Showing papers by "National Institute of Standards and Technology published in 1988"

Calculations of electron inelastic mean free paths for 31 materials

Abstract: We present new calculations of electron inelastic mean free paths (IMFPs) for 200–2000 eV electrons in 27 elements (C, Mg, Al, Si, Ti, V, Cr, Fe, Ni, Cu, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Os, Ir, Pt, Au and Bi) and four compounds (LiF, SiO2, ZnS and Al2O3). These calculations are based on an algorithm due to Penn which makes use of experimental optical data (to represent the dependence of the inelastic scattering probability on energy loss) and the theoretical Lindhard dielectric function (to represent the dependence of the scattering probability on momentum transfer). Our calculated IMFPs were fitted to the Bethe equation for inelastic electron scattering in matter; the two parameters in the Bethe equation were then empirically related to several material constants. The resulting general IMFP formula is believed to be useful for predicting the IMFP dependence on electron energy for a given material and the material-dependence for a given energy. The new formula also appears to be a reasonable but more approximate guide to electron attenuation lengths.

Observation of atoms laser cooled below the Doppler limit

Abstract: The generally accepted theory of laser cooling of free atoms predicts that the lowest achievable temperature is given by kaT = hγ/2, where kB is Boltzmann's constant arid γ is the natural linewidth of the transition for laser cooling. This "Doppler cooling limit" results from the minimization of the detuning-dependent temperature at low laser power1:

The parent structure of the layered high-temperature superconductors

Abstract: Oxide superconductors in the system Tl2Ba2Can–1CunO4+2n (ref. 1) have transition temperatures (Tc) above 100 K, increasing with n. So far, stacking sequences up to n = 3 have been found in small crystals, and sequences with n > 3 have been seen in electron microscopy studies1. For large n, the stoichiometry of Tl2Ba2Can–1CunO4+2n approaches CaCuO2, a structure expected to consist only of CuO2 planes separated by Ca atoms. By analogy with Tl2Ba2Can–1CunO4+2n, the unit cell of this hypothetical phase is expected to be tetragonal with a = 3.86 A. Such a compound is not known in the Ca–Cu–O system, but Roth2 recently reported that small amounts of Sr on the Ca site can stabilize this simple structure. Here we report the growth of small single crystals of this phase, with composition (Ca0.86Sr0.14)CuO2, and their characterization by single-crystal X-ray diffraction. The crystals are tetragonal with space group P4/mmm, and the structure contains planar [CuO2]∞ layers separated by Ca and Sr atoms. The structure is a simple defect perovskite with ordered oxygen vacancies and can be regarded as the n = ∞ parent of the A2B2Can–1CunO4+2n (A = Bi, Tl; B = Sr, Ba) superconductors.

Trace elements in human clinical specimens: evaluation of literature data to identify reference values.

Abstract: Reference values are proposed for the concentrations of As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Mo, Pb, Se, and Zn in whole blood, blood serum, urine, milk, liver, and hair from adult human subjects. For F, I, and Ni, it was not possible to evaluate reference intervals for all the specimens mentioned above. For several elements, including Al, B, Br, Cs, Li, Rb, U, and V, the present status of the literature does not provide an adequate basis for formulating baseline concentrations; therefore, results from selected investigations are listed for information only. For elements such as Cu, Fe, and Zn, which are known to be homeostatically controlled, the concentrations in whole blood and blood serum follow a gaussian-like frequency distribution, and we could consider both median and mean values for evaluation. On the other hand, elements whose concentrations in tissues and body fluids are influenced by dietary availability (e.g., As and Se) or environmental factors (e.g., Cd, Hg, and Pb) show wide scatter. In these cases, the median appeared to be a better indicator of the central tendency than the mean, when different populations are involved. These points are illustrated.

The DARPA 1000-word resource management database for continuous speech recognition

Abstract: A database of continuous read speech has been designed and recorded within the DARPA strategic computing speech recognition program The data is intended for use in designing and evaluating algorithms for speaker-independent, speaker-adaptive and speaker-dependent speech recognition The data consists of read sentences appropriate to a naval resource management task built around existing interactive database and graphics programs The 1000-word task vocabulary is intended to be logically complete and habitable The database, which represents over 21000 recorded utterances from 160 talkers with a variety of dialects, includes a partition of sentences and talkers for training and for testing purposes >

Amorphization and conductivity of silicon and germanium induced by indentation.

Abstract: We report the observation, by transmission electron microscopy, that single-crystal silicon and germanium are converted to an amorphous state at room temperature directly under both Vickers and Knoop indentations. The effect is seen for crystal orientations of [001], [011], and [111], and with applied loads between 0.1 and 0.5 N. We also observe that the materials become electrically conducting under load and that the process is reversible on subsequent unloading and reloading. Furthermore, the transformed phase is found to make Ohmic contact to the surrounding, untransformed, semiconductor.

RIR - Measurement and Use in Quantitative XRD

Abstract: The Reference Intensity Ratio (RIR) is a general, instrument-independent constant for use in quantitative phase analysis by the X-ray powder diffraction internal standard method. When the reference standard is corundum, RIR is known as I/Ic; These constants are collected in the Powder Diffraction File (1987), can be calculated, and can be measured. Recommended methods for accurate measurement of RIR constants are presented, and methods of using these constants for quantitative analysis are discussed. The numerous, complex constants in Copeland and Bragg's method introduced to account for superimposed lines can be simply expressed in terms of RIR constants and relative intensities. This formalism also permits introduction of constraints and supplemental equations based on elemental analysis.

Oxygen vacancies and defect electronic states on the SnO 2 (110)- 1×1 surface

Abstract: The ${\mathrm{SnO}}_{2}$(110)-1\ifmmode\times\else\texttimes\fi{}1 surface, a surface which is structurally similar to the more studied ${\mathrm{TiO}}_{2}$(110) surface, has been found to be an ideal system of study for the identification and characterization of surface oxygen vacancies. Heating a well-oxidized, nearly perfect (110) surface in UHV removes large amounts of surface lattice oxygen. Ion-scattering spectroscopy (ISS) and ultraviolet photoelectron spectroscopy (UPS) have shown conclusively that defect electronic states which appear low in the band gap for annealing temperatures less than 800 K arise from ``bridging'' oxygen vacancies [i.e., from the removal of oxygen anions from the terminal layer of an ideal, rutile-structure (110) surface]. UPS and four-point conductivity measurements indicate that heating at 800 K or above causes the formation of a second type of surface defect. It is argued that this second defect is an ``in-plane'' oxygen vacancy [i.e., the result of removing an oxygen anion from what is normally the second, tin-containing, atomic plane of an ideal (110) surface]. The in-plane oxygen vacancy is characterized by occupied states higher in the band gap which extend to the Fermi level.

The mathematics of nonlinear programming

Abstract: Unconstrained Optimization via Calculus. Convex Sets and Convex Functions.- Iterative Methods for Unconstrained Optimization.- Least Squares Optimization.- Convex Programming and the Karush-Kuhn-Tucker Conditions.- Penalty Methods.-Optimization with Equality Constraints.

Acuchem: A computer program for modeling complex chemical reaction systems

Abstract: Acuchem is a program for solving the system of differential equations describing the temporal behavior of spatially homogeneous, isothermal, multicomponent chemical reaction systems. It is designed to provide modelers, data evaluators, and laboratory scientists with an easy to use program for modeling complex chemical reactions, and for presenting the results in tabular or graphical form. The program is described and some examples of its application given. Acuchem is designed to operate on the IBM Personal Computer family and other compatible microcomputers, and is available in a compiled version on a floppy disk.

Structure of phosphate-free ribonuclease A refined at 1.26 A.

Abstract: The structure of phosphate-free bovine ribonuclease A has been refined at 1.26-A resolution by a restrained least-squares procedure to a final R factor of 0.15. X-ray diffraction data were collected with an electronic position-sensitive detector. The final model consists of all atoms in the polypeptide chain including hydrogens, 188 water sites with full or partial occupancy, and a single molecule of 2-methyl-2-propanol. Thirteen side chains were modeled with two alternate conformations. Major changes to the active site include the addition of two waters in the phosphate-binding pocket, disordering of Gln-11, and tilting of the imidazole ring of His-119. The structure of the protein and of the associated solvent was extensively compared with three other high-resolution, refined structures of this enzyme.

Effect of closed classical orbits on quantum spectra: Ionization of atoms in a magnetic field. I. Physical picture and calculations

Abstract: This is the first of two papers that develop the theory of oscillatory spectra. When an atom is placed in a magnetic field, and the absorption spectrum into states close to the ionization threshold is measured at finite resolution, so that individual energy levels are not resolved, it is found that the absorption as a function of energy is a superposition of sinusoidal oscillations. These papers present a quantitative theory of this phenomenon. In this first paper, we describe the physical ideas underlying the theory in the simplest possible way, and we present our first calculations based upon the theory. In the second paper, the theory is developed in full detail, proofs of all of the assertions are given, and we describe the algorithm that was used to make the calculations.

An analytical model for the steady-state and transient characteristics of the power insulated-gate bipolar transistor

Abstract: An analytical model for the power Insulated-Gate Bipolar Transistor (IGBT) is developed. The model consistently describes the IGBT steady-state current-voltage characteristics and switching transient current and voltage waveforms for all loading conditions. The model is based on the equivalent circuit of a MOSFET which supplies the base current to a low-gain, high-level injection, bipolar transistor with its base virtual contact at the collector end of the base. The basic element of the model is a detailed analysis of the bipolar transistor which uses ambipolar transport theory and does not assume the quasi-static condition for the transient analysis. This analysis differs from the previous bipolar transistor theory in that (1) the relatively large base current which flows from the collector end of the base is properly accounted for, and (2) the component of current due to the changing carrier distribution under the condition of a moving collector-base depletion edge during anode voltage transitions is accounted for. Experimental verification of the model using devices with different base lifetimes is presented for the on-state current-voltage characteristics, the steady-state saturation current, and the current and voltage waveforms for the constant voltage transient, the inductive load transient, and the series resistor-inductor load transient.

Mechanisms of inhibition of free-radical processes in mutagenesis and carcinogenesis.

Abstract: Mechanisms of the defense system against genotoxic damage, with emphasis on free-radical processes, are presented. Components of the defense system, interrelationships between components, and the stages in defense are discussed. Examples of free-radical mechanisms of protective agents (mannitol, DMSO) and repair agents (thiols and antioxidants), and the kinetics of their reactions are reviewed. Future trends in the development of novel protective agents are invoked.

Laser stabilization at the millihertz level

Abstract: The main task of this paper is to identify a number of physical problems that must be successfully addressed to achieve stabilized laser linewidths well below 1 Hz. After presentation of the basic stability characteristics of available laser sources, it is shown that if any of these lasers were optimally locked to a high-finesse Fabry-Perot cavity it would be theoretically possible to obtain a laser linewidth in the millihertz domain. Problems of optical feedback, modulation waveform errors, mechanical support and isolation of the reference cavity, thermal stabilization of the environment, etc. are considered, and interim solutions are discussed. Experimentally, locking accuracy to successive cavity orders of less than 0.00002 linewidths (+ or - 1.5 Hz) was achieved; mirror birefringence pulled the lock point approximately 10-fold more. Relative phase coherence between two independent lasers locked onto adjacent cavity orders was preserved for 8 sec, corresponding to a linewidth of each optical source of about 50 mHz.

Josephson-junction model of critical current in granular Y 1 Ba 2 Cu 3 O 7 − δ superconductors

Abstract: We calculate the transport critical-current density in a granular superconductor in magnetic fields below about 5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}3}$ T The field dependence in this region is assumed to be controlled by intragranular or intergranular Josephson junctions Various model calculations are fitted to transport critical-current data on bulk ${\mathrm{Y}}_{1}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ ceramic superconductors, whose average grain size somewhat exceeds 10 \ensuremath{\mu}m The results yield an average junction cross-sectional area (thickness x length) of 4-6 \ensuremath{\mu}${\mathrm{m}}^{2}$ If the junctions are at the grain boundaries, a London penetration depth of about 150-300 nm is inferred, consistent with other estimates We conclude that Josephson junctions are limiting the transport critical current in these samples and that they lie at the grain boundaries The parameters of the fit are not consistent with Josephson junctions at twinning boundaries

Crystal structure of octacalcium bis(hydrogenphosphate) tetrakis(phosphate)pentahydrate, Ca8(HP04)2(PO4)4·5H2O

Abstract: The crystal structure of octacalcium bis(hydrogenphosphate) tetrakis(phosphate)pentahydrate, Ca8(HPO4)2(PO4)4·5H2O, has been refined toR=0.051 for 4502 reflections. The crystals are triclinic:P¯1,a=19.692(4),b=9.523(2),c=6.835(2) A,α=90.15(2),β=92.54(2), and γ=108.65(1)°,Z=2. The structure can be considered to consist of two layers parallel to (100), an apatitic layer which corresponds very closely to that of hydroxyapatite and a hydrated layer which contains all the water molecules. Several atoms in the hydrated layer are probably disordered. Possibilities for the hydrogen-bonding arrangement are discussed.

Ostwald ripening in a system with a high volume fraction of coarsening phase

Abstract: Experiments on the coarsening behavior of two-phase mixtures in a model Pb-Sn system are reported. This system fulfills most of the assumptions of theory and has the particular advantage that all the materials parameters necessary for a comparison between the experimentally measured and theoretically predicted coarsening kinetics are known. We have examined the coarsening of Sn-rich and Pb-rich solid phases in contact with eutectic liquid in the volume fraction solid range above approximately 0.6 where the development of a solid skeletal structure inhibits sedimentation. Particle intercept distributions are measured and found to be time independent when scaled by the average intercept. This invariance is interpreted as evidence that scale factor coarsening is present. The intercept distributions are in good agreement with the predictions of theory. Measurements of average intercept diameter as a function of time establish unambiguously that the coarsening follows the theoretically predictedt 1/3 kinetics. The coarsening rate constants are measured as a function of volume fraction solid and are found to exceed the values calculated from theory using the known thermophysical properties of the Pb-Sn system by factors ranging from approximately 2 to 5.

Optically driven surface reactions: Evidence for the role of hot electrons.

Abstract: Evidence is presented for the role of excited conduction electrons in the laser-induced desorption of NO from Pt(111). State-specific detection of the desorbed NO establishes that the rotational distributions are non-Boltzmann, that the spin-orbit population is inverted, and that both the translational and vibrational distributions are uncorrelated with the laser-induced surface-temperature jump. The role of optically excited substrate electrons in driving the desorption process is evidenced by a dramatic dependence of the vibrational and translation energy distributions on the desorption-laser wavelength.

Electrodeposited Cu‐Ni Textured Superlattices

Abstract: Artificially layered materials of alternating 2–3 nm thick copper and nickel layers have been produced by electrodeposition on oriented single‐crystal copper substrates. X‐ray diffraction examination of thin layers chemically removed from the substrate showed that a coherent, relatively homogeneous coating, with a strong epitaxial relationship, had been produced. In particular, artificially layered Cu‐Ni alloys displaying a textured [110] orientation were obtained by electrodeposition. Noncoherent nucleation leads to the presence of various amounts of randomly oriented alloy grains on all substrates but, in particular, for the [100] orientation.