. In that purpose we present a diffractometric determination of the triclinic lattice parameters between 13 and 300 K. Our results agree with those previously reported by B.Gallois et al. [2]. However the better resolution reveals anomalies below 50 K in the thermal dependence of these parameters which could be related to the freezing of the CH/sub 3/ groups of the TMTSF molecule. We have also performed low temperature measurements of the triclinic lattice parameters in R and Q samples, cooled respectively at 2K/h and 30K/min around 24 K, and found significant differences for the c (CR-cQ/c = - 0.2% ) and Y (YR-YQ/Y = + 0.3%) lattice parameters. The c parameter controls the coupling between (a,b) conducting planes, while the Y angle controls the balance between interchain transfer integrals within these planes. The relative variation of these parameters amounts to that observed[3] in (TMTSF)/sub 2/PF/sub/6 pressurized by several Kbars at low temperature.

Temperature effects on the 13C high resolution NMR in (TMTSF)2X salts

Abstract We report the results of the 13 C NMR experiments performed on TMTSF molecules of the organic conductors (TMTSF) 2 ClO 4 and (TMTSF) 2 ReO 4 . This investigation has allowed to resolve all individual sites of the organic molecule. Strong and widely distributed enhanced nuclear relaxation rates on different sites of the molecule suggest a picture where Coulomb correlations play a major role for the sites at the corners of the molecule. On the system (TMTSF) 2 ReO 4 , continuous variations of the resonance positions are observed when the temperature is decreased down to 182 K, temperature at which the system exhibits a structural transition, leading to a semiconducting state at lower temperature. This experiment constitutes the first observation in direct space of the collapse of the conduction electron density at a metal-semiconductor transition.