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We report the studies of the elasticity of ${\mathrm{O}}_{60}$ single crystals. For sublimed fcc crystals, Young's modulus has an 8% jump at the first-order transition at 260 K. At \ensuremath{\sim}160 K there is a frequency-dependent elastic anomaly resulting from time dependent stress relaxation. Comparison with rotation rates seen in NMR suggests that the dynamics below 260 K is more complex than jumps between equivalent molecular configurations. Solvent grown monoclinic crystals have a second-order transition at 242 K, while the 160-K glass transition remains unchanged.

Crystals of C60 or C70 cocrystallized with n-pentane grow as elongated, ten-sided columns. X-ray diffraction shows ordering of C60 or C70 molecules along the column and a remarkable tenfold symmetry normal to the column. The ratio of the x-ray-diffraction vectors of the two lowest-order diffraction spots is nearly equal to the “golden ratio,” τ. Despite these similarities with decagonal, quasicrystalline order, the diffraction indicates crystalline order with a twinned unit cell.

Composite As2Se3/polyethersulfone fibers are fabricated that have As2Se3 core diameters between 0.5 and 3 microns surrounded by a PES cladding 50 to 150 microns in diameter. The small core areas along with the large nonlinear coefficients found for As2Se3 make these fibers excellent candidates for compact low-cost nonlinear devices.

We report optical reflectivity data for single crystals of C 60 in the energy range 1.8–4.8 eV. A Kramers—Kronig analysis of the reflectivity spectrum yields an absorption spectrum similar to that of thin films of polycrystalline C 60 . Based on the general features of the linear absorption spectrum, it would appear unlikely that crystalline C 60 manifests exceptionally large second- or third-order optical nonlinearities.

THE recent syntheses1,2 of macroscopic quantities of C60 have suggested possible applications in host–guest and organic chemistry, tribology, electrochemistry and semiconductor tech-nology. Here we report the preparation of alkali-metal-doped films of C60 and C70 which have electrical conductivities at room temperature that are comparable to those attained by n-type doped polyacetylene. The highest conductivities observed in the doped films are: 4Scm−1 (Cs/C60), 100 (Rb/C60), 500 (K/C60), 20 (Na/C60), 10 (Li/C60), 2 (K/C70). The doping process is reversed on exposure of the films to the atmosphere. At high doping levels, the films become more resistive. We attribute the conductivity induced in these films to the formation of energy bands from the π orbitals of C60 or C70, which become partially filled with carriers on doping. The smaller alkali metal ions should be able to fit into the interstices in the lattice without disrupting the network of contacts between the carbon spheroids. In the case of C60, this would allow the development of an isotropic band structure, and we therefore propose that these materials may constitute the first three-dimensional 'organic' conductors.

Non-oxide glasses1-5 find an increasing use in fiber optic applications. Most of the current research activity is in the halide glass fibers used as active hosts for rare-earth ion doping. These include, 1.3 and 1.5 μm optical amplifiers, frequency upconversion devices, fiber lasers, permanent gratings, etc. Other non-oxide glasses, chalcogenides and mixed chalcogenide halide glasses have potential in fiber applications in IR laser power deliverers, image-guides and sensors. Some of the important issues that require further research include, the glass purification, stable glass compositions, hermetic coatings for fibers, better methods for preform and fiber fabrication.

We have grown linearly compositionally graded GexSi1−x structures at high temperatures (700–900°C) on Si substrates to form a surface which resembles a GexSi1−x substrate. We have obtained completely relaxed structures with x≤0.50 and threading dislocation densities in the 105cm−2 - 106cm−2 range. Because of the very low threading dislocation densities, the structures appear dislocation free in conventional transmission electron microscopy (TEM) cross-section and plan view. Employing the electron beam induced current technique (EBIC), we were able to consistently measure these low threading dislocation densities. A direct comparison of two x=0.35 films, one graded in Ge content and one uniform in Ge content, shows that compositional grading decreases the dislocation density by a factor of 100–1000. These. higher quality graded buffers have been used as templates for the subsequent growth of InGaP light emitting diodes (LED) and GexSi1−x/Si two-dimensional electron gas (2DEG) structures. Room temperature operation of orange-red LEDs were obtained at current densities of =600A/cm, and mobilities as high as 96,000 cm2/V-s were achieved at 4.2K in the 2DEG structures.

The elasticity tensor of a single-grain quasicrystal of composition ${\mathrm{Al}}_{5.1}$${\mathrm{CuLi}}_{3}$ has been determined. The longitudinal and transverse mode velocities have been obtained at ultrasonic frequencies (20 MHz) for several propagation and polarization directions. We find that the quasicrystal is elastically isotropic within the experimental uncertainty of 2%. This finding supports hydrodynamic analyses which predict isotropy. At 300 K, the measurements yield 6.4\ifmmode\times\else\texttimes\fi{}${10}^{5}$ cm ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ and 3.8\ifmmode\times\else\texttimes\fi{}${10}^{5}$ cm ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ for longitudinal and transverse velocities, respectively, and a Debye temperature FTHETA of 493 K.

High-quality single-crystal Sc2O3 films a few nanometer thick have been grown epitaxially on Si (111) despite a huge lattice mismatch. The films were electron-beam evaporated from a Sc2O3 target. Structural and morphological studies were carried out by x-ray diffraction and reflectivity, atomic force microscopy, high-resolution transmission electron microscopy, and medium-energy ion scattering, with the initial epitaxial growth monitored by in situ reflection high-energy electron diffraction. The films have the cubic bixbyite phase with a remarkably uniform thickness and high structural perfection. The film surfaces are very smooth and the oxide/Si interfaces are atomically sharp with a low average roughness of 0.06nm. The films are well aligned with the Si substrate with an orientation relationship of Si(111)‖Sc2O3(111), and an in-plane expitaxy of Si[1¯10]‖Sc2O3[1¯01].

If a material is very nearly isotropic, quantifying the residual anisotropy may be quite difficult using methods such as pulse superposition or neutron scattering, when only a small sample is available. However, resonant ultrasound spectroscopy using piezoelectric film transducers allows precise (

A brief review is given on some of the most interesting structural and electrical properties in the C60 fullerenes. The elasticity study is reported on for C60 single crystals of different lattice symmetries. The temperature dependence of the sound velocity in the fcc crystals shows a discontinuous jump corresponding to about 8% increase in the modulus at an orientational ordering transition temperature T=260 K. This large change in the elastic modulus indicates a drastic change in the interaction mechanism between the molecules. At T∼160 K, the sound velocity is observed to increase considerably accompanied by a profound peak in the sound attenuation whose position is frequency dependent. The 160 K anomaly is associated with relaxations of the C60 molecules to symmetry‐inequivalent orientations in response to the stress field with the characteristic relaxation frequency following an activated temperature dependence and coinciding with the measurement frequency of about 20 kHz around 160 K. In the monocli...

We have carried out an extensive high resolution x-ray scattering study of the nematic-smectic A (N-S A) and reentrant nematic-smectic A (RN-SA) phase transition behaviour in mixtures of octyloxycyanobiphenyl (8OCB and hexyloxycyanobiphenyl (60CB). The smectic A phase boundary is found to be parabolic in the temperature-concentration plane with a median temperature TM = 38.06 °C and a critical concentration y0 = 0.427; here y is the 60CB :80CB molecular ratio. Detailed studies of the smectic fluctuations in the nematic phase are reported for y = 0.33, 0.413, 0.420, 0.429, 0.440 and 0.443. The first three concentrations exhibit N-SA and RN-SA transitions, whereas in the latter three samples with decreasing temperature the smectic correlation lengths and susceptibility exhibit maxima at TM and then decrease with a further decrease in temperature. The data are analysed using an extension of the Pershan-Prost optimal density theory. All of the data are well-described by the phenomenological theory; the critical exponents so-obtained are v ∥ = 0.76 ± 0.03, v⊥ = 0.62 ± 0.05 and γ = 1.49 ± 0.07. These agree quantitatively with values obtained in single layer materials with comparable nematic ranges; thus the N-SA and RN-SA transitions are identical in character to conventional N-SA transitions provided that one includes the crossover effects inherent in the parabolic phase boundary. Studies of the in-plane fluid structure factor in the N, SA and RN phases show that the mean molecular spacings and positional correlations are closely similar to those in single layer materials; in each phase the structure factor is well described by a circularly-averaged Lorentzian with a correlation length of 6.9 A. These results argue strongly against pairing models for the reentrant behaviour. We conclude with a phenomenological discussion of the optimal density model which is so successful here and models based on competing order parameters which are required to describe the varied behaviour observed in other polar materials.

We report stable ${\mathrm{Al}}_{65}$${\mathrm{Cu}}_{15}$${\mathrm{Co}}_{20}$ decagonal quasicrystals which exhibit faceted columnar morphology. The column axis is along the tenfold direction, and it is faceted by the ten twofold planes. Faceted single grains, several millimeters in size were grown and characterized. Peritectic growth is fastest along the periodic tenfold axis and slowest along the nucleation controlled, quasiperiodic twofold directions. Single-crystal x-ray scattering verifies the two-dimensional quasiperiodic structure. The two-dimensional quasicrystalline structure is defect free on a length scale of 2000 A\r{} and is layered with a thickness of 8.26 A\r{}.

Etude experimentale du mouvement force d'un defaut lineaire, a vitesse constante, sous l'action d'une tension appliquee V. Etude sur le 5CB

The melting transition of the two-dimensional stripe-domain phase of bromine layers intercalated into graphite has been investigated by high-resolution x-ray diffraction. The transition is characterized by the progressive vanishing of successive harmonics so that near melting the density is described by a two-dimensional sine wave. Surprisingly, the sine-wave order parameter is best characterized by the power-law form, r/sup -//sup eta//sup =/, with eta/sub 1/ evolving continuously from 0.04 to --2.0 through melting.

NANOMETRE-SCALE semiconductor quantum crystallites exhibit size-dependent and discrete excited electronic states which occur at energies higher than the band gap of the corresponding bulk solid1–4. These crystallites are too small to have continuous energy bands, even though a bulk crystal structure is present. The onset of such quantum properties sets a fundamental limit to device miniaturization in microelectronics5. Structures with either one, two or all three dimensions on the nanometer scale are of particular interest in solid state physics6. We report here our discovery of the biosynthesis of quantum crystallites in yeasts Candida glabrata and Schizosaccharomyces pombe, cultured in the presence of cad-mium salts. Short chelating peptides of general structure (γ-Glu-Cys)n-Gly control the nucleation and growth of CdS crystallites to peptide-capped intracellular particles of diameter 20 A. These quantum CdS crystallites are more monodisperse than CdS par-ticles synthesized chemically. X-ray data indicate that, at this small size, the CdS structure differs from that of bulk CdS and tends towards a six-coordinate rock-salt structure.