bluuebunny/crossref_metadata_2025 huggingface dataset:
A low-temperature, whole-wafer-imaging system for defect and impurity mapping
Authors: Steiner T. W. , Thewalt M. L. W. , Maciaszek M. , Bult R. P.
Published: April 2011

We have developed a whole-wafer, low-temperature imaging system for making absorption and photoluminescence maps of wafers up to 100 mm (4 in) in diameter. The cryostat requires no cryogens and can maintain a temperature of 15 K in the sample chamber indefinitely. He gas provides the thermal contact with the samples, allowing rapid (15 min) interchange of samples. The transmitted or emitted light from the wafer is imaged on a CCD (charge-coupled device) camera rather than using a raster scanning mechanism. The resultant parallel collection advantage allows images to be acquired in much less time. The system is ideally suited for the routine characterization of large numbers of wafers. This apparatus has been used to map the neutral EL2 concentration and near gap photoluminescence efficiency for a number of semi-insulating GaAs wafers.
Characterization of a two-dimensional electron gas in GaAs–AlGaAs by surface acoustic waves
Authors: Guillion F. , Sachrajda A. , D'Iorio M. , Boulet R. , Coleridge P.
Published: April 2011

We report simultaneous magnetotransport and surface acoustic-wave attenuation measurements performed at 1.25 K on a two-dimensional electron gas (2-DEG) GaAs–AlGaAs heterojunction sample. The carrier density was varied by means of the persistent photoconductivity effect. A comparison of the conductivity deduced from the two types of measurements is used to obtain information about the homogeneity of the 2-DEG sample. For this specific sample a simple model shows that there exists a 20% variation in the carrier density at some intermediate carrier densities, while prior to illumination and after saturation with light the spread in carrier density in the sample was only a few percent.
Contribution des impuretés aux coefficients de transport des superréseaux, en absence de champ magnétique
Authors: Habti D. Ait el , Vasilopoulos P. , Currie J. F.
Published: April 2011

Using the transfer matrix method, effective mass approximation, and a Kronig–Penney potential, we have claculated the analytical normalized wave functions and the band structure of a superlattice. We have also studied the nonparabolicity effect for the conduction band. This latter effect was negligible for a GaAs–GaAlAs superlattice. We have used these wave functions to calculate the transport coefficients of superlattices. The linear response theory and the relaxation time (electron–impurity collisions) approximation were used. At very low temperatures the parallel (σzz) and perpendicular (σxx) conductivities do not vary with temperature, but increase with the electron concentration. The thermoelectric power (Qzz, Qxx) increases with temperature and decreases with the electron concentration. The thermal conductivities (Kzz,Kxx) increase with both temperature and electron density. These results are in agreement with the diffusive behavior of the transport coefficients. σzz, and Kzz increase with the width of the miniband. The perpendicular transport coefficients are larger than the parallel ones, verifying the anisotropy of the superlattice.
Experimental and theoretical electroabsorption in an InGaAs–GaAs strained-layer superlattice, and the performance of a wave-guide modulator
Authors: Hunt N. E. J. , Jessop P. E. , Wasilewski Z. R.
Published: April 2011

Excitonic electroabsorption was investigated in an InGaAs–GaAs strained-layer superlattice (SLS) wave-guide structure. The structure consisted of a SLS region embedded in a 1.2 μm thick single-mode wave guide. The SLS formed the intrinsic region of a pin diode structure and contained 40 periods of 10 nm thick In0.14Ga0.86As wells alternating with 10 nm thick GaAs barrier layers. Zero-bias transmission and photocurrent spectra were recorded for light incident normal to the surface, showing a sharp exciton feature centred at 955 nm. With increasing reverse-bias voltage this absorption peak shifted to longer wavelength, and became broader and weaker. The electric-field-induced changes in the exciton position were modelled using two different exactly solvable methods. Theoretical absorption strengths and widths were also determined. Calculation and experiment agreed reasonably well, though some differences were noted. Slab wave-guide samples, 360 μm long, were prepared, and transmission spectra for various reverse-bias voltages were obtained. At 982 nm, a transverse electric-polarization extinction ratio of 25 dB was measured between 0 and 8 V bias, with a zero-bias absorption loss of 2.8 dB. At 993.5 nm the zero-bias absorption was reduced to 0.8 dB but the extinction ratio fell to 17 dB.
A high-resolution Raman gain spectrometer for spectral lineshape studies
Authors: Forsman J. W. , Sinclair P. M. , Duggan P. , Drummond J. R. , May A. D.
Published: April 2011

The details and performance of a shot-noise-limited high-resolution (≤ 2 MHz) Raman gain spectrometer are discussed and compared to a "standard" continuous wave (cw) spectrometer. For a single pass cell and 1 s integration time, a signal-to-noise ratio greater than 1000 is obtained for the Q(2) line of D2 at pressures above a few atmospheres. The quality of the spectrometer for the determination of spectral profiles is demonstrated by examining the Dicke narrowing of the line.
Hamiltonian formulation of the Freedman–Townsend model of massive vector mesons
Authors: McKeon D. G. C.
Published: April 2011

A gauge invariant theory of massive vector mesons, formulated by Freedman and Townsend, is quantized using the Hamiltonian theory for reducible constrained systems of Batalin, Fradkin, and Vilkovisky. The effective action is Lorentz covariant in the gauge in which we work. All propagators have an ultraviolet behaviour that is consistent with power-counting renormalizability. We take this formulation of the Freedman–Townsend model to be consistent with unitarity.
Observation of translational-to-vibrational energy transfer in charge-transfer reactions of CO+ with CO using crossed-beam techniques
Authors: Howard Stephen L. , Wang Jaiming , Rockwood Alan L.
Published: April 2011

The crossed-beam method was used to investigate the charge-transfer reaction of CO+ (X2Σ+, ν = 0) with CO (X1Σ+, ν = 0). Scattering of CO+ demonstrates that several vibrational states are populated. At a collisional energy of 1.1 eV, transfer of translational energy to vibrational energy is shown to occur by changes of an even number of vibrational quanta.
Electron transmission in one-dimensional crystals with interfaces
Authors: Patitsas S. N. , Barrie Robert
Published: April 2011

An analysis is made of the transmission coefficient for electrons at a sharp interface separating two periodic one-dimensional potentials. Numerical results are presented for the case that the cell potentials are square wells. The transmission coefficient is sensitive to the position of the interface relative to the cells in the two crystals. Results are also presented for two interfaces (a barrier).
Cancellation effects in alkalilike doublet lifetimes
Authors: Curtis Lorenzo J.
Published: April 2011

A recent lifetime measurement of the 7p levels in Pb IV yielded an anomalous ratio [Formula: see text]. This result is shown to be a consequence of a nearly complete cancellation in the 6s–7p radial transition moment of the 2P1/2 level, coupled with the influence of wavelength-cubed factors on the 7s–7p, 6d–7p, and 6s2–7p transition probabilities. Cancellation effects that suppress the intensity of one branch of the decay of a level are a common feature of Δn > 0 transitions in alkali sequences, but this measurement demonstrates that these effects can also produce strong lifetime anomalies. The cancellation effects that produce this specific lifetime anomaly are examined isoelectronically, and the use of lifetime measurements near such cancellations as a probe of secondary interactions is discussed.
Collision-induced absorption in gaseous sulphur hexafluoride
Authors: Joslin C. G. , Gray C. G. , Dagg I. R. , Anderson A. , Gmach T. F. , Smith W.
Published: April 2011

The absorption spectrum of gaseous sulphur hexafluoride has been measured in the spectral region from approximately 20 to 140 cm−1 at 339 K. The experiments were performed over a range of densities from 65 to 109.6 amagat (for SF6, 1 amagat = 45.3 mol m−3), using FTIR spectroscopy. From the density dependence of the absorption, the collision-induced spectrum was clearly separated from the strong allowed transitions of vibrational difference bands at 26 and 91 cm−1. Our results for the induced spectrum differ substantially, both in overall magnitude and in shape, from earlier data. We interpret the absorption as due to anisotropic overlap induction; however, a substantial contribution from a hexadecapolar mechanism cannot be ruled out. We can place an upper limit of approximately 7 × 10−42 esu on the hexadecapole moment. The induced line shape is calculated using information theory, based on our knowledge of the zeroth and second moment of the frequency spectrum. Reasonable agreement is found between theory and experiment.
Stability of hydrogenated amorphous silicon prepared by reactive evaporation
Authors: Craigen D. C. , Audas R. D. , Brodie D. E.
Published: April 2011

Hydrogenated amorphous silicon (a-Si:H) was prepared by evaporating Si in a controlled ambient of reactive gases. Contamination of the samples by exposure to air affects both the dark conductivity and the photoconductivity. Some of the contamination effects can be removed by annealing, but some changes are not reversible. The irreversible changes are mainly due to the chemisorption of oxygen obtained from water vapour when the samples are stored in air. The Staebler–Wronski effect is observed in all samples whose photoconductivity is at least an order of magnitude higher than the dark conductivity. The photoconductivity versus time curve displays at t−1/3 dependence, typical of the Staebler–Wronski effect, but the degradation is much slower than that reported for glow discharge a-Si:H. The activation energy for the effect is 0.12 eV, which is larger than the 0.04 eV expected for the bond-switching model.
A solvable two-body Dirac equation in one space dimension
Authors: Dominguez-Adame F. , Méndez B.
Published: April 2011

A solvable Hamiltonian for two Dirac particles interacting by instantaneous linear potentials in (1 + 1) dimensions is discussed. The system presents no Klein paradox even if the coupling is rather strong, so particles remain bound. The four independent components of the wave function describing the system resemble the nonrelativistic oscillator eigenfunctions. Although the Hamiltonian is not fully covariant, the effective frequency of the oscillator obeys a typical relativistic Doppler law. In contrast to the nonrelativistic treatment, eigenstates are intrinsically coupled with the overall translational motion of the system.
Comparison of plasma-density and electron-temperature profiles during the auroral modelling campaign ARIES
Authors: Margot Joëlle , McNamara A. G.
Published: April 2011

Plasma-density and electron-temperature profiles were measured during the auroral modelling campaign ARIES. This campaign consisted of two rockets launched in the auroral E region under different geophysical conditions. The plasma-density and electron-temperature behaviours were tentatively related to the energy and intensity of the ionizing primary-electron fluxes. It is concluded that the plasma-density height distribution can be used to estimate the primary-electrons energy. The set of data presented is sufficiently complete to allow, when used together with other types of experiments such as the height distribution of the optical intensity and the high-energy electron spectra, the achievement of the objective of the ARIES multi-instrument campaign, i.e., refinement of the auroral model.
Possible influence of long-term sea surface temperature anomalies in the tropical Pacific on global ozone
Authors: Komhyr W. D. , Oltmans S. J. , Grass R. D. , Leonard R. K.
Published: April 2011

A significant negative correlation exists between June–August sea surface temperatures (SSTs) in the eastern equatorial Pacific and 15–31 October total ozone values at South Pole, Antarctica. SSTs in the eastern equatorial Pacific were anomalously warmer by 0.67 °C during 1976–1987 compared with 1962–1975. Quasi-biennial oscillation (QBO) easterly winds in the equatorial Pacific stratosphere were generally stronger after 1975 than they were before that time. Prior to the early-to-mid 1970s the trend in global ozone was generally upward, but then turned downward. Total ozone at Hawaii and Samoa, which had been decreasing at a rate of about 0.35% yr−1 during 1976–1987, showed recovery to mid-1970s values in 1988–1989 following a drop in SSTs in the eastern equatorial Pacific to low values last observed there prior to 1976. During 15–31 October 1988, total ozone at South Pole, which had decreased from about 280 Dobson units (DU) prior to 1980 to 140 DU in 1987, suddenly recovered to 250 DU, though substantial ozone depletion by heterogeneous photochemical processes involving polar stratospheric clouds was still evident in the South Pole ozone vertical profiles. These observations suggest that the downward trend in ozone observed over the globe in recent years may have been at least partially meteorologically induced, possibly through modulation by the warmer tropical Pacific ocean waters of QBO easterly winds at the equator, of planetary waves in the extratropics, of the interaction of QBO winds and planetary waves, and of Hadley Cell circulation. A cursory analysis of geostrophic wind flow around the Baffin Island low suggests a meteorological influence on the observed downward trend in ozone over North America during the past decade. Because ozone has a lifetime that varies from minutes to hours in the primary ozone production region at high altitudes in the tropical stratosphere to months and years in the low stratosphere, changes in atmospheric dynamics have the potential for not only redistributing ozone over the globe, but also changing global ozone abundance.
Polar ozone depletion: Current status
Authors: Henderson G. S. , McConnell J. C. , Beagley S. R. , Evans W. F. J.
Published: April 2011

Rapid springtime depletion of column ozone (O3) is observed over the Antarctic during the austral spring. A much weaker springtime depletion is observed in the Arctic region. This depletion results from a complex chemical mechanism that involves the catalytic destruction of stratospheric ozone by chlorine. The chemical mechanism appears to operate between ~12–25 km in the colder regions of the polar winter vortices. During the polar night heterogeneous chemical reactions occur on the surface of polar stratospheric clouds that convert relatively inert reservoir Cl species such as HCl to active Cl species. These clouds form when temperatures drop below about 197 K and are ubiquitous throughout the polar winter region. At polar sunrise the reactive Cl species are photolysed, liberating large quantities of free Cl that subsequently catalytically destroys O3 with a mechanism involving the formation of the Cl2O2 dimer. The magnitude of the spring depletion is much greater in the Antarctic relative to the Arctic owing to the greater stability and longer duration of the southern polar vortex. Breakup of the intense high-latitude vortices in late (Antarctic) or early (Arctic) spring results in infilling of the ozone holes but adversely affects midlatitude ozone levels by diluting them with O3-depleted, ClO-rich high-latitude air. The magnitude of the Antarctic ozone depletion has been increasing since 1979 and its current depletion in October 1990 amounts to 60%. The increase in the size of the depletion is anticorrelated with increasing anthropogenic chlorofluorocarbon (CFCs) release. Adherence to the revised Montréal Protocol should result in a reduction of stratospheric halogen levels with subsequent amelioration of polar ozone depletion but the time constant for the atmosphere to return to pre-CFC levels is ~60–100 years.
Atomic oxygen effects on polymer-based materials
Authors: Tennyson R. C.
Published: April 2011

This paper describes the operation and performance of an atomic-oxygen (AO) beam facility developed at the University of Toronto Institute for Aerospace Studies (UTIAS), capable of providing ground-state neutral oxygen atoms at ~ 2.2 eV for flux levels as high as ~ 1016 atoms (cm2 s)−1. Results are presented on the AO erosion of polymer thin films and composite materials containing graphite and aramid fibres in epoxy matrices. Comparisons with space flight tests are also given, including studies of samples recently retrieved from the UTIAS composite-materials experiment on the NASA Long Duration Exposure Facility after 70 months exposure in low Earth orbit. Parameters that have been investigated include synergistic effects of UV radiation, surface-morphology changes, and accelerated testing.
Exact geometrical realization of the Cabibbo–Kobayashi–Maskawa matrix: The catamorphy of quark–lepton families
Authors: Wilkinson D. H.
Published: April 2011

It is suggested that the number of quark–lepton families is three as the result of a catamorphy; this is illustrated by a geometrical representation that limits the number of families to three, that most naturally generates 15 quark–lepton fields per family, and that exactly realizes the Cabibbo–Kobayashi–Maskawa matrix. The geometrical representation uniquely requires a 1:λ:λ2:λ3 hierarchy for the weak quark–quark linkages and, together with a supplementary Ansatz that derives from the geometrical representation, entrains CP violation in the [Formula: see text]-system and yields mt = 123 ± 65 GeV/c2 from an analysis of Cabibbo–Kobayashi–Maskawa phenomenology.
Painlevé analysis and special solutions for a class of Cahn–Hilliard equations
Authors: Choudhury S. Roy
Published: April 2011

A Painlevé analysis of a family of Cahn–Hilliard equations (with diffusion coefficients of the form [Formula: see text] is performed. For m = 2, the equation fails the Painlevé test. Of the other cases of physical interest, the case m = 1 has only the conditional Painlevé property; while the cases m = 3, 4 are weak Painlevé, suggesting that the equation is partially integrable for these cases. For the cases m = 1 and m = 2, an auto-Bäcklund transformation between two solutions is constructed, leading to classes of analytical solutions of the equations. Special analytical solutions for all [Formula: see text] are also constructed.
A solvable four-dimensional model with gauge symmetry
Authors: McKeon D. G. C. , Sherry T. N.
Published: April 2011

Recently, interest has been focused on quantum field theories in which a Lagrange multiplier field occurs in the classical Lagrangian. This has the effect of restricting the sum over classical paths in the path integral to solutions of the classical field equation. Examples of such theories are the dynamical theory of two-dimensional gravity proposed by Jackiw and Teitelboim, the Chern–Simons formulation of gravity in 2 + 1 dimensions by Witten, and the path-integral formulation of classical systems by Gozzi. We examine, in this paper, a gauge theory in which a vector field [Formula: see text] acts as a Lagrange multiplier in the classical Lagrangian, ensuring that a vector field [Formula: see text] satisfies the Yang–Mills equations of motion. Quantization can be carried out either using BRST quantization or by using the Faddeev–Popov procedure. Either by explicitly integrating over the field [Formula: see text] and its associated ghost fields, or by directly examining the Feynman perturbation theory, it can be established that all diagrams beyond one-loop order vanish, allowing one to compute the one-particle irreducible generating functional exactly. Background-field quantization is introduced to simplify the renormalization program. The β function is computed in closed form. In an appendix we show how our interaction can be derived from Yang–Mills theory based on a group G or by considering the Yang–Mills theory for a group IG. This can be extended to deal with four interacting gauge fields. A second appendix deals with a scalar model that superficially resembles our vector model.
The analysis of the 5s25p2, 5s5p3, 5s25p5d, and 5s25p6s configurations of Te III
Authors: Joshi Y. N. , Tauheed A. , Davison I. G.
Published: April 2011

The spectrum of tellurium was photographed in the wavelength region 400–2000 Å (1 Å = 10−10 m) on a 3 m normal incidence spectrograph. The new measurements and configuration interaction calculations lead to major revisions in the earlier analysis of Te III. For the ground state 5s25p2, all but one of the previously known levels have been confirmed. Out of the 26 levels belonging to the three excited configurations, viz. 5s5p3, 5s25p5d, and 5s25p6s, 14 levels have been confirmed, 4 have been rejected, and for 8 of the levels, either the designation, J value or configuration assignments have been revised. Configuration interaction calculations were carried out to interpret the observed spectrum. Seventy-two lines have been classified in the 5s25p2 − (5s5p3 + 5s25p5d + 5s25p6s) transition array.
InGaAs/GaAs single quantum well lasers with monolithically integrated multilayer wave guides for surface-emitted sum-frequency generation
Authors: Dai H. , Janz S. , Normandin R. , Chatenoud F.
Published: April 2011

We report on strained layer InGaAs/GaAs single quantum well (SQW) lasers with monolithically integrated nonlinear wave guides for surface-emitting sum-frequency generation. Broad-area, ridge wave-guide and segmented cavity lasers with uncoated facets were processed from a molecular beam epitaxy grown AlxGa1 − xAs multilayer embedded with an InGaAs SQW separate cofinement heterostructure and tested at room temperature. Broad-area threshold current density of 87.5 A cm−2 was obtained for a cavity length of 3.3 mm and ridge lasers had a minimum threshold current of ~8 mA. The differential internal quantum efficiency, wave-guide loss, and lasing wavelength variation with cavity length were investigated. The sum-frequency generation (SFG) was studied in an active/passive segmented cavity device by coupling 1.06 μm light into the cavity as the transverse magnetic guided wave to interact with the transverse electric SQW laser mode. The measured conversion efficiency in terms of the nonlinear cross section An1 is about 2 × 10−7 W−1, which agrees well with the calculated value. Applications of the integrated devices as WDM demultiplexers and spatially addressable coherent detectors were demonstrated.
Electronic passivation of GaP surfaces using (NH4)2S treatment
Authors: Jedral L. , Ruda H. E. , Sodhi R. , Ma H. , Mannik L.
Published: April 2011

The influence of (NH4)2S treatment on the surface properties of GaP is presented for the first time. Changes in the chemical composition and Fermi level position are characterized using X-ray photoelectron spectroscopy. These measurements show that after surface treatment, the GaP surface is free of native oxides. A strong enhancement (~ 50 times) in the cathodoluminescent efficiency was observed for sulfur passivated samples. Schottky diode characteristics were used to reveal changes in the barrier height and interface state density due to surface treatment. Changes in carrier concentration were also found to be reflected in measured Raman spectra. The passivation mechanism is also briefly discussed.
New experimental method for determining the S1 → Sn absorption spectrum of highly fluorescent dyes
Authors: Gazeau Marie-Claire , Wintgens Véronique , Valat Pierre , Kossanyi Jean , Doizi Denis , Salvetat Gerard , Jaraudias Jean
Published: April 2011

A new method has been developed to measure the excited-state absorption cross sections σ1n, of highly fluorescent dyes. which gives excited singlet-state absorption (ESSA) spectra over most of the UV–visible region, including the one where the fluorescence occurs. The error in the σ1n values depends on that for the singlet excited-state concentration. Our method used benzophenone as an actinometer to determine the excited-state concentration. Rhodamine 6G was used to test the method; the ESSA spectrum obtained shows a maximum around 530 nm with an excited-state absorption cross section σ1n value of 2.08 × 10−16 cm2. Our results are discussed and compared with those reported in the literature.
Sputtering of cobalt and chromium by argon and xenon ions near the threshold energy region
Authors: Handoo A. K. , Ray P. K.
Published: April 2011

Sputtering yields of cobalt and chromium by argon and xenon ions with energies below 50 eV are reported. The targets were electroplated on copper substrates. Measurable sputtering yields were obtained from cobalt with ion energies as low as 10 eV. The ion beams were produced by an ion gun. A radioactive tracer technique was used for the quantitative measurement of the sputtering yield.57Co and 51Cr were used as tracers. The yield–energy curves are observed to be concave, which brings into question the practice of finding threshold energies by linear extrapolation.
(1/ω) Noise and the dynamical Casimir effect
Authors: Widom A. , Sassaroli E. , Srivastava Y. N.
Published: April 2011

The dynamical Casimir effect in quantum electrodynamics, which occurs for a frequency-modulated electromagnetic oscillator, is described in terms of reflection backward in time of the oscillator mode. From an experimental viewpoint, baekward-in-time reflections appear as radiated photons. The distribution of the electromagnetic radiation due to a modulation pulse is computed as a function of the frequency ω and is shown to exhbit a (1/ω) singularity in the limit ω → 0.
Hard thermal loops and their Noether currents
Authors: Weldon H. Arthur
Published: April 2011

The effective action found by Braaten and Pisarski, and by Frenkel, Taylor, and Wong that summarizes all hard thermal loops is investigated. Nonlocal field equations for ψ and Aμ are derived. Nonlocal forms are constructed for the U(1)-vector and axial-vector currents, the SU(N)-vector and axial-vector currents, the energy-momentum tensor, and the angular momentum density.
Characteristics of Thomas-Fermi solutions for nuclei with specified angular momenta
Authors: Gallego J. , Gupta S. Das
Published: April 2011

We obtain Thomas–Fermi solutions for nuclei in the mass range [Formula: see text]. Solutions for a given nucleus are obtained from angular momentum zero to the maximum value at which the nucleus breaks up. On one hand, we can establish contact with the liquid-drop model; on the other hand since Thomas–Fermi theory can be regarded as an approximation to Hartree-Fock theory and does use an underlying force (the same force can be used in Hartree–Fock theory), it is also instructive to compare results with the quantum cranking calculations carried out by other researchers. It appears that for low-mass nuclei quantum effects make the two results very different even at high spins but for medium-mass nuclei there are similarities that are seen at high-spin states. Rather large deformations are obtained at high-spin states but the magnitudes are sensitive to the actual force used. We present graphs indicating the change of shape in the Q – γ plane as angular momentum is cranked up.
Maximal Abelian subalgebras of complex Euclidean Lie algebras
Authors: Kalnins E. G. , Winternitz P.
Published: April 2011

Maximal Abelian subalgebras (MASAs) of the complex Euclidean Lie algebra [Formula: see text] are classified into conjugacy classes under the action of the Lie group [Formula: see text] Use is made of an earlier classification of MASAs of the orthogonal Lie algebra [Formula: see text] These are then extended to nonsplitting MASAs of [Formula: see text] in which maximal Abelian nilpotent subalgebras of [Formula: see text] are coupled with translations in a nontrivial manner. The methods presented are applicable to the classification of MASAs of any affine Lie algebra.
Some results on the pseudorapidity and clusters produced in 50 and 400 GeV particle–nucleus interactions
Authors: Khushnood H. , Ansari A. R.
Published: April 2011

The characterization of clusters produced in 50 GeV π−–nucleus and 400 GeV proton–nucleus interactions were investigated. The results reveal that the leading particle effect is more pronounced in 400 GeV proton–nucleus interactions. Moreover, from the study of minimum rapidity gaps distributions, it is observed that about 90% of the events possess at least one cluster at 400 GeV that is independent of the number of encounters made by the impinging hadron inside the struck nucleus, v, Finally, this result at 400 GeV is in complete accord with the predictions of multiperipheral model.
The effect of magnetic fields on the nonlinear instability of two superposed magnetic streaming fluids, each of a finite depth
Authors: Elhefnawy Abdel Raouf F.
Published: April 2011

The nonlinear Kelvin–Helmholtz instability of a horizontal interface separating two flowing superposed magnetic fluids of finite depths is described in the presence of a normal magnetic field. The fluids are taken to be incompressible and inviscid and the motion is assumed to be irrotational. The method of multiple-scale perturbations is used to obtain two nonlinear Schrödinger equations describing the behaviour of the perturbed system. The stability of the system is discussed both theoretically and numerically and the stability diagrams are obtained. The nonlinear cutoff magnetic field that separates the regions of instability from those of stability is also obtained.
Two-body perturbed problem revisited
Authors: Vrbik Jan
Published: April 2011

Quaternion formulation of a perturbed two-body problem is extended to include nonconservative forces, and the resulting algorithm is demonstrated using the classical example of the precession of apses.
Ouvertures de traces développées de particules α dans un détecteur polymérique (CR39) : modélisation et vérification expérimentale
Authors: Khalil A. , Membrey F. , Fromm M. , Chambaudet A.
Published: April 2011

With the aim of establishing a method for the dosage of α emitting radioactive elements present in natural phosphates, by using a polymeric detector of nuclear tracks, in our case a polycarbonate sold under the trade name CR39, we present, in this paper, a formalism for the development of nuclear tracks. This development widens the latent track (the zone of damage produced by the passage of the α particle) to make it visible by optical microscopy. The model uses two velocities of attack, a constant one (VB) and a variable one (VT). The evaluation of VT is obtained from the spherical phase of overdevelopment. A comparison is afterwards made between the apertures of the experimental tracks and of those simulated with the proposed formalism and the expressions of VB and VT.
Onsager theory of geminate recombination: A single series expansion
Authors: Que W.
Published: April 2011

The Onsager theory of geminate recombination has been widely applied to low-mobility materials such as anthracene and amorphous selenium. The photogeneration efficiency in this theory is usually expressed as a triple series expansion. A single series expansion has been previously derived, but it makes use of nonexplicit functions. Here we derive a single series expansion that allows rapid and accurate computation of the photogeneration efficiency, and eliminates the need for nonexplicit functions.
Characterizing experimental pressure and temperature conditions in multi-anvil apparatus
Authors: Walter M. J. , Thibault Y. , Wei K. , Luth R. W.
Published: April 2011

Octahedral media made of MgO–5%Cr2O3, with edge lengths of 18, 14, and 10 mm are used as pressure cells in experiments in a multi-anvil solid media apparatus at pressures of 4 to 27 GPa and temperatures to > 2700 °C. Calibrations of press-load versus sample pressure are based on room-temperature and high-temperature phase transitions, and are accurate to within ± 0.5 GPa. Calibrations of the temperature distribution were made in the central portion of the furnaces (graphite or LaCrO3) in the various sample assemblies used routinely in this laboratory. The following gradients away from the furnace midlines were observed: 18 mm: high-T straight graphite (−100 °C mm−1), high-T stepped graphite (+ 25 °C mm−1), low-T stepped graphite (−20 °C mm−1), high-T stepped LaCrO3 (−50 °C mm−1); 14 mm: high-T stepped LaCrO3 (−70 °C mm−1); 10 mm: straight LaCrO3 (−200 °C mm−1). The effect of increasing the wall thickness of the central segment of the furnace ("stepping") is to reduce the temperature gradient relative to a straight design. The relative effect of pressure on W3Re–W25Re and Pt–Pt13Rh thermocouples was measured by comparison of apparent temperatures recorded by each type in a given experiment. Corrections for the effect of pressure on thermocouple emf depend on the temperature distribution in the gasket regions surrounding the pressure cell, where pressure is reduced to ambient conditions. The temperature of this pressure seal controls the magnitude of the effect of pressure on thermocouple emf. Because this temperature will vary depending on the assembly, no universal pressure correction can be derived.
Crack velocities in concrete subjected to impact loading
Authors: Mindess Sidney
Published: April 2011

A high-speed video camera, operating at 1000 frames per second, was used to determine crack velocities in concrete blocks subjected to impact loading. The impact loads were generated using a large, instrumented drop-weight impact machine, capable of dropping a 504 kg mass through heights of up to 2.5 m. Specimens in the form of 254 mm cubes were subjected to splitting tension using the impact machine. Plain concrete, and concretes reinforced with two different types of discontinuous steel fibres (fibre volumes of 0.77%) were tested. It was found that steel fibres both reduced the observed crack velocities, and increased the fracture energies.
BRST formulation of the Christ–Lee model
Authors: Kulshreshtha Usha , Kulshreshtha D. S. , Müller-Kirsten H. J. W.
Published: April 2011

The BRST formulation of the Christ–Lee model describing a particle moving in a central potential V, but being observed in a frame rotating with angular velocity ξ, is investigated.
Broadening and shifting of the depolarized component of the Raman Q branch in D2 at room temperature
Authors: Sinclair P. M. , Duggan P. , Drummond J. R. , May A. D.
Published: April 2011

We report the first direct measurement of the broadening and shifting of the depolarized component of the Raman Q branch in D2. Spectral data were recorded from 1.5 to 12 amagat at 305.2 K. Both the measured broadening and shifting of the depolarized component are significantly different from that for the polarized Q branch. These results can be used as a check of theoretical calculations when they become available. Here they are used to correct previously reported values of the broadening of the polarized part of the Q-branch spectrum. Measured values of the ratio of the depolarized to polarized intensity for the Q-branch lines are also given.
New analytical expression for the rotational factor in Raman transitions
Authors: Korek M. , Kobeissi H.
Published: April 2011

The matrix elements of the polarizability anisotropy γ in the Raman spectra of diatomic molecules are investigated. These matrix elements are given by [Formula: see text] where Gνν′(m) is the rotational factor with m = [(J′(J′ + 1) − J(J + 1)]/2 and J′ − J = ±2. By using a nonconventional approach to the Rayleigh–Schrödinger perturbation theory the rotational factor can be written as Gνν′(m) = A0 + A1m + A2m2 where the coefficients A0, A1, and A2 are given by simple analytical expressions in terms of the integrals [Formula: see text] and [Formula: see text] where Y stands for Ψ(0) (the pure vibration wave function), or Ψ(0) (the first rotational perturbative correction to Ψ(0), or Ψ(2) (the second correction). A numerical application is presented for the ground states of CO and H2 molecules. A comparison with a numerical and direct calculation of the rotational factor Gνν′(m) shows the accuracy of the present formalism.
Time-dependent critical spin fluctuations in magnetic salts
Authors: Lovesey Stephen W.
Published: April 2011

A short perspective of dynamic critical scattering is given, together with an orientation to modern theories of static and dynamic critical phenomena. The materials discussed are magnetic salts that are believed to spontaneously order through correlations induced by a Heisenberg exchange interaction between atomic spin moments.
Energy distributions of neutrons scattered from solid C60 by the beryllium detector method
Authors: Copley J.R. D. , Neumann D. A. , Kamitakahara W. A.
Published: April 2011

We measured the energy distribution of neutrons scattered from polycrystalline C60, using a high-resolution filter-analyzer spectrometer. In the energy range 30–90 meV (242–726 cm−1) we observed a rich spectrum that we fitted to a sum of 15 Gaussian functions, each of which is assigned to one or a set of several degenerate normal vibrational modes of the C60 molecule. We also observed two broad features in the energy range from 90–130 meV (726–1049 cm−1). Our results are generally in excellent agreement with published spectroscopic data. Detailed comparisons with the results of several first-principles calculations suggest that present-day theories can predict the internal vibrational frequencies of C60 rather well, at least in the 30–90 meV range of energies.
Neutron scattering near the order-disorder transition in Cu3Au: Evidence for a lower spinodal temperature
Authors: Rogge R. B. , Svensson E. C. , Wei W. , Rogge R. B. , Gaulin B. D. , Hallman E. D.
Published: April 2011

The binary alloy Cu3Au undergoes a first-order phase transition at 667 ± 3 K Within the context of the Landau theory of phase transitions, there exist, in addition to the order–disorder temperature, Tc, upper and lower spinodal temperatures, Tsu and Ts1. These mark the first temperatures, upon approaching the phase transition from above and below, respectively, at which metastable droplets of the second phase can fluctuate out of the first phase. Until recently, there has, however, been little physical evidence supporting the existence of the spinodal temperatures. Elastic and inelastic neutron-scattering measurements have been carried out on Cu3Au over an extended temperature range with particular emphasis on temperatures near Tc. The lattice constant data, order-parameter data, and phonon data provided by these measurements all indicate that there are two temperature regimes just below Tc with a crossover between these regimes in the range of (Tc – 35) to (Tc – 25) K. This crossover temperature is interpreted as the lower spinodal temperature of Cu3Au.
Positronium formation in low-energy positron–helium scattering
Authors: Humberston J. W. , Reeth P. Van
Published: April 2011

Detailed investigations were made of positron collisions with helium atoms in the Ore gap using a two-channel version of the Kohn variational method with very flexible trial wave functions and accurate correlated helium-target wave functions. Accurate values of the s-wave elastic scattering and positronium-formation cross sections are presented, together with preliminary values of the cross sections for s-wave scattering. The s-wave positronium-formation cross section in helium displays a remarkable similarity in both magnitude and energy dependence to the corresponding cross section in positron–hydrogen scattering, with a rapid rise from the positronium-formation threshold and a more gradual rise thereafter. Its small magnitude is consistent with the findings of a recent analysis of experimental measurements of the positronium-formation cross section. Calculations of the s-wave elastic-scattering cross section above and below the positronium-formation threshold reveal a Wigner "rounded step" at the threshold itself.
The e+–H experiment of the Bielefeld–Brookhaven collaboration
Authors: Raith W. , Hofmann A. , Weber M. , Lynn K. G.
Published: April 2011

The Bielefeld–Brookhaven collaboration began in 1989 and was originally scheduled for a duration of three and then six years and has recently been extended for about one more year. It will end in 1996 for reasons of manpower and funding. The goal of this collaboration was to measure integral and differential e+–H cross sections by employing the high-intensity positron beam (HIP) of the Brookhaven National Laboratory, anticipated to provide an electrostatically guided beam of 109 moderated positrons s−1. This goal has not yet been reached. Over all these years, the HIP operation has been suffering from a variety of technical difficulties, despite the great efforts of all parties involved. Nevertheless, since the HIP situation is improving, we will continue this collaboration and try to reach the goal within the remaining time. Our experimental results obtained with low-current positron beams thus far are discussed together with related experimental and theoretical work of other groups. Particular attention is given to open questions. The present experimental setup at the Brookhaven National Laboratory is described in detail. The planned measurements are outlined in order of their priority.
Positron annihilation in a simulated interstellar medium
Authors: Iwata K. , Greaves R. G. , Surko C. M.
Published: April 2011

The annihilation of positrons in a simulated interstellar medium consisting of molecular hydrogen and a small admixture of the polycyclic aromatic hydrocarbon, naphthalene, was studied in a Penning trap containing cold positrons. The experiments demonstrate the ability to distinguish two distinct components in the annihilation γ-ray spectra. The significance of the experiment for the analysis of γ-ray spectra of astrophysical origin is discussed.
Positron-molecule scattering at low and medium energies
Authors: Ghosh A. S. , Mukherjee T.
Published: April 2011

This review surveys the recent developments carried out by us on positron–molecule scattering at low and intermediate energies. The relative merits between the laboratory frame rotational close-coupling (rotational LFCC) and the adiabatic nuclei rotation models near the electronic and rotational excitation threshold energies are discussed. The importance of the use of the rotational LFCC model for polar molecules is also reviewed. The rovibrational LFCC and the adiabatic nuclei rovibration models are used to calculate the scattering cross sections for e+–H2 scattering. The results demonstrate the necessity of the inclusion of the vibrational motion dynamically in the theory.
Differential cross-section measurements in positron–argon collisions
Authors: Finch R. M. , Kövér Á. , Charlton M. , Laricchia G.
Published: April 2011

Differential cross sections for elastic scattering and ionization in positron–argon collisions as a function of energy (40–150 eV) are reported at 60°. Of particular interest is the energy range 55–60 eV, where earlier measurements by the Detroit group found a drop in the elastic-scattering cross section of a factor of 2. This structure has been tentatively attributed to a cross channel-coupling effect with an open inelastic-scattering channel, most likely ionization. Our results indicate that ionization remains an important channel over the same energy range and only begins to decrease at an energy above 60 eV.
Coupled-channel calculations of e+–Mg scattering
Authors: Hewitt R. N. , Joachain C. J. , Noble C. J. , Bransden B. H.
Published: April 2011

Close-coupled calculations are presented for positron scattering by Mg atoms. We examine the effect upon elastic scattering and excitation of the target when the coupling between the target and the Ps(n = 1, 2) channels is taken into account. The calculations are performed within the single-active-electron approximation. The final cross sections are obtained by implementing the independent particle model. The inclusion of the coupling between the target and Ps channels is found to significantly change the behaviour of the target cross sections for impact energies below about 30 eV. Positronium formation is found to be the dominant inelastic process in the impact energy range below about 15 eV. The present partial cross section estimates are of interest in the light of the first experimental work recently performed for positron–Mg scattering.
Studies of autoionization with (e,2e) spectroscopy
Authors: Crowe Albert , McDonald Denis G. , Martin Shouna E. , Balashov Vsevolod V.
Published: April 2011

Recent advances in the study of double excitation-autoionization processes in helium using the (e,2e) technique are reviewed. Experiments at Flinders (Australia) and Newcastle (UK) provide the first reliable experimental data and have provoked the development of new theoretical approaches to the problem. Comparisons with recent calculations of the coincident electron spectra and parameters characterising the resonance states are presented for different kinematic conditions. Since the resonance parameters contain complex amplitudes relating to the resonance excitation, and direct and resonance ionization, they are extremely sensitive to the dynamics of the reaction and the atomic structure. Hence they provide the most rigorous test of the theory of ionization by electron impact.
Equivalence between the formulas for inductance calculation
Authors: Bueno M , Assis AKT
Published: April 2011

We demonstrate the equivalence for the self-inductance of closed circuits, with the formulas of Neumann, Weber, Maxwell, and Graneau.
Two spectrometers for threshold photoelectron coincidence studies of double photoionization
Authors: Hochlaf M. , Kjeldsen H. , Penent F. , Hall R. I. , Lablanquie P. , Lavollée M. , Eland J. H. D.
Published: April 2011

We present here two simple instruments for the study of double photoionization processes at threshold. They rely on the coincidence measurement of two near-zero energy electrons, which are collected by a penetrating field technique, and energy selected either by time-of-flight analysis (1st setup) or by electrostatic filtering in a hemispherical analyzer (2nd setup). Performance of the apparatus is demonstrated on Ar threshold double photoionization, and used to show the importance of two step double photoionization routes at Ar and Kr double ionization threshold.
Relativistic distorted-wave calculation of electron impact excitation of silver and gold
Authors: Zeman V. , McEachran R. P. , Stauffer A. D.
Published: April 2011

Relativistic distorted-wave calculations were performed for the electron impact excitation of silver and gold to their first (np) 2P1/2,3/3 levels at incident electron energies of 30–100 eV. Results are reported for the differential and integrated cross sections, differential and integrated Stokes parameters, and generalized STU parameters. Mott scattering is found to play an increasingly dominant role at larger energies and scattering angles.
Behaviour of the nonlinear Dirac soliton in an external field of the Lorentz scalar type
Authors: Toyama F M
Published: April 2011

The nonlinear Dirac soliton in an external field of the Lorentz scalar type in (1+1) dimensions is examined by solving the nonlinear Dirac equation numerically. A remarkable feature of the behaviour of the soliton is found, namely, for the linear potential the motion of the soliton centroid is independent of the strength of the nonlinear interaction within the accuracy of the calculation. This feature can be understood by means of a version of the so-called collective variable Ansatz, which leads to an approximate equation of motion for the soliton centroid. It is argued that this feature is characteristic of the case with the linear external potential of the Lorentz scalar type. For the quadratic potential, however, the independence of the motion of the soliton centroid on the strength of the nonlinear interaction is broken very slightly.PACS Nos.: 03.50-w; 03.40.Kf; 11.10.Qr