The fullerene C(60) can be converted into two different structures by high pressure and temperature. They are metastable and revert to pristine C(60) on reheating to 300 degrees C at ambient pressure. For synthesis temperatures between 300 degrees and 400 degrees C and pressures of 5 gigapascals, a nominal face-centered-cubic structure is produced with a lattice parameter a(o) = 13.6 angstroms. When treated at 500 degrees to 800 degrees C at the same pressure, C(60) transforms into a rhombohedral structure with hexagonal lattice parameters of a(o) = 9.22 angstroms and c(o) = 24.6 angstroms. The intermolecular distance is small enough that a chemical bond can form, in accord with the reduced solubility of the pressure-induced phases. Infrared, Raman, and nuclear magnetic resonance studies show a drastic reduction of icosahedral symmetry, as might occur if the C(60) molecules are linked.

No abstract

We study the transfer of spectral weight in the optical spectra of a strongly correlated electron system as a function of temperature and interaction strength. Within a dynamical mean field theory of the Hubbard model that becomes exact in the limit of large lattice coordination, we predict an anomalous enhancement of spectral weight as a function of temperature in the correlated metallic state and report on experimental measurements which agree with this prediction in V 2 O 3 . We argue that the optical conductivity anomalies in the metal are connected to the proximity to a crossover region in the phase diagram of the model. 71.27.+a, 71.30.+h, 78 Typeset using REVT E X 1

No abstract

Millikelvin measurements of the conductivity as a function of donor density and uniaxial stress in bulk samples of phosphorus-doped silicon establish that the transition from metal to insulator is continuous, but sharper than predicted by scaling theories of localization. The divergence of the dielectric susceptibility as the transition is approached from below also points out problems in current scaling theories. The temperature dependence of the conductivity and the magnetoresistance in the metal indicate the importance of Coulomb interactions in describing the behavior of disordered systems.

A low temperature mobility up to 10 000 cm2 V−1 s−1 at 1.7 K was observed for a new polymorphic form of pentacene (C22H14, see picture). Optical measurements show a band gap of the order of 2 eV, and transport measurements revealed a mobility above 3 cm2 V−1 s−1 at room temperature.

No abstract

A 204, 406 (1950