Nicole Coustel

Compressibility of M3C60 Fullerene Superconductors: Relation Between Tc and Lattice Parameter

X-ray diffraction and diamond anvil techniques were used to measure the isothermal compressibility of K3C60 and Rb3C60, the superconducting, binary alkali-metal intercalation compounds of solid buckminsterfullerene. These results, combined with the pressure dependence of the superconducting onset temperature Tc measured by other groups, establish a universal first-order relation between Tc and the lattice parameter a over a broad range, between 13.9 and 14.5 angstroms. A small secondorder intercalate-specific effect was observed that appears to rule out the participation of intercalate-fullerene optic modes in the pairing interaction.

X-Ray Diffaction Evidence for Nonstoichiometric Rubidium-C60 Intercalation Compounds

Powder x-ray diffraction at 300 K on equilibrated samples of several nominal compositions χ in RbχC60 is reported. In addition to the face-centered cubic (fcc) (χ = 3, superconducting), body-centered tetragonal (χ = 4), and body-centered cubic (bcc) (χ = 6) stoichiometric phases, direct evidence for a dilute fcc doped phase, 0 x c ≤ 1, and for a substoichiometric bcc phase, χ ∼ 5, is presented. In contrast, χ = 3 and χ = 4 appear to be line phases with nearly zero solubility of Rb vacancies and interstitials at 300 K. These results are summarized in a provisional binary phase diagram.

Rotational dynamics and orientational melting of C60: A neutron scattering study

Well‐defined librational excitations have been observed at energies of 2–3 meV in the low temperature ordered phase of solid C60. These relatively high energies imply a stiff orientational potential below the transition. The sharpness of the peaks indicates that this potential does not depend strongly on the axis of the angular displacement. The modes soften and broaden as the temperature approaches that of the orientational melting transition which occurs when the librational amplitude is a considerable fraction of nearest‐neighbor interatomic angles.

Structure and low energy dynamics of solid C60

Abstract A simple model explains both the Bragg peaks and the diffuse scattering observed in neutron diffraction measurements on Solid C 60 at 295 and 14 K. Neutron inelastic scattering measurements reveal dynamic orientational disorder above the order-disorder transition temperature. An analysis of the diffraction results shows that significant disorder remains at 14 K.

Photoluminescence in solid films of C60

Abstract Photoluminescence has been measured for solid films of nominally pure C 60 and C 60 /C 70 mixtures, as a function of temperature and excitation energy. The photoluminescence spectrum shows vibronic structure, and there is evidence for the involvement of three modes. Trace amounts ( 70 appear to be clearly visible in the spectrum of the nominally pure C 60 film.

Spin cast thin films of fullerenes and fluorinated fullerenes: Preparation and characterization by x‐ray reflectivity and surface diffuse x‐ray scattering

We demonstrate that high quality thin films of fullerenes (C60) and fluorinated fullerenes can be prepared from solution by the spin coating technique on float glass or silicon wafers. The films were characterized by x‐ray reflectivity and diffuse x‐ray scattering. A systematic study of films of different thicknesses allows estimation of the distance between the fullerenes spheres to be 10.9±1 A in our C60 films. The C60 film‐air surface is very smooth. Annealing in air above 90 °C leads to an irreversible increase of the film thickness, which is attributed, at present, to oxidation.

High-resolution equilibrium conductivity vs concentration in [(CH)Ky]x

An oriented (CH)x electrode has been studied during five potassium doping/dedoping cycles. Evidence for phase transitions in σ and Voc at y=0.06, 0.12, and 0.16 agree with previous results. We also find new reproducible anomalies in the range 0.02<y<0.06, which occur only during dedoping and are only apparent in conductivity (σ(y) reproducibly exhibits large hysteresis in the 0−0.06 range). The evolution of electronic properties suggests the existence of phase transitions involving “hard” and “soft” degrees of freedom, respectively in the (a,b) plane (perpendicular to the chain axis) and along the c-axis (parallel to the chain axis). For instance, a Voc study performed more slowly exhibits new singularities in the 0.12−0.16 range which we suggest are due to c-axis interactions. Percolation models involving two phases of different but finite conductivities qualitatively reproduce the observed σ(y) behavior. This will be demonstrated both on a simple 1D toy problem and Landauers effective medium theory.

Percolation in Conducting Polymers — Well and Alive!

The existence of ordered dopant structures with well-defined electronic properties in K-doped polyacetylene, transforming into each other through first order phase transitions, implies an important contribution of percolation effects to the doping-dependent conductivity. An effective medium theory of percolation of a good in a bad conductor is successfully used to describe the observed conductivity over most of the accessible doping range and several orders of magnitude in conductivity. Percolation at low doping apparently takes place in reduced dimensions as borne out by the existence of a highly conducting phase around 3% evidenced by ESR.