Dielectric response of charge induced correlated state in the quasi-one-dimensional conductor (TMTTF)2PF6

Conductivity and permittivity of the quasi-one-dimensionsional organic transfer salt (TMTTF)2PF6 have been measured at low frequencies (10^3-10^7 Hz) between room temperature down to below the temperature of transition into the spin-Peierls state. We interpret the huge real part of the dielectric permittivity (up to 10^6) in the localized state as the realization in this compound of a charge ordered state of Wigner crystal type due to long range Coulomb interaction.Comment: 11 pages, 3 .eps figure

X-ray Investigation of the Magneto-elastic Instability of alpha'-NaV2O5

We present an X-ray diffuse scattering study of the pretransitional structural fluctuations of the magneto-elastic transition in alpha'-NaV2O5. This transition is characterized by the appearance below Tsp~35K of satellite reflections at the reduced wave vector (1/2,1/2,1/4). A large regime of structural fluctuations is measured up to 90 K. These fluctuations are three dimensional between Tsp and ~50K and quasi-one dimensional above ~60K. At 40 K the anisotropy ratio is found to be (xib :xia :xic)= (3.8 : 1.8 : 1), which reveals the importance of transverse interactions in the stabilization of the low temperature phase. We discuss our results within the framework of recent theories dealing with the simultaneous occurrence of a charge ordering, a spin gap and a lattice distortion in this intriguing compound.Comment: Accepted in PRB Rapid.comm. Corrected typos, references added, figures improve

SDW VECTOR AND AMPLITUDE IN (TMTTF)2 SbF6 AND (TMTST)2 ClO4 BY NMR AND ANISOTROPY BY EPR

By NMR, the nesting vector Q and amplitude d of the SDW state in the organic conductors with sulfur (TMTTF)2 SbF6 and selenium (TMTST)2 ClO4 are respectively Q = -0.05b* and 0.12b* and d = 0.18 and 0.12 µB. By EPR the, up to this date, unknown orientation of easy axis has been determined toward crystal axis a in agreement with the theory of anisotropy

Effects of coordination and pressure on sound attenuation, boson peak and elasticity in amorphous solids

Connectedness and applied stress strongly affect elasticity in solids. In various amorphous materials, mechanical stability can be lost either by reducing connectedness or by increasing pressure. We present an effective medium theory of elasticity that extends previous approaches by incorporating the effect of compression, of amplitude e, allowing one to describe quantitative features of sound propagation, transport, the boson peak, and elastic moduli near the elastic instability occurring at a compression ec. The theory disentangles several frequencies characterizing the vibrational spectrum: the onset frequency Image ID:c4sm00561a-t2.gif where strongly-scattered modes appear in the vibrational spectrum, the pressure-independent frequency ω* where the density of states displays a plateau, the boson peak frequency ωBP found to scale as Image ID:c4sm00561a-t3.gif, and the Ioffe-Regel frequency ωIR where scattering length and wavelength become equal. We predict that sound attenuation crosses over from ω4 to ω2 behaviour at ω0, consistent with observations in glasses. We predict that a frequency-dependent length scale ls(ω) and speed of sound ν(ω) characterize vibrational modes, and could be extracted from scattering data. One key result is the prediction of a flat diffusivity above ω0, in agreement with previously unexplained observations. We find that the shear modulus does not vanish at the elastic instability, but drops by a factor of 2. We check our predictions in packings of soft particles and study the case of covalent networks and silica, for which we predict ωIR ≈ ωBP. Overall, our approach unifies sound attenuation, transport and length scales entering elasticity in a single framework where disorder is not the main parameter controlling the boson peak, in agreement with observations. This framework leads to a phase diagram where various glasses can be placed, connecting microscopic structure to vibrational properties

Propriétés orientationnelles et conductrices de polypyrroles mésomorphes

Des polypyrroles à groupes latéraux mésogènes ont été synthétisés dans le but d'avoir des matériaux conducteurs intrinsèquement anisotropes. Les conditions de polymérisation ont été adaptées pour obtenir des polymères mésomorphes. Ces polymères peuvent être orientés macroscopiquement, soit en cours de synthèse en phase orientée, soit à l'issue de la polymérisation. L'orientation est confirmée au niveau des mésogènes par diffraction des rayons X, et au niveau du squelette polymère par diffusion des neutrons aux petits angles. Les conductivités des polypyrroles mésomorphes obtenus sont de l'ordre de 10-3-10-4 S.cm-1. Des mesures effectuées sur des films orientés ont mis nettement en évidence une anisotropie de la conductivité statique