Showing papers on "Crystal published in 1994"

Plasma crystal: Coulomb crystallization in a dusty plasma.

Abstract: A macroscopic Coulomb crystal of solid particles in a plasma has been observed Images of a cloud of $7\ensuremath{-}\ensuremath{\mu}m$ "dust" particles, which are charged and levitated in a weakly ionized argon plasma, reveal a hexagonal crystal structure The crystal is visible to the unaided eye The particles are cooled by neutral gas to 310 K, and their charge is $g9800e$, corresponding to a Coulomb coupling parameter $\ensuremath{\Gamma}g20 700$ For such a large $\ensuremath{\Gamma}$ value, strongly coupled plasma theory predicts that the particles should organize in a Coulomb solid, in agreement with our observations

Atomic Control of the SrTiO3 Crystal Surface

Abstract: The atomically smooth SrTiO3 (100) with steps one unit cell in height was obtained by treating the crystal surface with a pH-controlled NH4F-HF solution. The homoepitaxy of SrTiO3 film on the crystal surface proceeds in a perfect layer-by-layer mode as verified by reflection high-energy electron diffraction and atomic force microscopy. Ion scattering spectroscopy revealed that the TiO2 atomic plane terminated the as-treated clean surface and that the terminating atomic layer could be tuned to the SrO atomic plane by homooepitaxial growth. This technology provides a well-defined substrate surface for atomically regulated epitaxial growth of such perovskite oxide films as YBa2Cu3O7-δ.

Crystal field effects on the topological properties of the electron density in molecular crystals: The case of urea

Abstract: The Quantum Theory of Atoms in Molecules, due to Bader, is applied to periodic systems. Results for molecular and crystalline urea are presented. Changes in both bond critical points and atomic properties due to changes of chemical environment are described. A rationale for the different lengths of the in‐plane and out‐of‐plane hydrogen bonds and for the lengthening of the CO bond in bulk urea is provided in terms of the properties of the Laplacian of the oxygen atom electron density distribution. An evaluation of molecular and atomic volume changes indicates that the decrease of molecular volume upon change of phase from gas to solid originates primarily from a contraction of the atomic basins directly involved in hydrogen bonds. Other atoms show a small expansion. The considerable decrease of oxygen and hydrogen atomic volumes is related to the mutual penetration of their van der Waals envelopes following hydrogen bond formation. The results confirm that urea is more polar in the solid phase.

Observation of Coulomb-Crystal Formation from Carbon Particles Grown in a Methane Plasma

Abstract: A Coulomb crystal was successfully formed as a result of the growth of spherical and monodisperse carbon particles suspended in a methane plasma. The crystal structure was confirmed to be hexagonal from top- and side-view photographs. The particle growth was monitored by Mie-scattering ellipsometry and correlated with the formation process of the Coulomb crystal. The liquid-to-solid phase transition occurred when particle diameter grew to 1.3 µm, and when the Wigner-Seitz radius was about 90 µm.

Two crystal structures of the B1 immunoglobulin-binding domain of streptococcal protein G and comparison with NMR.

Abstract: The structure of the 56-residue B1 immunoglobulin-binding domain from streptococcal protein G has been determined in two different crystal forms. The crystal structures were deduced by molecular replacement, based on the structure of the B2 domain (Brookhaven accession code 1PGX). Final R values are 0.174 and 0.198 for orthorhombic and trigonal forms, for diffraction data from 6.0 to 2.07 A and from 6 to 1.92 A, respectively. The orthorhombic crystals have an unusually high packing density for protein crystals, with Vm = 1.66 and a solvent content of 26%. The protein structure is found to be very similar (rms deviation 0.25 A for 56 C alpha's) in the two crystal forms, with an efficiently packed hydrophobic core between a four-stranded beta-sheet and a four-turn alpha-helix. The B1 domain has the same fold and general structure as the B2 domain (rms deviations 0.36 and 0.39 A), despite the six residue differences between them. The crystallographic models differ from NMR-derived models in several local regions, primarily in the loop involving residues 46-51; other significant variations are observed in the helix and in the structure of bound water. The primary crystal contact is the same in both crystal forms, involving both sheet edges to form extended beta-sheets throughout the crystals.

Spectroscopic characterization and laser performance of diode-laser-pumped Nd: GdVO4

Abstract: We have carried out a detailed spectroscopic characterization of Nd: GdVO4, a new laser crystal with high effective absorption and emission cross sections. The accidental degeneracy of the upper4 F 3/2 laser level decreases the number of emission lines and creates -together with the anisotropic crystal field - high emission cross sections (7.6 × 10−19 cm2 at 1.06 µm and 300 K). In addition, the lines are strongly homogeneously broadened (1.6 nm for the 808.4 nm diode-laser pump transition). The temperature dependences of lifetime, linewidths, and cross sections have been determined. Slope efficiencies up to 57% with respect to the absorbed diode-laser pump power and output powers up to 0.8 W have been achieved at 1.06 µm from a 2 mm long crystal. Intracavity second-harmonic generation, using a KTP crystal, is demonstrated.

Measurement of a three-dimensional photonic band gap in a crystal structure made of dielectric rods.

Abstract: We have designed, fabricated, and tested a three-dimensional photonic-band-gap structure. The structure is built by an orderly stacking of dielectric rods. A network analyzer is used to measure the transmission and dispersion relations of electromagnetic waves along several crystal directions. The experimental results are in good agreement with the theoretical calculations. The structure is different from that previously suggested, as it is already used to improve the midgap frequency by almost two orders of magnitude, and it offers a viable geometry to extend the photonic band gap to optical regimes.

Density of amorphous si

Abstract: The density of amorphous Si has been measured Multiple Si implants, at energies up to 80 MeV, were made through a contact mask to produce alternating amorphous/crystalline Si stripes with amorphous thicknesses up to ∼50 μm For layers up to 34 μm (5 MeV), the amorphous Si is constrained laterally and deforms plastically Above 5 MeV, plastic deformation of the surrounding crystal matrix is observed Height differences between the amorphous and crystalline regions were measured for as‐implanted, thermally relaxed, and partially recrystallized samples using a surface profilometer Combined with ion channeling measurements of the layer thickness, amorphous Si was determined to be 18±01% less dense than crystalline Si (490×1022 atom/cm3 at 300 K) Both relaxed and unrelaxed amorphous Si show identical densities within experimental error (<01% density difference)

Chemical order in amorphous silicon carbide.

Abstract: While ordering in alloy crystals is well understood, short-range ordering in amorphous alloys remains controversial. Here, by studying computer-generated models of amorphous SiC, we show that there are two principal factors controlling the degree of chemical order in amorphous covalent alloys. One, the chemical preference for mixed bonds, is much the same in crystalline and amorphous materials. However, the other factor, the atomic size difference, is far less effective at driving ordering in amorphous material than in the crystal. As a result, the amorphous phase may show either strong ordering (as in GaAs), or weaker ordering (as in SiC), depending upon the relative importance of these two factors.

Studies of the radiative properties of ice and mixed-phase clouds

Abstract: Radiative parametrizations for both ice and water clouds are developed in terms of liquid/ice water content, based on Mie scattering theory. For ice crystals the application of Mie theory is guided by the hexagonal-crystal/equivalent-spheres comparison of Takano and Liou. These parametrizations are extensively tested against measurements from aircraft and are shown to perform satisfactorily, although corrections for unobserved small crystals and the effect of crystal shape are large and not currently well defined. The parametrizations are then used to investigate the effect of mixed-phase clouds on radiative transfer. It is found that, because the radiative properties of ice crystals and liquid droplets are significantly different, the radiative properties of mixed-phase clouds cannot be simulated successfully if the ice in clouds is converted into liquid water. Both the albedo and the rate of change of albedo with ice fraction are significantly dependent on the method by which the phases are mixed; these factors may be of especial importance in climate-sensitivity experiments that incorporate mixed-phase clouds. The presence of ice in clouds below the cirrus level is often ignored in climate-model and radiation-budget studies. The calculations presented here indicate that this neglect may lead to a serious bias in cloud albedo for a given path of condensed water.

A Model Predicting the Evolution of Ice Particle Size Spectra and Radiative Properties of Cirrus Clouds. Part II: Dependence of Absorption and Extinction on Ice Crystal Morphology.

Abstract: This study builds upon the microphysical modeling described in Part 1 by deriving formulations for the extinction and absorption coefficients in terms of the size distribution parameters predicted from the micro-physical model. The optical depth and single scatter albedo of a cirrus cloud can then be determined, which, along with the asymmetry parameter, are the input parameters needed by cloud radiation models. Through the use of anomalous diffraction theory, analytical expressions were developed describing the absorption and extinction coefficients and the single scatter albedo as functions of size distribution parameters, ice crystal shapes (or habits), wavelength, and refractive index. The extinction coefficient was formulated in terms of the projected area of the size distribution, while the absorption coefficient was formulated in terms of both the projected area and mass of the size distribution. These properties were formulated as explicit functions of ice crystal geometry and were not based on an 'effective radius.' Based on simulations of the second cirrus case study described in Part 1, absorption coefficients predicted in the near infrared for hexagonal columns and rosettes were up to 47% and 71% lower, respectively, than absorption coefficients predicted by using equivalent area spheres. This resulted in single scatter albedos in the near-infrared that were considerably greater than those predicted by the equivalent area sphere method. Reflectances in this region should therefore be underestimated using the equivalent area sphere approach. Cloud optical depth was found to depend on ice crystal habit. When the simulated cirrus cloud contained only bullet rosettes, the optical depth was 142% greater than when the cloud contained only hexagonal columns. This increase produced a doubling in cloud albedo. In the near-infrared (IR), the single scatter albedo also exhibited a significant dependence on ice crystal habit. More research is needed on the geometrical properties of ice crystals before the influence of ice crystal shape on cirrus radiative properties can be adequately understood. This study provides a way of coupling the radiative properties of absorption, extinction, and single scatter albedo to the microphysical properties of cirrus clouds. The dependence of extinction and absorption on ice crystal shape was not just due to geometrical differences between crystal types, but was also due to the effect these differences had on the evolution of ice particle size spectra. The ice particle growth model in Part 1 and the radiative properties treated here are based on analytical formulations, and thus represent a computationally efficient means of modeling the microphysical and radiative properties of cirrus clouds.

Compound semi-conductors and controlled doping thereof.

Abstract: A method of controlling the amount of impurity incorporation in a crystal grown by a chemical vapor deposition process. Conducted in a growth chamber, the method includes the controlling of the concentration of the crystal growing components in the growth chamber to affect the demand of particular growth sites within the growing crystal thereby controlling impurity incorporation into the growth sites.

Study of the Raman spectrum of nanometer SnO2

Abstract: The Raman spectra of nanometer‐sized particles of SnO2 (3–90 nm) at room temperature are reported. In the Raman spectra of the particles of SnO2, which are quite different from that of single‐crystal SnO2, there appear two new characteristic peaks, and their intensities decrease gradually with the increase of crystal size. It is concluded that the new peaks can be explained by the surface phonon modes of nanometer SnO2, consequently there is a relation between the structures of surface layers and bulk. It is believed that it is possible to determine the exact positions of atoms in surface layers of some nanometer crystals from their laser Raman spectra.

A mechanism for preferential H 2 O leakage from fluid inclusions in quartz, based on TEM observations

Abstract: Preferential leakage of H2O from fluid inclusions containing multiple gas components has been suspected in natural metamorphic rocks and has been demonstrated experimentally for synthetic H2O-CO2-rich inclusions in natural quartz. Knowledge of the physical and chemical characteristics of the leakage mechanism, which may be very complex, increases the value of natural fluid inclusions to metamorphic geology. It is proposed that crystal defects play a major role in nondecrepitative preferential H2O leakage through quartz, and remain effective during metamorphism. Inclusions with either an internal overpressure or underpressure produce strain in the adjacent quartz crystal via the nucleation of many dislocations and planar defects (like Dauphine twin boundaries). These defects allow preferential loss of H2O from H2O-CO2-rich inclusions at supercritical conditions. The transport capacity of this leakage mechanism is enhanced by nucleation of small bubbles on defect structures. The nucleation of these bubbles seems to be a recovery process in strained crystals. Solubility gradients of quartz in water in a crystal with internally underpressurized inclusions may result in optical visible implosion halos in a three dimensional spatial arrangement, caused by the growth of small bubbles at the expense of the larger original fluid inclusion. Natural fluid inclusions from Naxos (Greece) are always associated with numerous interlinked dislocations. These dislocations may have been produced by plastic derormation or by crystal growth related processes (e.g. crack healing). The presence of small bubbles on these dislocations indicates that a similar leakage mechanism for H2O must have occurred in these rocks.

Detection of ultrasonic motion of a scattering surface by two‐wave mixing in a photorefractive GaAs crystal

Abstract: The performance of an interferometric system based on two‐wave mixing at 1.06 μm in undoped GaAs crystal, for the remote detection of transient motion of a scattering surface, is described. In this system, the wave scattered by the surface is mixed inside the photorefractive crystal with a pump wave directly derived from the laser to provide the reference wave of the interferometer. The system shows several features appropriate to industrial applications, although its sensitivity is less than passive interferometric systems of the confocal Fabry–Perot type presently in use, except in the low frequency range (below 1 MHz).

Semiconductor device including a plurality of thin film transistors at least some of which have a crystalline silicon film crystal-grown substantially in parallel to the surface of a substrate for the transistor

Abstract: Nickel is introduced to a predetermined region of a peripheral circuit section, other than a picture element section, on an amorphous silicon film to crystallize from that region. After forming gate electrodes and others, sources, drains and channels are formed by doping impurities, and laser is irradiated to improve the crystallization. After that, electrodes/wires are formed. Thereby an active matrix type liquid crystal display whose thin film transistors (TFT) in the peripheral circuit section are composed of the crystalline silicon film whose crystal is grown in the direction parallel to the flow of carriers and whose TFTs in the picture element section are composed of the amorphous silicon film can be obtained.

Coupling between adjacent crystal planes in heterogeneous catalysis by propagating reaction–diffusion waves

Abstract: UNDERSTANDING of the mechanisms and kinetics of heterogeneous catalytic reactions has come largely from the study of gas–solid interactions on well defined single-crystal surfaces1,2. But real catalysts usually consist of nanometre-sized particles on which several different crystal planes are exposed. In general, it has been assumed that their properties can be regarded as a superposition of the contributions from each individual structural element. Here we show that this assumption may be invalid, even qualitatively, in certain cases. We have studied the oxidation of hydrogen on platinum surfaces at low pressure and room temperature. On a macroscopic Pt(lOO) single crystal the reaction reaches a steady state with a uniform distribution of adsorbates. But on the platinum tip of a field ion microscope, on which several different crystal planes are exposed, the reaction has a very different character. The tip contains a region of the (100) plane just 40 nm in diameter, on which the reaction rate displays sustained temporal oscillations. This effect is associated with continuously changing distributions of the adsorbed species in the form of propagating waves, which are generated by coupling of reactions occurring on adjacent crystal planes. This kind of interaction between different crystal planes may exert a profound influence on the kinetics of heterogeneous catalysis.

Bone Crystal Sizes: A Comparison of Transmission Electron Microscopic and X-Ray Diffraction Line Width Broadening Techniques

Abstract: Bone crystals are particularly small and hence estimating their sizes have proved to be difficult and values obtained inconsistent. Here we use rat bone crystals of different ages, as well as different synthetic carbonate apatite crystals, to compare two methods commonly used for determining bone crystal sizes. One method involves direct measurement of crystal lengths and widths, but not thicknesses, from transmission electron microscope (TEM) photographs of dispersed crystals, The second method utilizes X-ray diffraction line width broadening to estimate the average length of crystals. We conclude, that line width broadening values tend to reflect crystal coherence lengths rather than the physical dimensions of the whole crystal. TEM measurements provide reliable estimates of average crystal lengths and widths and their ranges. Sample preparation procedures, however, cause breakage of the fragile crystals, which probably results in underestimates of in vivo crystal sizes.

1.681-ev luminescence center in chemical-vapor-deposited homoepitaxial diamond films

Abstract: The 1.681-eV luminescence center characteristically observed in chemical-vapor-deposited diamond films is studied in a homoepitaxially grown diamond film. Homoepitaxial growth relaxes the strain typical for films grown on heterosubstrates with lattice mismatch, thus reducing dramatically the optical linewidths down to 0.2 meV. The no-phonon luminescence transition that we observe exhibits fine structure consisting of a fully resolved doublet with line components at 1.6820 and 1.6828 eV. The doublet thermalizes with an activation energy of (0.80\ifmmode\pm\else\textpm\fi{}0.04) meV equal to the spectroscopic spacing of 0.8 meV. In addition, either doublet component has itself an associated close satellite in a mirrorlike arrangement. Three other partly resolved lines enhance the total number of components in the no-phonon transition to at least seven. Photoluminescence and photoluminescence excitation measurements under uniaxial stress along the 〈001〉 crystal direction reveal a splitting of the no-phonon structure into four main components. These are studied at varying temperatures and stress values for their thermalization behavior. We deduce an electronic level scheme of two excited states from which electrons radiatively relax to two lower states. The data are not consistent with excitonic recombination or electron-to-hole recombination. They indicate that the optical center is under uniaxial internal overpressure of approximately 0.06 GPa, probably due to its large size. The luminescence decay time of the optical center was measured to be 4 ns (5 K) through 2.7 ns (300 K) in the homoepitaxial film and \ensuremath{\approxeq}1 ns nearly independent of temperature in a polycrystalline diamond film.

Numerical modeling of crystal shapes in thin sections: Estimation of crystal habit and true size

Abstract: Although the size and shape of crystals in thin sections have been measured in a number of studies, it has not been possible so far to calculate from these data the true, three-dimensional shape and size of the crystals. A numerical model, based on orthogonal solids, has been developed to attack this problem. This model shows that crystal habit can be calculated from width to length ratio distributions for most crystals in massive rocks and for all crystals in laminated or lineated rocks. Variations in the habit of minerals can reveal aspects of the physicochemical conditions of crystallization. The same model has also been used to develop the equations necessary to transform two-dimensional crystal size istributions into true crystal size distributions. Corrections for the cut effect and the intersection probability effect both require a knowledge of the crystal habit.

Nonstoichiometry and the Long‐Range Cation Ordering in Crystals of (Na1/2Bi1/2) TiO3

Abstract: Crystals of (Na1/2Bi1/2)TiO3 have grown by the flux technique and the Czochralski method. Nonstoichiometry, twin configurations, and long-range cation ordering in the crystals have been investigated using X-ray diffraction and an optical polarizing microscope. It has been found that nonstoichiometry was induced in the crystal grown by the Czochralski method owing to the volatilization of Bi-rich phase during the crystal growth. This nonstoichiometry in the crystal resulted in less lattice distortion from cubic symmetry, a lower degree of cation ordering, and a larger domain width in twin configuration. Variations in twinning with temperature and isotropization have been investigated.

Temperature-tuned 90/spl deg/ phase-matching properties of LiB/sub 3/O/sub 5/

Abstract: A single LiB/sub 3/O/sub 5/ crystal cut normal to x(=a) and z(=b) can be used for the 90/spl deg/ phase-matched SHG at 0.475-0.875 /spl mu/m by heating the crystal from /spl sim/20 to 320/spl deg/C. The improved Sellmeier's equations and new formula of the temperature variation of the refractive indexes are presented. >