bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular characterization of a Penicillium chrysogenum exo‐1,5‐α‐L‐arabinanase that is structurally distinct from other arabinan‐degrading enzymes
Authors: Sakamoto Tatsuji , Ihara Hideshi , Shibano Asako , Kasai Naoya , Inui Hiroshi , Kawasaki Haruhiko
Published: February 2004

The nucleotide sequence of the abnx cDNA gene, which encodes an exo‐arabinanase (Abnx) of Penicillium chrysogenum 31B, was determined. Abnx was found to be structurally distinct from known arabinan‐degrading enzymes based on its amino acid sequence and a hydrophobic cluster analysis. The protein in the protein database with the highest similarity to Abnx was the Neurospora crassa conserved hypothetical protein. The abnx cDNA gene product expressed in Escherichia coli catalyzed the release of arabinobiose from α‐1,5‐L‐arabinan. The activity of the recombinant Abnx towards a series of arabino‐oligosaccharides, as expressed by k
cat/K
m value, was greatest with arabinohexaose.
Potential for interactions between the carboxy‐ and amino‐termini of Rubisco activase subunits
Authors: Salvucci Michael E.
Published: February 2004

The subunit interactions of Rubisco activase were investigated using mutants containing an introduced Cys near the N‐ and/or C‐terminus. Chemical cross‐linking of the C‐terminal and double insertion mutant produced subunit dimers and dimers plus high ordered oligomers, respectively. Fluorescence measurements with N,N′‐dimethyl‐N‐(iodoacetyl)‐N′‐(7‐nitrobenz‐2‐oxa‐1,3‐diazol‐4‐yl)ethylenediamine showed that the environment around the introduced Cys near the C‐terminus becomes more hydrophilic upon nucleotide binding. The Cys insertion mutants catalyzed Rubisco activation and ATP hydrolysis even when the subunits of the C‐terminal or double insertion mutants were completely cross‐linked. The results indicate that the termini of adjacent activase subunits are in close proximity and can be modified and even joined without affecting enzyme function.
Redox regulation of PTEN and protein tyrosine phosphatases in H2O2‐mediated cell signaling
Authors: Cho Seung-Hyun , Lee Chang-Hun , Ahn Younghee , Kim Hyunjung , Kim Hoeon , Ahn Chi-Young , Yang Kap-Seok , Lee Seung-Rock
Published: February 2004

Protein tyrosine phosphatase (PTP) is a family of enzymes important for regulating cellular phosphorylation state. The oxidation and consequent inactivation of several PTPs by H2O2 are well demonstrated. It is also shown that recovery of enzymatic activity depends on the availability of cellular reductants. Among these redox‐regulated PTPs, PTEN, Cdc25 and low molecular weight PTP are known to form a disulfide bond between two cysteines, one in the active site and the other nearby, during oxidation by H2O2. The disulfide bond likely confers efficiency in the redox regulation of the PTPs and protects cysteine‐sulfenic acid of PTPs from further oxidation. In this review, through a comparative analysis of the oxidation process of Yap1 and PTPs, we propose the mechanism of disulfide bond formation in the PTPs.
Specific inhibition of gene expression and transactivation functions of hepatitis B virus X protein and c‐myc by small interfering RNAs
Authors: Hung Le , Kumar Vijay
Published: February 2004

With a view to developing therapeutic strategies against hepatocellular carcinoma (HCC), we have recently shown that co‐expression of c‐myc and the X protein of hepatitis B virus (HBx) resulted in the development of HCC in the X‐myc transgenic mice [Lakhtakia et al., J. Gastroenterol. Hepatol. 18 (2003) 80–91]. We now show in cell culture‐based studies that small interfering RNA (siRNA) corresponding to HBx and c‐myc can regulate expression and transactivation of the target genes. Expression vectors for small hairpin RNAs (shRNAs) against two different regions each of the HBx and c‐myc open reading frames were constructed and their regulatory effects were investigated in COS‐1 cells. A dose‐dependent specific inhibition in the expression levels of HBx and c‐myc was observed with individual shRNAs. Further, the recombinantly expressed shRNAs also blocked the transactivation functions of their cognate genes. Though each shRNA worked at a different efficiency, the inhibitory effects with two different shRNAs were cumulative. These results appear promising for developing a siRNA‐based therapy for HCC.
Constitutively active Src facilitates NGF‐induced phosphorylation of TrkA and causes enhancement of the MAPK signaling in SK‐N‐MC cells
Authors: Tsuruda Akinori , Suzuki Shingo , Maekawa Takaaki , Oka Syuichi
Published: February 2004

Here we investigated a biological association of constitutively active Src with TrkA in SK‐N‐MC human neuroblastoma cells. Activation of TrkA and extracellular signal‐regulated kinase (ERK) by nerve growth factor (NGF) was inhibited by pretreatment with PP2, an inhibitor of Src family kinases. Moreover, NGF‐induced phosphorylation of TrkA and ERK was also attenuated by the transfection with a dominant‐negative src construct. On the other hand, the transfection with a constitutively active src construct enhanced these phosphorylations. In addition, we showed that active Src phosphorylates TrkA directly in vitro, and that Src associates with TrkA through Grb2 after NGF stimulation. These results suggest that constitutively active Src that associates with TrkA through Grb2 after NGF stimulation participates in TrkA phosphorylation and in turn enhances the mitogen‐activated protein kinase signaling in SK‐N‐MC cells.
Prediction of training performance for diesel mechanics
Authors: Aramburu-Zabala Luis , Casals Marina
Published: February 2004

The new systems of electronic fuel injection have changed the content of the job of diesel automotive mechanic. In this article we analyse the predictive value and the adverse impact of a trainability test based on a diesel mechanic minicourse. The minicourse AC had null adverse impact and was significantly related to training performance in the complete 300 hour auto mechanics course. Results are discussed and one promising alternative based on the dynamic assessment notion is suggested.
Effects of metacognitive thinking and knowledge acquisition in dyads on individual problem solving and transfer performance
Authors: Brand S. , Reimer T. , Opwis K.
Published: February 2004

We investigated if metacognitive thinking and knowledge acquisition in dyads improve individual problem solving performance and transfer to new problems. In the learning phase, participants solved several Tower of Hanoi problems and half of them were stimulated to metacognitive thinking. A second variable studied was if the learning tasks were solved individually or in dyads. The subsequent individually completed test phase consisted of two structurally similar and of two dissimilar transfer tasks. Metacognitive stimulation enhanced performance in all cases. Those participants who had been stimulated to metacognitive thinking, whether individually or in dyads, performed better on every task than did the individuals in the control group. Dyads proved better at solving the learning tasks than did the individuals, although this advantage did not affect individual performance on the transfer tasks.
Multiyear analysis of amazonian biomass burning smoke radiative forcing of climate
Authors: Procopio A. S. , Artaxo P. , Kaufman Y. J. , Remer L. A. , Schafer J. S. , Holben B. N.
Published: February 2004

Seven years analysis of clear‐sky aerosol direct radiative forcing is presented for two locations in the Amazon Region heavily impacted by biomass burning emissions. During the dry season, the monthly average direct forcing at the top of the atmosphere, deduced from measurements, varied from ‐5 to ‐12 W/m2 and at the surface from ‐21 to ‐74 W/m2, with the difference associated with absorption of sunlight by the smoke aerosol layer. The spatial distribution of the forcings over Amazonia showed that they affect an area over 1.2–2.6 million square kilometers.
The 1997 Kagoshima (Japan) earthquake doublet: A quantitative analysis of aftershock rate changes
Authors: Woessner J. , Hauksson E. , Wiemer S. , Neukomm S.
Published: February 2004

We quantitatively map relative rate changes for the aftershock sequence following the second mainshock of the 1997 earthquake doublet (MW = 6.1, MW = 6.0) in the Kagoshima province (Japan). Using the spatial distribution of the modified Omori law parameters for aftershocks that occurred during the 47.8 days between the two mainshocks, we forecast the aftershock activity in the next 50 days and compare it to the actually observed rates. The relative rate change map reveals four regions with statistically significant relative rate changes ‐ three negative and one positive. “Our analysis suggests that the coseismic rate changes for off‐fault aftershocks could be explained by changes in static stress. However, to explain the activation and deactivation of on‐fault seismicity, other mechanism such as unusual crustal properties and the presence of abundant crustal fluids are required.”
Forecasting methods for occurrence and magnitude of proton storms with solar soft X rays
Authors: Garcia H. A.
Published: February 2004

Solar energetic proton (SEP) events in the vicinity of Earth have the potential of affecting the performance of civilian, military, and research satellites, including such diverse functions as communications, spacecraft operations, surveillance, navigation, and life support systems. The National Oceanic and Atmospheric Administration's (NOAA) Space Environment Center and the U. S. Air Force Weather Agency cooperate to provide advance warnings of SEP events. Their explicit duties include the need to continually upgrade and improve the accuracy, timeliness, and scope of SEP forecasts. Previous work on this topic established the empirical connection between SEPs and low‐temperature X‐ray flares. The main focus of the present work is to improve the quality of SEP forecasts by enhancing the size and content of the flare database used to quantify the probability model, tuning the model with imposed operational constraints, and augmenting each SEP prediction with an estimate of the magnitude of the particle event itself.
Craik–Criminale solutions and elliptic instability in nonlinear-reactive closure models for turbulence
Authors: Fabijonas Bruce R. , Holm Darryl D.
Published: February 2004

The Craik–Criminale class of exact solutions is examined for a nonlinear-reactive fluids theory that includes a family of turbulence closure models. These may be formally regarded as either large eddy simulation or Reynolds-averaged Navier–Stokes models of turbulence. All of the turbulence closure models in the class under investigation preserve the existence of elliptic instability, although they shift its angle of critical stability as a function of the rotation rate Ω of the coordinate system, the wave number β of the Kelvin wave, and the model parameter α, the turbulence correlation length. Elliptic instability allows a comparison among the properties of these models. It is emphasized that the physical mechanism for this instability is not wave–wave interaction, but rather wave, mean-flow interaction as governed by the choice of a model’s nonlinearity.
Investigation of Neutron-Irradiation Damage in Silicon by Transmission Electron Microscopy
Authors: Pankratz J. M. , Sprague J. A. , Rudee M. L.
Published: February 2004

Dislocation-free single crystals of three types of silicon have been irradiated by 6×1017 and 5×1019 nvt of fast neutrons and studied by bright- and dark-field transmission electron microscopy. Mean defect image size has been found to depend on the impurity content of the silicon and annealing treatment, ranging from 22 Å in the most nearly pure silicon in the as-irradiated state to a maximum of 40 Å in the n-type silicon after a 30-min anneal at 700°C. Defect density ranges from 1017 images/cm3 in the as-irradiated state to less than 1015 after a 1000°C anneal for 30 min. The density is not noticeably affected by the type of impurity, but is approximately proportional to neutron dosage. The properties of the images formed by diffraction contrast in thin silicon foils have been studied by extending the solutions of the basic equations of the dynamical theory of diffraction contrast with anomalous absorption to defects of the small size encountered in the research. An anomalously wide black-white image is predicted in dark field from which it is possible to determine whether a defect has interstitial or vacancy character. A ratio in the order of 2 to 1 interstitial type to vacancy type was found in the n-type silicon; the ratio in the other two types of silicon appeared to be more nearly unity.
Scattering of Electromagnetic Waves from a Moving Layer in a Transmission Line
Authors: Tsai C. S.
Published: February 2004

The scattering of a traveling electromagnetic wave from a small moving layer in a nonlinear L-C transmission line is treated in detail by the technique of Green's function. First-order scattered waves, toward both directions, are calculated as the radiations arising from the secondary current sources induced in the region of the moving layer. A Doppler-frequency shift is found in the scattered wave propagating in the direction opposite to the incident wave but not in the scattered wave in the other direction. The power variation of the frequency-shifted scattered wave is an oscillatory function of both the velocity and width of the layer. This analysis illustrates the physical mechanism of the double-Doppler effect in stationary media and indicates the possibility of employing a nonlinear electrical or acoustical transmission line for double-Doppler effect experiments.
Thermodynamic Force on a Dislocation
Authors: deWit Roland
Published: February 2004

The thermodynamic force on a dislocation is defined as the negative gradient of the Gibbs free energy of the crystal with respect to dislocation motion. This is shown to lead to a consistent force expression which contains two terms: the Peach-Koehler force due to the stress, and the Bardeen-Herring force due to the vacancy concentration. The precise definition of these two terms is arbitrary, though their sum is unique. Various forms of the force expression are derived in terms of different reference vacancy concentrations, including the original forms of Weertman and Lothe and Hirth.
High-Field Conduction in Films of Mylar and Teflon
Authors: Lilly A. C. , McDowell John R.
Published: February 2004

Conduction currents in Mylar and Teflon films were measured at applied field strengths of 2–1400 kV·cm−1 and at a number of temperatures in the range 70°–163°C. The samples were carefully conditioned.

The current varied with voltage as exp (αV01/2), and its temperature behavior was consistent with an activation process, giving an activation energy for conduction essentially independent of sample thickness, except at low fields. This change in activation energy with thickness at low fields, along with breaks in the Schottky plots are attributed to space-charge effects. The data did not fit a hyperbolic sine relationship previously applied to this system in support of an ionic mechanism. The results are discussed in terms of available theories of conduction in solids.
Electrical Behavior of Copper-Diffused Insulating Layers on CdS Crystals
Authors: Simhony M. , Williams R. , Willis A.
Published: February 2004

Insulating CdS layers are formed on conducting CdS crystals by diffusion of copper. The layers are 1–10 μm thick and have resistivity of 108 Ω·cm. The layer thickness as determined from capacitance measurements does not depend on the applied voltage. Current-voltage curves have Ohmic and square-law regions in fields below 104 V/cm, and thereafter, they rise very steeply (j∝V5). Breakdown occurs at average fields of 5×105 V/cm. Pulsing to 105 V/cm greatly reduces the low-field resistance, frequently increasing the capacitance; this is shown to be due to a peripheral expansion of the diffused layer together with a formation of breakdown channels. Field-controlled freeing of carriers from traps is indicated by the rise of current with time at voltages above threshold, while below threshold the current pulse is decaying, due to trapping. Observations made along and perpendicular to c axis are similar.
Stacking-Fault Energy in Dilute Silver Alloys from Extended Node Measurements
Authors: Gallagher P. C. J.
Published: February 2004

Extended node measurements have been made on alloys of Ag–Zn, Ag–Al, Ag–Mg, Ag–Ge, and Ag–Au with electron-to-atom ratio up to 1.06. In all cases, the magnitude of the stacking-fault energy was found to be the same within±15% of that in pure silver. This result is in conflict with recently published data obtained using the τ3 method, but follows the same pattern as other recent node work on dilute alloys in the Ag–In and Ag–Sn series. The present findings, together with other comparisons between results from the direct observation of defects and those from the τ3 method, suggest that under some circumstances values of the stacking-fault energy determined by the τ3 method must be considered unreliable.
Compressions of Isotopic Lithium Hydrides
Authors: Stephens D. R. , Lilley E. M.
Published: February 2004

The compressions of Li7H, Li7D, Li6H, and Li6D were measured to 40 kbar at 23°C. The data fit well to both the simple Born-Mayer model and the Murnaghan equation. The 1-atm compressibilities varied from 2.8 to 2.9 mbar−1. The data do not agree well with other reported results.
Measurement of rf Ionization Rates in High-Temperature Air
Authors: Taylor W. C. , Chown J. B. , Morita T.
Published: February 2004

High-frequency net ionization rates in hot air were determined as a function of field strength by measurements in a shock tube. The equilibrium air temperature was 3350°K±350°. The rates were deduced from time-resolved measurements of the electron density in hot air when exposed to the pulsed fields on microwave slot antennas. The rates in equilibrium hot air were found to be higher than the rates in room-temperature air by more than a factor of ten. The ionization rates are shown to increase from the room-temperature values to the equilibrium hot-air values during the first few mean free paths behind the shock front.
Perpendicular Anisotropy in Electrodeposited Nickel Films
Authors: Mayadas A. F. , Klokholm E.
Published: February 2004

The existence of a perpendicular anisotropy in electrodeposited nickel films, comparable in magnitude to that found in films evaporated in a nominal vacuum is reported. Measurements of the magnetization, stress, and anisotropy indicate that the perpendicular anisotropy constant K⊥ is a sum of two terms: a magneto-stress term which may be reduced in magnitude or even eliminated entirely by adding saccharin to the plating bath, and a term equal to the shape demagnetizing energy with magnetization M, roughly 90% that of bulk nickel. Since additional shape effects due to grain boundaries do not seem to be present, it is concluded that the lowering of the magnetization is due to randomly distributed defects.

Ferromagnetic resonance measurements normal to the film plane give a perpendicular anisotropy field that is lower than the corresponding anisotropy field as measured by quasistatic methods. This discrepancy is attributed to the suppression of the uniform-precession spin mode for perpendicular polarization.
Direct Comparison of the Kelvin and the Electron-Beam Methods of Contact-Potential-Difference Measurement
Authors: Hopkins B. J. , Smith B. J.
Published: February 2004

Both the Kelvin and the retarding potential, or electron beam, techniques have been used to measure the contact potential differences between (110) and (100) oriented tungsten single-crystal surfaces clean and covered with vapor deposited barium films. The evaporation of barium in an ultra-high vacuum is difficult and it was necessary to double evaporate the metal before the final evaporation could be made without a pressure increase inside the experimental tube above, at best, 3×10−10 Torr. Contrary to the result obtained from a previous similar experiment, the contact-potential-differences measured by the two methods differed by 130 mV for the (100) crystal face and by 150 mV for the (110) crystal face. Differences of this magnitude can be explained either in terms of a two-patch model, provided there is a work function difference of up to 1.0 eV between the patches, or by a change in the reflection coefficient for electrons. To account for the difference in terms of changes in the reflection coefficient only, a change of from zero for the clean crystal to about 0.6 for the barium-covered surface would be required.
Microwave Conductivity of Silicon and Germanium
Authors: Holm John D. , Champlin Keith S.
Published: February 2004

This paper presents measurements of the complex microwave conductivity of lightly-doped n- and p-type silicon and genmanium as functions of impurity concentration and of temperature between 77° and 300°K. The measurements were obtained by observing the reflectivity of the circular T E01 mode at 48 GHz and are thus free of errors caused by the impedance of the waveguide-sample contact. Analysis utilized currently accepted scattering models including ionized impurity scattering, intravalley acoustic and optical mode scattering, and intervalley scattering. The results are used to investigate the temperature and doping dependence of the average relaxation time and the conductivity ``effective'' mass.
Pressure-Induced Phase Transformation in NaCl
Authors: Bassett William A. , Takahashi Taro , Mao Ho-Kwang , Weaver J. Scott
Published: February 2004

A pressure-induced phase transformation in NaCl which occurs rapidly and reversibly at approx 300 kbar and room temperature has been observed in a diamond-anvil high-pressure cell. X-ray diffraction data indicate that the high-pressure polymorph has the cesium chloride (B2) structure. The lattice parameters of the low- (B1) and high- (B2) pressure phases at the transformation pressure are, respectively, 4.872±0.004 Å and 2.997±0.004 Å, and the volume change for the transformation is − 1.00±0.05 cm3 mole−1. The entropy change for the phase transformation has been calculated from the volume change and from the high-temperature-pressure data obtained by the shock experiments of Fritz et al. and found to be 1.5±0.3 cal deg−1 mole−1. Comparison with other alkali chlorides indicates that a linear relationship exists between the entropy change and the volume change for the B1-B2 phase transformation. A thermodynamic equation accounting for this relationship has been derived under the assumption that the Grüneisen parameter is proportional to the Ath power of the volume. An equation which relates this factor A to the adiabatic bulk modulus and its pressure and temperature derivatives has also been derived.
Second-Order Strain Accumulation at Ultrasonic Frequency
Authors: Mason Warren P. , Ronay Maria
Published: February 2004

Permanent axial extension is produced by reversed cyclic straining in tension-compression at ultrasonic frequency and in torsion at mechanically induced (low) frequency. It is shown that in both ranges the axial extension is a quadratic function of the cyclic strain amplitude, which is characteristic of second-order effects that exist independently of the frequency of the cyclic strain.
Longitudinal Optical Phonon-Plasmon Coupling in CdS
Authors: Bell Robert J. , McMahon Thomas J. , Rathbun Donald G.
Published: February 2004

The longitudinal optical phonon-plasmon interaction theory of Varga is shown to be applicable to Gadoped CdS through the region of the first reflectivity minumum (2.5 to about 30 μ). The theoretical reflectivity vs wavelength for several concentrations of Ga-doped CdS are compared with experiment. The agreement between theory and experiment is particularly good for high dopant concentrations. A plot of the wavelength at minimum reflectivity vs impurity concentrations shows that the Varga theory does well in explaining the data. When phonon damping is introduced into the Varga dielectric function, the reflectivity over the whole region of resonance is more realistic. Finite plasmon lifetime is not considered since no data are available for CdS.
Laser-Induced Breakdown in Transparent Dielectrics
Authors: Olness D.
Published: February 2004

Quantitative data for the threshold of damage and some observations concerned with the damage phenomena in transparent solids are presented and discussed.
Photoelectric Correlation Measurements with Time-to-Amplitude Converters
Authors: Davidson F. , Mandel L.
Published: February 2004

Photoelectric delayed coincidence measurements of an optical field with a time-to-amplitude converter are now becoming common, and yield results which are related to certain correlation functions of the field. The equations governing the measurements are obtained and some computational methods are developed which allow the correlations to be derived from measured data. The methods are illustrated by an example.
Phenomenological Relation between Thermophysical Properties
Authors: Mitvalský V.
Published: February 2004

In the region in which no phase transition occurs, the temperature variations of some properties (e.g., density, viscosity, sound velocity, specific heat, thermal conductivity) of a simple substance appear to be tied together in a remarkable way: the ratios of their values corresponding to any pair of temperatures of the region are all uniquely fixed when any two of them are given. This assumption implies a simple correlation of three such properties at four temperatures. It represents experimental data almost if not quite in the limits of experimental errors, so that it permits extrapolation, e.g., the anomalous behavior of sound velocity and specific heat of water with a maximum deviation of 0.02% and 0.04%, respectively. Similar extrapolations for gaseous CO2 and solid Fe are also in excellent agreement with experimental data.
Magnetoacoustic Excitation of Radio-Frequency Resonances and Echoes in Magnetic Materials
Authors: Rubinstein M. , Stauss G. H.
Published: February 2004

Continuous spectra of resonances are observed when some ferromagnetic substances are examined with cw or spin-echo rf spectrometers. These resonances have previously been assigned to domain-wall motion; evidence is presented here to show rather that they are magnetically excited acoustic vibrations. An explanation of the manner in which a nonlinear magnetoacoustic interaction allows the production of echoes in an otherwise linear system is presented. A means of suppressing these resonances to facilitate the study of nuclear magnetic resonances in magnetic materials is demonstrated.
Non-Maxwellian Form of the Electron Velocity Distribution in Nitrogen Plasmas
Authors: Noon J. H. , Blaszuk P. R. , Holt E. H.
Published: February 2004

Changes in the form of the electron velocity distribution with time in the active discharge and in the afterglow of a low-energy nitrogen plasma immersed in a magnetic field, have been investigated. A gated microwave radiometer was used to monitor the radiation temperature of the plasma electrons which drops rapidly from 15 000° to 5000°K following termination of the active discharge. The radiation temperature then remains an order of magnitude higher than that of the neutral gas molecules for a time an order of magnitude greater than the calculated relaxation time of the electron temperature. The magnetic field was swept through a range around the value which produces an electron cyclotron frequency equal to the radiometer frequency. Observation of the radiation temperature as a function of magnetic field strength shows that, for certain values of the discharge parameters, the electron velocity distribution in the active discharge is Maxwellian. In the afterglow, the form of the distribution function is found to change rapidly.
Growth of Tungsten Carbide Monocrystals
Authors: Gerk A. P. , Gilman J. J.
Published: February 2004

Tungsten carbide monocrystals have been grown from a Co flux using a modified Czochralski technique. These crystals are about 1 cm in size, show little internal strain, and contain few Co inclusions. The highest-quality crystals grow in the [0001] direction with a melt composition of 16 mole% WC-84 mole% Co, a surface temperature of 1475°C, and a pulling rate of 0.2 mm/h.
The Grid Probe as an Ionic Diagnostic Tool in Cesium Plasma
Authors: Haug R. , Felden M. , Schirmann D.
Published: February 2004

A plane grid probe is operated in a cesium-vapor-discharge plasma. The collected electron and ion currents are separated. The characteristic curve and its second derivative are analyzed. The densities, temperatures, and plasma potentials deduced from these curves are given and compared.
Temperature Dependence of the Widths and Positions of the R Lines in Li2Ge7O15:Cr3+
Authors: Powell Richard C.
Published: February 2004

The widths and positions of the R1 fluorescence lines of trivalent chromium ions in nonequivalent lattice sites in lithium germanate crystals have been measured at temperatures from 24°K up to 200°K. The temperature dependences of the linewidths are explained in terms of microscopic strains, direct phonon processes, and two-phonon Raman scattering processes. The temperature dependences of the lineshifts are attributed to two-phonon virtual processes and direct phonon processes. The theraml changes in the widths and positions are found to be of the same order of magnitude. These results are compared with those from similar measurements on ruby- and chromium-doped MgO.
Stress Fields Associated with Coaxial Circular Glide-Dislocation Arrays
Authors: Marcinkowski M. J.
Published: February 2004

A detailed numerical analysis has been made of the six components of stress associated with three distinct glide dislocation loop arrays. The treatment is exact and is based on the isotropic elastic dislocation theory as developed by Kröner. The arrays considered were plate-shaped, equiaxed, and rod-shaped. Planar cuts were made through the center of these arrays so as to show the salient features of the six stress components and plotted. These stress fields have been discussed in relation to twin and martensite lamellas, precipitates and grains and their ability to nucleate slip and fracture.
Measured and Computed rf Energy Distribution in Velocity-Modulated Electron Beams
Authors: Gunshor Robert L. , Mihran Theodore G.
Published: February 2004

Direct experimental measurements have been made of the time-averaged rf energy distribution in velocity-modulated electron beams. Measurements were made as a function of axial distance for a variety of drive powers from near-ballistic to near-space-charge-wave levels. Two types of velocity analyzers were used: a pinhole, giving the velocity distribution of near-axis electrons, and a mesh, accepting electrons from the entire cross section of the beam. The experimental measurements are compared with the predictions of three theories. In increasing order of accuracy, these are: (1) third-order, Lagrangian, disk-electron analysis, (2) single-beam disk-electron computer model, and (3) multiple-beam disk-electron computer model. The multiple-beam model, which takes into account the principal dc velocity spread of 1.5% in the experiment, gives excellent agreement with measured behavior at intermediate and high drive levels. At low drive levels the existence of a group of extra low velocity electrons extending below the main dc beam velocity is found to account for discrepancies beyond the rf velocity spread minimum.
Surface Processes in the Growth of Silicon on (111) Silicon in Ultrahigh Vacuum
Authors: Abbink H. C. , Broudy R. M. , McCarthy G. P.
Published: February 2004

A replication technique for electron microscopy has been developed which is capable of detecting 3 Å steps on surfaces. This technique was used to observe the growth of silicon on a (111) silicon surface in ultrahigh vacuum. On a substrate surface that is ``clean'' by current standards of silicon surface study, three-dimensional growth centers developed. However, on a cleaner surface consisting of a continuous layer of freshly deposited silicon, growth proceeded by a step-motion mechanism. By the use of a simple model to calculate the critical supersaturation ratio and the equilibrium adatom concentration, and from the observed absence of nucleation on atomically flat terraces between steps, a surface self-diffusion coefficient of 10−3 cm3/sec at 800°C was estimated. Straight steps which propagate in the 〈1̄1̄2〉 direction were observed, in contradiction of the simple bond model of step edges which predicts straight steps that propagate in the〈112̄〉 direction. Step arrays and step separation were observed and interpreted by the recent theory of adatom capture at steps by Schwoebel and Shipsey.
Thermal Radiation from Rough Tungsten Surfaces in Normal and Off-Normal Directions
Authors: Thomas Ludwig K.
Published: February 2004

The effect of surface roughness on the normal spectral emittance and on the spectral angular dependence of thermal radiation was investigated for tungsten at 2800°K over a wavelength range of 0.3–6 μm. The normal emittance of rough surfaces is higher than of flat surfaces for all wavelengths, the largest increase occurring in the infrared. In the angular dependence of thermal radiation a minimum was found which depends on the surface geometry and on the wavelength. The values are interpreted in terms of emission from flat facets, which do not radiate according to Lambert's cosine law.
Theory of Laser Action Involving Free Excitons and LO-Phonon-Assisted Transitions
Authors: Haug H.
Published: February 2004

The laser system is described by the equations of motion for the creation operators (amplitude equations) of the excitons in the lowest band, the longitudinal optical phonons, and the photons in the laser mode. These equations include damping, pump, and fluctuation terms. By eliminating the `virtual excitons' with wavevector q=0 one finds the effective amplitude equations for the indirect transitions (A1-LO line). From these equations quantum mechanical rate equations are derived, whose fluctuation operators have shot-noise character. The mean stationary rate equations are treated in the limit of a short LO-phonon lifetime and of a fast relaxation of the excitons into the thermal distribution in the lowest band. For the example of CdS, we calculated the temperature dependence of (a) the maximum gain frequency at threshold, (b) the threshold pump rate Pth, (c) the spectrum of the gain function at threshold, and (d) the output power in the region 0.1≤P/Pth≤2.0, where P is the pump rate. Finally, the light intensities of the A1 line (direct transitions) and of the A1-LO line are calculated simultaneously in the same pump range. The calculated slopes of the intensity curves are in good agreement with those measured by Nikitine et al. in CdS.
An Electron-Microscope Study of Shock-Synthesized Diamond
Authors: Trueb Lucien F.
Published: February 2004

The microstructure of diamonds formed from the graphite in cast iron by an explosive shock process was investigated by means of transmission electron microscopy electron, diffraction, and selected beam microscopy. Morphologically, the material consists of a mixture of two different forms: compact aggregates of of usually acicular crystallites with a strong preferred orientation, and single crystals. The latter partly consist of hexagonal diamond, the recently discovered modification of carbon, and contain many stacking faults. Hexagonal diamond is also randomly distributed in the polycrystalline fraction.
Generation of the Two Pure Modes of Transverse Ultrasonic Waves in an x-Cut Quartz Bar at 9.4 GHz
Authors: Ogawa Tomoya , Sakamoto Akimasa
Published: February 2004

The two pure transverse modes of ultrasonic waves along the x direction were generated in an x-cut quartz bar by use of a TE110 cavity with an inhomogeneous electric field which was produced by a metal post of off-axial cut. The sound velocities of transverse waves were (3.18±0.01) and (4.95±0.05)×105 cm/sec, and that of the longitudinal wave was (5.59±0.01)×105 cm/sec at 4.2°K. The elastic stiffness constants of quartz crystal at 4.2°K are estimated, by use of the density 2.65 g/cm3, which is the value at room temperature, to be c11D=8.31×1011 dyn/cm2, c12=2.88×1011 dyn/cm2, c44D=6.63×1011 dyn/cm2.
Semiconducting Properties of Single Crystals of n- and p-Type Tungsten Diselenide (WSe2)
Authors: Upadhyayula L. C. , Loferski J. J. , Wold A. , Giriat W. , Kershaw R.
Published: February 2004

The optical absorption, photoconductivity, contact photovoltage, electrical conductivity, and Hall coefficient of single crystals of WSe2 have been studied over the temperature range 77°–295°K. It was found that the forbidden energy gap Eg was 1.35 eV at 295°K and that the temperature dependence of Eg was given by dEg/dT=−4.6×10−4 eV/°K. The material as grown by iodine vapor transport in a sealed ampule is p-type with hole mobility μh∼80 cm2/V·sec and p∼1016/cc at 295°K. The carrier concentration could be reduced by pumping out excess selenium. Doping with rhenium during the crystal growth process resulted in n-WSe2 with a carrier concentration n∼1017/cc and electron mobility μn∼100 cm2/V·sec at 295°K.
Effects of 3He Irradiation on the Anisotropy-Field Inhomogeneity and Coercive Force in Thin Permalloy Films
Authors: Williams Conrad M.
Published: February 2004

The effects of 3He irradiation on the inhomogeneity in the anisotropy-field magnitude, Δ90, the angular dispersion, α90, and the coercive force, Hc, have been examined for vacuum-deposited thin Premalloy films, with compositions ranging between 43% and 83% nickel. The changes induced in these parameters were developed at 60°C by exposing the films to 2 MeV 3He irradiation, in the presence of a 500 Oe orienting magnetic field applied parallel to the easy-axis directions of the films. The total dosage received by the films was 2×1017 particles/cm2. As a result of this exposure, the values for both α90 and Δ90 decreased for all compositions, with the decrease a minimum in the vicinity of 78% nickel. Similarly, the coercive force decreased for most compositions, but was insensitive to irradiation for compositions around 81% nickel. Decreases in α90, Δ90, and Hc. were as much as 46% for some compositions; however, much of the decrease in Δ90 was primarily the result of an irradiation-induced increase in the average uniaxial anisotropy field, Hk. On the basis of these results it was concluded that the irradiation induces an over-all decrease in the internal strain in the film.
Band Structure and the Titanium LII, III X-Ray Emission and Absorption Spectra from Pure Metal, Oxides, Nitride, Carbide, and Boride
Authors: Fischer David W. , Baun William L.
Published: February 2004

The titanium LII, III x-ray emission and absorption spectra (λ∼27.5 Å) from pure metal, oxides, nitride, carbide, and boride have been investigated using a plane-crystal vacuum spectrometer with electron-beam excitation and flow-proportional counter detection. Emission spectra were studied over a wide range of accelerating voltages and takeoff angles, showing that satellite emission and self-absorption effects can significantly distort the band shapes and energy positions of intensity maxima. A replica of the LII, III absorption spectrum can be constructed solely from emission spectra affiicted with widely different amounts of self-absorption. The LII, III emission spectra from the oxides, nitride, and carbide exhibit an important crossover transition from the 2p level of the anion to the LII and LIII levels of titanium. Results indicate formation of a 3d band in titanium compounds which is only partially filled, giving rise to metallic conduction. X-ray data is compared to density of states calculations for TiO, TiN, and TiC. The energy shifts of emission bands and absorption edges indicate a transfer of charge from titanium to anion in the oxides, nitride, and boride but that in TiC there is a transfer of charge from carbon to titanium. Energy separations between anion 2p band and titanium 3d band are measured and it is shown that both bands contribute significantly to the TiLII, III spectra from the compounds.
Electron Momenta in Monte Carlo X-Ray Transport
Authors: Veigele Wm. J.
Published: February 2004

Monte Carlo x-ray transport codes presently in use neglect momenta of scattering electrons. These momenta cause the Compton scattered wavelength to assume a distribution of wavelengths which is observed as a scattered photon linewidth. This linewidth was included in a Monte Carlo code using a point-source monoenergetic emitter in a homogeneous, infinite, high-atomic-number material, and transport parameters were calculated from one to ten mean free paths of the source photon. Results indicate that linewidth does not significantly influence energy deposition or fluence calculations for the cases considered.
A Microscopic Theory for the Prediction of Vapor Condensation
Authors: Abraham Farid F.
Published: February 2004

Abraham has recently proposed an empirically determined surface-energy correction to the critical size droplet obtained by fitting Band's equation of state for the saturated vapor to the experimentally measured compressibility factor. He has shown good agreement between his E(d) theory and experiment. In this paper, we present a complete development of this method based on Wergeland's ``gas'' model of a cluster. We never have to introduce the capillarity approximation, in which macroscopic thermodynamic properties of the bulk liquid phase are ascribed to a small cluster. The success of the theory is demonstrated by predicting the critical supersaturation for water vapor and ammonia for a range of temperatures. Finally, the empirically determined intensive parameters of clusters are compared with the capillarity approximation.
Dislocation Arrangements Resulting from the Diffusion of Zn into Cu: Etch-Pit Studies
Authors: Ayres P. S. , Winchell P. G.
Published: February 2004

Zinc was diffused for 100 h at 780°C into the (111) surface of thick copper single crystals from zinc vapor in equilibrium with 10 wt.% zinc copper. Dislocations were produced in the crystals and were revealed by etch-pitting isoconcentration (111) planes in the region of concentration change. The dislocation density was larger than 107 cm−2 near the exposed surface and decreased to about 5×105 cm−2 in the interior. Many of the dislocations were in the boundaries of subgrains which increased continuously in size from less than 20 to about 300 μ as the distance from the surface increased to 3.7 (Dt)1/2 (400 μ). The interior of many of the subgrains appeared to be free of dislocations.
Time Behavior of a Laser with a Saturable Absorber as Q Switch
Authors: Hofelich-Abate E. , Hofelich F.
Published: February 2004

The time behavior of solutions of a system of three coupled nonlinear rate equations, describing the photon density and the population inversion of lasering and absorbing centers within the resonator, is discussed qualitatively. It is shown that aperiodic and oscillatory solutions occur. By introducing the condition that the absorber follows the radiation field adiabatically, the original system can be reduced to a system of only two variables. In this case it is demonstrated that for certain parameter values, a stable limit cycle exists.
Hertzian Fracture in Single Crystals with the Diamond Structure
Authors: Lawn B. R.
Published: February 2004

Extension of an earlier theory of Hertzian fracture in brittle isotropic materials is here made to include the case of brittle single crystals, with particular reference to crystals having the diamond structure. A detailed description is first given of the inhomogeneous stress field in a flat, elastic specimen loaded normally with a hard sphere. The geometry of cracks growing in such a stress field is then discussed, taking into account the anisotropy in surface energy relevant to the diamond-structure crystals. Analyses of the mechanics of crack growth into the crystals subsequently indicate that for a certain range of indenter size, the Hertzian crack passes through four equilibrium stages, as it does in glass, before reaching its fully developed length. As a result Auerbach's law, which states that the critical load on a spherical indenter necessary to produce a fully developed Hertzian fracture is proportional to the radius of the indenter, holds within this certain range of indenter size. This law is experimentally confirmed Hertzian fracture tests on single crystals of silicon. The mechanism of crack initiation and growth outlined in this paper is then discussed in terms of conclusions made by previous workers on the nature of the Hertzian cracks in the diamond structure crystals. Finally, possible application of the Hertzian test to the study of some mechanical properties related to the fracture surface energy in brittle single crystals is indicated.
Numerical Solution of the Poynting-Effect Problem
Authors: Partom Yehuda
Published: February 2004

The nonlinear behavior of a thin long elastic bar under combined tension and torsion (known as the Poynting-effect problem) is investigated by numerical methods. Finite differences are used, and the problem is formulated as a system of nonlinear algebraic equations, which is solved by the Newton-Raphson iterative method. The stress-strain relations used are derived from an energy function of a special form. The numerical scheme is, however, designed in such a way that any other stress-strain relations may be readily introduced. A detailed study of the Poynting effect has been conducted, and the dependence of the effect on the various parameters is presented graphically. For a bar made of steel, radial distributions of displacement, volume change, and stress components are given.
Stress Softening and Strain Softening of Poly(Methyl Methacrylate) in Yielding under Constant Load
Authors: Ender Dieter H.
Published: February 2004

The strain rate of poly(methyl methacrylate) during yielding under constant load was determined experimentally for various tensile loads at temperatures from 70° to 100°C. In the constant-load test, the polymer strains uniformly beyond the yield point up to several times the yield strain. The yield point is evident as a minimum of the strain rate. Intermittent superposition of load increments affects the strain rate reversibly. The observed strain-rate increase under constant load in the uniform-strain post-yield region, results essentially from strain softening with some additional softening from the stress increase due to specimen thinning. The stress influence on the strain rate is expressible as a stress-shift factor of the strain rate. A theoretical stress-shift factor is derived from Doolittle's viscosity equation and the assumption that Poisson's volume dilatation contributes entirely to the free volume. Good agreement between the predicted and the observed stress-shift of the strain rate is found. An apparent anomaly of the stress-shift factor at 100°C is probably caused by recovery from strain softening in the vicinity of the glass transition temperature.
The Relationship between Morphology and Light-Scattering Patterns for Polytetrafluoroethylene
Authors: Rhodes Marion B. , Stein Richard S.
Published: February 2004

Two types of light-scattering patterns have been observed for polytetrafluoroethylene films. It has been postulated that these patterns arise from differences in the orientation of the optic axis with respect to the morphological unit of the structure. Films studied by the light-scattering technique have been examined by electron microscopy and the differences between the patterns are ascribed to differences in the orientation of the lamella with respect to the plane of the film.