bluuebunny/crossref_metadata_2025 huggingface dataset:
GaAs epitaxial layers obtained by close-spaced vapor transport in H2 + H2O and H2 + CO2 ambients: Fine control of the growth rate and its effect on the electrical properties of the layers
Authors: Cossement D. , Huang Z. , Perron G. , Jean B. , Dodelet J. P.
Published: April 2011

In view of developing the close-spaced vapor transport technique (CSVT) to obtain III/V homojunction solar cells, it is necessary to finely control the growth rate of GaAs epitaxial layers. This has been performed either by controlling the water vapor pressure, [Formula: see text] injected in the reactor along with H2, in H2 + H2O ambient, or by controlling the water vapor pressure generated in situ by the reaction of H2 + CO2 in the reactor. For H2 + CO2 ambient, [Formula: see text], controls [Formula: see text] according to the following reaction: [Formula: see text]. The growth rates calculated with a diffusion controlled model are in agreement with the experimental values for both ambients, including the observation of a maximum in the evolution of the growth rate with [Formula: see text], Controlling the growth rate of GaAs by changing [Formula: see text] affects the carrier density (NA–ND) of p-type layers grown from Zn-doped GaAs sources. In both ambients (NA–ND) is a function of [Formula: see text]. Such a behavior is also obtained for the calculated carrier densities. It is the result of the transport of Zn as ZnO in CSVT. In H2 + CO2 ambient, where H2O and C are generated in situ, carbon is not incorporated as a major p-type doping impurity, contrarily to expectations, n-type GaAs layers were also obtained from Te-doped GaAs sources. In that case, the measured NA–ND values are not affected by changes in [Formula: see text] because water is not involved in the transport of Te in CSVT.
Transition probabilities for lines arising from the 3d4 4p configuration of Cr(II)
Authors: Gonzalez A. M. , Ortiz M. , Campos J.
Published: April 2011

Relative transition probabilities for 43 lines arising from the 3d4 4p configuration of Cr(II) with cores 3d4 (5D), 3d4 (3P), and 3d4 (3H) were determined from measurements of emission-line intensities in a laser produced plasma. This experiment was carried out with Al–Cr alloys with a Cr content of about 0.1%, in order to have an optically thin plasma. Transition probabilities were placed on an absolute scale by using, where possible, accurate experimental lifetimes from the literature and line-strength sum rules. Comparison of the present results with the available theoretical and experimental data are made.
Daytime observations of chorus emissions at low latitude
Authors: Singh R. P. , Singh U. P. , Singh Ashok Kumar , Singh D. P.
Published: April 2011

Discrete very-low-frequency chorus emissions observed during the daytime at ground station Gulmarg (geomag. lat. 24°10′N) are reported. The generation of these emissions is explained in terms of transverse resonance interaction between whistler waves and counter-streaming energetic electrons. The theory is tested by evaluating different parameters.
Collisional broadening and shifting of the pure rotational Raman lines S0(J = 0–4) of H2 at room temperature
Authors: Flohic M. P. Le , Duggan P. , Sinclair P. M. , Drummond J. R. , May A. D.
Published: April 2011

The pressure broadening and shifting, of the S0(J = 0–4) pure rotational Raman lines of normal hydrogen, have been determined from 1 to 35 amagat units of density, at 29.8 °C. Over the range of densities explored, the width and shift of the lines are linear functions of the density. Our precise results may be reconciled with earlier measurements at higher densities by adding a term cubic in the density for the widths and quadratic in the density for the shifts. It is argued that the present results are safely within the impact regime and may be compared with calculations when they become available. The accuracy obtained is about 0.2 MHz per amagat [Formula: see text], for the broadening, and ranges from 0.1 to 0.3 MHz per amagat [Formula: see text] for the shifting coefficients. An analysis of the results indicate that it may be possible to combine the shifts of the pure vibrational and the pure rotational Raman lines to disentangle the contributions of the isotropic and anisotropic intermolecular forces to the perturbation of molecular spectra. Departures from linear broadening and shifting, at high density, are discussed.
Interference effects in the spectrum of HD: VI: HD–HD, and HD–Kr at room temperature
Authors: McKellar A. R. W.
Published: April 2011

The absorption spectrum of the fundamental band of hydrogen deuteride (λ ≈ 2.7 μm) has been studied in pure HD and in mixtures with krypton at moderate densities (1–45 amagat) and room temperature, using a high-resolution Fourier transform infrared spectrometer. The effects that arise from interference between the allowed dipole transition moments of free HD and the dipoles induced during collisions were studied. For HD–HD collisions, the eight transitions from P1(3) to R1(4) were analyzed to determine line positions, intensities, shift and broadening coefficients, and the phase shift parameters that govern the interference effects. Thus the interference phenomenon was studied over a wider range of initial- and final-J values than previously possible, and the systematic dependence of the phase shifts on transition was determined. For HD–Kr collisions, the R1(0) and R1(1) transitions were examined in detail. The spectrum in the region of R1(1) exhibited a realtively broad underlying "plateau" feature that was shown to be due to the presence of impurity CF4 molecules in the Kr sample.
Optimum growth conditions for the epitaxy of GaAs on Ge by close-spaced vapor transport
Authors: Lalande G. , Guelton N. , Cossement D. , Saint-Jacques R. G. , Dodelet J. P.
Published: April 2011

GaAs epitaxial layers are grown by close-spaced vapor transport (CSVT) on (100)Ge substrates and (100)Ge substrates misoriented 1.5° and 3° toward (011). Water vapor is used as the transport agent. When the temperatures of the GaAs source (T1) and of the Ge substrate (T2) are 800 and 750 °C, respectively, the growth rate is about 3 μm h−1. When an optimum source–substrate temperature evolution is followed, it is possible to grow specular layers of GaAs/Ge that contain only a small number (< 105 cm−2) of threading dislocations. All antiphase boundaries (APBs) annihilate close to the interface (from about 230 nm for (100)Ge substrates to about 65 nm for vicinal (3° off) (100)Ge substrates). The GaAs growth occurs via the coalescence of 3D nuclei that are formed on an arsenic prelayer n-type GaAs layers are always obtained. By encapsulating the Ge substrate, it is possible to drastically decrease the autodoping resulting from the transport of Ge by water vapor in the same growth conditions as those prevailing for GaAs. After encapsulation, uncompensated doping densities ND – NA in the order of 5 × 1016 cm−3 are easily obtained for GaAs/Ge films grown from undoped semi-insulating GaAs sources. These GaAs/Ge layers can be used as bases for solar cells.
Energy bounds for the generalized exponential–cosine screened Coulomb potential
Authors: Côté Michel , Hall Richard L.
Published: April 2011

In this paper the energy eigenvalues corresponding to the generalized exponential–cosine screened-Coulomb potential, V(r) =−alr exp(−λr) cos(μr), are studied. Variational characterization of eigenvalues and the method of potential envelopes are used to determine general eigenvalue bounds and also bounds on the critical-screening parameters.
The renormalization group functions and the effective potential
Authors: McKeon D. G. C. , Kotikov A.
Published: April 2011

It is first demonstrated how the effective potential Veff in a self-interacting scalar theory can be computed using operator regularization. We examine [Formula: see text] and [Formula: see text] theories, recovering the usual results in the former case and showing how Veff is a power series in the square root of the coupling λ in the latter. Scheme dependence of Veff is considered. Since no explicit divergences occur when one uses operator regularization, the renormalization group functions (β and γ) associated with the dependence of λ and [Formula: see text] on the radiatively induced scale parameter μ must be determined by considering the finite effective potential. It is shown that one must in fact compute Vefff to a higher power in the perturbative expansion than if β and γ were to be computed using Green's functions. The usual results to lowest order are recovered in the [Formula: see text] model. Finally, a nonperturbative β function is determined by requiring that the mass generated by radiative effects be independent of μ2; it is found that both [Formula: see text] and [Formula: see text] are asymptoticly free with this β function. In the appendix we explicitly compute a two-loop integral encountered in the evaluation of Veff.
An algebraic construction of some self-conjugate operators in U(5)
Authors: Leroy C. , Michelot F.
Published: April 2011

The group U(5) may be taken as the degeneracy (respectively, dynamical) group of five (respectively, four) equivalent oscillators or particles. Practical applications require the knowledge of a complete set of independent tensors of arbitrary degree acting within a totally symmetric irreducible representation of U(5). These are built in the canonical chain and their components explicitly determined through a simple realization of semimaximal tensors.
Effects of the magnetic field on a non-Newtonian conducting fluid past a stretching plate
Authors: Helmy K. A.
Published: April 2011

The boundary-layer flow past a stretching plate in a viscoelastic conducting fluid in the presence of a magnetic field is considered. The solution for the boundary-layer equations is obtained using the similarity parameter method. An analytic solution for the temperature distribution of the plate undergoing cooling in this fluid and in the boundary layer is developed. The effect of the visco-elastic parameter k0 and the magnetic field on the flow velocity is investigated. The effect of the magnetic field on the convergence of the solution is also discussed.
Simple Cn modules with multiplicities 1 and applications
Authors: Britten D. J. , Hooper J. , Lemire F. W.
Published: April 2011

In this paper we show that there exist exactly two nonequivalent simple infinite dimensional highest weight Cn modules having the property that every weight space is one dimensional. The tensor products of these modules with any finite-dimensional simple Cn module are proven to be completely reducible and we provide an explicit decomposition for such tensor products. As an application of these decompositions, we obtain two recursion formulas for computing the multiplicities of simple finite dimensional Cn modules. These formulas involve a sum over subgroups of index 2 in the Weyl group of Cn.
A relativistic cockroach nest
Authors: Moshinsky M. , Mesa A. del Sol
Published: April 2011

This paper does not deal with cockroaches running, as they sometimes seem to do, at a speed close to that of light. Rather its subject is a cockroach nest, i.e., a normally innocuous crack in the wall, but from which cockroaches start pouring out if some food is put near it. The physical problem concerns first the possible values of the energy of two relativistic free particles. An elementary classical calculation, in the center of mass frame of reference, shows that energy levels E appear at E ≥ 2mc2 and E ≤ −2mc2but also at E = 0. The latter is our crack in the wall. Turning to quantum mechanics for two Dirac free particles we obtain explicitly the infinite number of states present at E = 0. We consider then a Poincaré-invariant two-body problem, along the lines suggested by Barut, and see how we can pass from two free particles, to the ones in which different types of interaction are present, i.e., put food near our cockroach nest in the wall. In particular when the two particles have a Dirac oscillator type of interaction, we can see exactly that in some cases the cockroach nest remains inert, in others, the cockroaches pour out, i.e., levels start pouring out from the one with E = 0. A variational procedure allows us to carry out numerical calculations for a particle–antiparticle system with Dirac oscillator interactions as well as what we call "positronium." In both cases all the cockroaches come out of the crack, i.e., all the degenerate states abandon the original E = 0. In the conclusion we indicate that this type of phenomenon also appears in the relativistic Hamiltonians employed by atomic physicists, and that the awareness of its existence may be of use in these types of calculations.
On the calculation of invariants of Lie algebras
Authors: Pecina-Cruz J. N. , Ne'eman Yuval
Published: April 2011

We discuss methods for the evaluation of generalized Casimir invariants of a Lie group and evaluate these invariants for solvable Lie algebras of dimension six with five-dimensional nil-radicals, thereby continuing a list published by R.T. Sharp and colleagues in 1975.
U(3) symmetry of the Sp(3, R) model
Authors: Rosensteel G.
Published: April 2011

The functionally independent U(3) scalars in the enveloping algebra of Sp(3, R) are given in terms of a generating function. The U(3) branching rules for the Sp(3, R) Verma modules are determined by another generating function. The U(3) scalar part of the nuclear symplectic collective model Hamiltonian is written explicitly in terms of a U(3) integrity basis. This scalar component of the Hamiltonian determines the average energy or centroid for each irreducible U(3) representation within an infinite-dimensional irreducible representation of the symplectic Lie algebra. The amplitude of the ground-state wave function for each U(3) representation space is expected to be a monotonically decreasing function of the centroid energy. A finite-dimensional truncation scheme for the nuclear shell-model excludes U(3) representation spaces with large centroid energies.
On factorization of the normalization SU(3) Clebsch–Gordan coefficients
Authors: Smirnov Yu. F.
Published: April 2011

The use of the factorized form of the extremal projection operator for the U(3) algebra allows one to prove the factorization of normalization Clebsch–Gordan coefficients [Formula: see text] (H is the highest weight) into the product of three simple SU(2) normalization Clebsch–Gordan coefficients in multiplicity free cases. A similar conclusion is conjectured to be valid for arbitrary simple Lie algebras (superalgebras) and their q analogs.
Laser ablation of silicon and aluminum. A photoacoustic and time-resolved-reflectivity study
Authors: Singleton Donald L. , Paraskevopoulos George , Irwin Robert S.
Published: April 2011

The interaction of KrF (248 nm) excimer laser pulses with silicon has been studied by photoacoustic and time-resolved-reflectivity techniques. Laser-induced acoustic pulses in aluminum and silicon were detected with a piezoelectric transducer. A sharp rise in amplitude of the longitudinal wave normalized to the incident fluence indicated the onset of ablation, thus permitting a fast and sensitive determination of ablation thresholds in solids. Threshold values of 0.50 and 0.95 J cm−2 were determined for aluminum and silicon, respectively. The same value is inferred for silicon by time-resolved reflectivity of the KrF laser beam that effects the ablation (self-reflectivity). Truncation of the reflected pulse starting at an incident fluence between 0.9 and 1 J cm−2 indicates the onset of ablation. Surface melting between ~ 0.2 and 0.9 J cm−2 was also observed in these experiments. Numerical simulation of the temperature evolution of the silicon surface gave values of the fluence required to raise the surface temperature to the melting and vaporization points in line with the observed values. The mechanism of pulse truncation is discussed in terms of time-dependent absorption and scattering from particles emerging from the surface during ablation. In agreement with the conclusion reached in another study, approximate calculations suggest that large particles are ejected initially that are continuously reduced in size during the laser pulse.
The spectroscopy of H2CS. 1. CS stretch potential curves for singlet states of planar H2CS, obtained by ab initio multireference CI methods
Authors: Hachey M. R. J. , Grein F.
Published: April 2011

For planar H2CS, (C2ν), the CS stretch potential curves were obtained for the four to six lowest singlet states of each symmetry species by using multireference CI methods. Included were the (n, 4s), (n, 4p), (n, 3d), (π, 4s), and (π, 4p) Rydberg as well as the (n, π*), (π, π*), (σ, π*), (n, σ*), (n0, π*2), and (nπ, π*2) valence states. Vertical and adiabatic excitation energies, equilibrium CS distances, vibrational frequencies for the CS stretching mode, dipole moments, oscillator strengths, and Franck–Condon factors were evaluated and found to be in good agreement with known experimental data. The role of the 1(π, π*) state that diabatically crosses all 1A1 states, including the n2 ground-state configuration, causing many interactions with other states, has been given special attention. The following reassignments and predictions are of interest. (i) A switch of Ẽ and [Formula: see text], with 1A1(n, 4py) corresponding to the Ẽ bands and 1B2(n, 4pz) corresponding to the [Formula: see text] bands is suggested, based on the energetic ordering. (ii) Because of strong Franck–Condon factors, hot bands are suggested to play an important role in the analysis of the CS stretch progression of [Formula: see text]. (iii) The [Formula: see text] system, only studied in low resolution, is predicted to have high intensity and be perturbed due to the crossing of (π, π*) with (n, 4py) in the vertical region. The CS stretch bands should be observable. (iv) Observed combination modes in the [Formula: see text] system may be due to vibronic mixing of (π, π*) with (σ, π*).
Can photoionization effects explain the solar-neutrino problem?
Authors: Hykawy J. G.
Published: April 2011

In a simple accretion model of the Sun, the inclusion of photoionizationlike phenomena leads to a set of initial conditions for solar burning that mimic a combination of the classic low-Z and high-Y models. When such initial conditions are used as input to the standard SunEV code, the predicted neutrino fluxes are in good agreement with those presently being observed in the Kamiokande, SAGE and GALLEX experiments. No new physics is required. It is possible that the combination of a low-Z and high-Y model, which is able to explain the solar neutrino deficit, might also provide good agreement with the presently observed helioseismic data. If data from the planned next generation of solar-neutrino experiments (e.g., SNO, SuperKamiokande) shows that the Mikheyer–Smirnov–Wolfenstein effect is not the solution to the solar-neutrino problem, experiments measuring the low-energy solar neutrino flux might show that a low-Z, high-Y combination model is valid.
Mechanics of a particle in a gauge field
Authors: Vatsya S. R.
Published: April 2011

Classical motion of a charged particle in an electromagnetic field is described by the Lorentz equation in terms of the field components. The field is defined by the infinitesimal gauge group elements associated with closed curves. The Lorentz equation may be derived from the action principle with an appropriate Lagrangian. The Lagrangian may also be used to associate group elements with curves in the space-time manifold. The action principle is shown here to be an equivalence relation between the infinitesimal elements so defined for a collection of closed curves and the identity element. This suggests a natural extension to require the equivalence of global elements with the identity and by considering all curves. The resulting equation, in addition to providing an extension of the Lorentz equation, also admits a straightforward generalization to non-Abelian gauge fields. The extended equation has an infinite number of trajectories as solutions. The properties of these paths are shown to impart wavelike properties to the particles in motion. In view of these results, the motion of a particle is formulated within the framework of the path integral formalism that yields a generalized Schrödinger type equation in a general gauge field. As a further implication of the properties of the trajectories assigned to a particle, this equation is shown to reduce to a set of equations, one of them being the Klein–Gordon equation.
Application of the interrupted field time-of-flight transient photoconductivity technique to investigating sample inhomogeneities: Cl-doped amorphous Se:Te and Se:As films
Authors: Kasap S. O. , Polischuk B.
Published: April 2011

The interrupted field time-of-flight (IFTOF) technique was applied to Cl-doped amorphous Se:Te and Se:As photoreceptor-type films to investigate the variation of the hole lifetime across the film due to sample inhomogeneities. By interrupting the hole photocurrent at different positions across the film, it was possible to obtain the variation of the hole lifetime, τ, across a photoreceptor film up to about two-thirds of the film thickness. For Cl-doped Se0.953Te0.047 films (46 at. ppm Cl) the Te content was uniform in the bulk, which resulted in the hole lifetime also being uniform. An important inference was that the Cl-content profile must therefore also be uniform. In the case of Cl-doped a-Se:0.2%As films (10 at. ppm Cl), the As concentration across the film was not uniform because of fractionation effects during vacuum deposition. The variation of the hole lifetime across the a-Se:As film correlated well with the As-concentration profile obtained from electron probe X-ray microanalysis. The present paper demonstrates the usefulness of the IFTOF technique for studying such inhomogeneities.
Liquid mirrors
Authors: Borra Ermanno F.
Published: April 2011

The surface of a spinning liquid takes the shape of a paraboloid that can be used as a reflecting mirror. This very old and nearly forgotten concept has recently been revived and I review its present status. Extensive interferometric tests of liquid mirrors (the largest one having a diameter of 2.5 m) show excellent optical qualities. I discuss the factors that can limit the optical quality of liquid mirrors, how to minimize them as well as the basic technology. A handful of liquid mirrors have now been built that are used for scientific work. I show representative data obtained from 2.65 m diameter liquid mirror telescopes used for astronomy and the atmospheric sciences (LIDAR). Section 5, of particular interest to cosmologists or astronomers using surveys, examines the expected performance of 4 m liquid mirror telescopes (LMTs) dedicated to cosmological surveys. It is rather impressive, due to the fact that the instruments work full-time on 4 year surveys: Spectrophotometry reaches B = 24 for all objects within over 100 square degrees and wide-band photometry reaches about B = 28. I consider the future of liquid mirror telescopes: limits to their sizes, engineering issues, as well as speculations on lunar or space LMTs. I briefly mention the possibility of nonrotating GRIN (gradient index) liquid mirrors. Finally, I address the issues of the field accessible to LMTs equipped with novel optical correctors. Optical design work, and some exploratory laboratory work, indicate that a single LMT should be able to access, with excellent images, small regions anywhere inside fields as large as 45°.
Anisotropic optical polaron in crystals with complex structure
Authors: Brancus D. E. N. , Mocuta A. C.
Published: April 2011

In the context of intermediate coupling theory, the forms of the self-energy and effective-mass tensor of the optical polaron, in appropriate uniaxial crystals with complex structure, are obtained. These forms contain all sources of anisotropy; the effective-mass tensor of the "bare" electron, the electron – optical-phonon interaction, and the angular dependence of the "true" phonoic modes. A numerical application is developed in the case of the CdS crystal.
A lattice dynamical study of Cu and Ni based on a new empirical many-body potential
Authors: Akgün I.
Published: April 2011

In the present work, a recently developed empirical many-body potential-energy function (PEF) is first used, as an application, to investigate the dynamical behaviors of the face-centred-cube d-band metals, Cu and Ni. The new PEF contains both two- and three-body atomic interactions. The two-body potential is a kind of hybrid function and the three-body potential is expressed in terms of the two-body interactions. The parameters defining the PEF for the metals are computed following a procedure similar to a method given by Girifalco and Weizer. The input data for evaluating the necessary parameters are independent of the phonon frequencies and elastic constants of the metals. The phonon frequencies along the principal symmetry directions of Cu and Ni are calculated using the computed two- and three-body force constants. The results are found to be in good agreement with the corresponding experimental values.
Edge states, band structure, and the Hall effect in two-dimensional lattice structures: quantum dot arrays and the tight-binding model
Authors: Akis R. , Barnes C. , Kirczenow G.
Published: April 2011

Using a model that is based on a transfer matrix formalism, we study the electronic structure and transport in two dimensional periodic arrays of quantum dots in magnetic fields. The quantum dots in our model are connected to each other via ballistic constrictions. The spectrum for this system has much in common with that with the tight-binding model. In particular, q bulk bands arise if the normalized magnetic flux per unit cell is p/q, where p and q are coprime integers. Working within an edge-state picture, we investigate if these similarities translate to a correspondence in the transport properties of the two systems. As we shall show, the answer to this question depends very much on the transmission probability of the constrictions. Our analysis also shows that, under certain circumstances, the Hall conductance within the context of the tight-binding model may take on fractional values.
Gravity currents and the electrolyte concentration field in electrochemical deposition
Authors: Linehan Kelly A. , Bruyn John R. de
Published: April 2011

Using shadowgraphs, we visualize local electrolyte concentration variations in quasi-two-dimensional electrochemical deposition experiments. Experiments have been performed in both circular and parallel-electrode geometries. Our results indicate that a region of ion depletion moves from the cathode towards the anode, while a region of enhanced concentration grows in from the anode. Large-scale gravity-driven flows, which drive the growth of these regions, are observed. The growth speed of the metal aggregate at the cathode is the same as the speed at which these flows move anions away from the cathode, except at the earliest stages of growth. By integrating the shadowgraph intensity, we calculate the concentration field in the parallel geometry.
Analyse du sillage proche d'une sphère par une méthode de reconnaissance de formes
Authors: Legentilhomme P. , Ferré J. A. , Giralt Francesc
Published: April 2011

A pattern recognition technique has been applied to analyse the near-wake of a sphere for a Reynolds (Re) criteria of 12 900 and 25 900. Measurements of the main components of the down-stream turbulent flow of the dynamic field of a sphere allowed to confirm literature data. Fluctuations of the streamwise velocity component were measured simultaneously by eight hot-wire anemometers arranged in a circular pattern with the radius corresponding to the position of the extrema of the Reynolds stress [Formula: see text]. For streamwise locations xld = 18 (d = 4 cm) and 36 (d = 2 cm) (d, diameter of the sphere; x, longitudinal position relative to the centre of the sphere), in the near-wake of the sphere, structures occur periodically. The Strouhal number characterizing this periodic activity is 0.18. These structures, which also exhibit some spatial periodicity, seem to constitute the counterpart of the Kármán vortex path detected in the wake of a cylinder perpendicular to the main flow. In the wake of a sphere, these structures might appear in the three-dimensional form of continuous crenellated rings for xld = 18, or interrupted in one or several points of their circumference for xld = 36.
The SAPPHIRE auroral radar system
Authors: Koehler J. A. , Sofko G. J. , André D. , Maguire M. , Osterried R. , McKibben M. , Mu J. , Danskin D. , Ortlepp A.
Published: April 2011

This paper describes a new, 50 MHz auroral radar system, named SAPPHIRE, which observes at two locations in Canada: SAPPHIRE SOUTH, in the auroral zone and SAPPHIRE NORTH at nominal cusp latitudes in the vicinity of the polar cap – auroral zone boundary. SAPPHIRE was designed to offer superior time and frequency resolution for studies of the geophysical phenomena in the observing regions and to provide a better insight into the plasma processes responsible for the radio scattering. The radars are continuous-wave, dual bistatic systems with two transmitters and two receivers (both at a common receiver location) in each radar system. This configuration provides two-dimensional estimates of the phase velocities in the region of the scatterers. The systems were designed for unattended operation at remote sites and are controlled via modems using conventional telephone service. There are a number of innovative parts to the design of this radar. All frequencies used in the transmitter and receivers are generated using direct digital synthesis which gives independent and accurate control of both frequency and phase. The data collection system uses extensive parallel processing via a custom design of INMOS "transputers" in order to handle the large amount of data received per day. This data collection system only stores data for those times when the observed echoes fulfill some minimum threshold conditions. The most interesting aspects of the electronic design are described and some representative data from SAPPHIRE SOUTH are presented.
Thermodynamic properties of a finite one-dimensional system of bosons with repulsive delta-function interaction
Authors: Poon K. L. , Young K. , Kiang D.
Published: April 2011

The thermodynamics of N bosons in a length L in one dimension, with repulsive delta-function interaction, is studied numerically for finite N, L. The results show the nature of finite-size corrections and how the thermodynamic limit is approached, and hopefully will be of some guidance in seeking the solution of a more general model.
Theoretical studies of structural stability at high pressure
Authors: Tse John S. , Klug Dennis D.
Published: April 2011

Theoretical methods are indispensible for the study of matter at high pressure. In the last decade the development of accurate intermolecular potentials and the methodologies in classical molecular dynamics (MD) simulations have greatly facililated the applications of these methods to the study of structural phase transformamtions of solids at high pressures. More recently, it has been possible to incorporate quantum mechanical effects into MD calculations. This method eliminates a great deal of empiricism. These first principles calculations have not only reproduced the experimental results for phase transformations but also provided detailed mechanisms and in some cases predicted new structures that may be found at high pressures. The success of MD calculations is illustrated through a review of our studies of pressure-induced amorphization and phase transitions in SiO2 and TiO2, and the structural memory effect in several materials. Current applications using quantum molecular dynamics on ice are discussed.
A microscopic theory of chemical reactions under high pressures
Authors: Basilevsky Michael V. , Weinberg Noham N.
Published: April 2011

A new approach to the microscopic description of high-pressure processes is formulated where the pressure effect on a chemical reaction system is represented by ah effective pressure-dependent potential of a compressed medium. The method is applied to discuss reaction kinetics and chemical equilibria under pressures from 0 to 45 kbar (1 bar = 105 Pa). It was found that under pressures exceeding 50 kbar the deformation of the original potential-energy surface of a chemical reaction caused by pressure can be so strong that the reaction becomes barrierless. It is also shown that at high pressures reaction kinetics can demonstrate considerably nonequilibrium (not obeying transition-state theory) behavior as a result of increased solvent viscosity.
High p, T physical property studies of Earth's interior: Thermoelectric power of solid and liquid Fe up to 6.4 GPa
Authors: Secco R. A.
Published: April 2011

High p,T experiments provide indirect access to the deep interior of Earth. High p, T research at the University of Western Ontario using large volume cubic anvil presses has focused on in-situ measurements of physical properties of mantle and core related substances as they relate to Earth's interior physical processes. Thermoelectric power measurements of solid and liquid Fe, using twisted Fe/Pt–10%Rh junctions, have been made at p up to 6.4 GPa. By isolating the sign determining energy derivative of the density of states at the Fermi energy, ∂N(εF)/∂ε, in the expression for the Seebeck coefficient of Fe, these measurements show that ∂N(εF)/∂ε is positive for liquid Fe at p below approximately 4.0 GPa but ∂N(εF)/∂ε is negative for p above 4.0 GPa. The solid phase polymorphism which Fe exhibits in this p region, and its influence on the liquid state, is suggested as the explanation for the short range bcc-like and fcc-like properties of the liquid. This finding corroborates our earlier measurements on the p-dependence of the T-coefficient of electrical resistivity of liquid Fe, which showed a similar discontinuity at the δ(bcc)–γ(fcc)-liquid triple point pressure.
Canola paste extrusion for feeding supercritical extractors
Authors: Wang Fangzhi , Meisen Axel
Published: April 2011

A custom-built, single-screw extruder (barrelf 0.44 m long, 36.75 mm ID; screw flight angles: 5.98°, 7.97°; or 9.94°; average screw flight: 4.9 mm high, 3 mm wide; flight-top to barrel clearance: 0.0254 mm) was developed for continuously feeding precrushed Canola seed pastes into supercritical fluid extractors. The screw was driven by two variable-speed hydraulic motors and rotated typically at 66.5, 88.5, or 100.5 rpm. The pressures and temperatures along the extruder barrel were measured with six iron–Constantan thermocouples and four pressure transducers, respectively. The discharge rate of the paste was determined gravimetrically and was found to vary from approximately 1.7 to 9.5 kg h−1 for average discharge pressures ranging from 30 to 10 MPa. The average pressure along the barrel was found to increase almost linearly. A three-component velocity model was developed to simulate the extruder performance and verified by comparison with experimental data obtained by using a Newtonian fluid having a viscosity of 130 Pa s. The model over predicted the discharge rates of Canola pastes unless allowance is made for slip. The practical implications for supercritical extraction processes are discussed.
"Nonintegral" expressions for the diatomic centrifugal distortion constants
Authors: Kobeissi Hafez , Trad Chafia H.
Published: April 2011

The problem of the centrifugal distortion constants (CDC), Dν, Hν, … for a diatomic molecule is considered. It is shown that a new formulation of the standard Rayleigh–Schrödinger perturbation theory can give simple and compact analytical expressions of the CDC (up to any order). Thus, the constants e1 = Bν, e2 = −Dν, e3 = Hν,…, en,… are all of the form en = lim σn(r)/σ0(r) as r → ∞. σ0 is the particular solution of the nonhomogeneous equation y″ + k(Eν – U)y = s, with s = ψν, where (Eν, ψν) is the eigenvector corresponding to the rotationless potential U(r) and to the vibrational level ν; and where σ0(0) = σ′0(0) = 0. σn is the particular solution of the above equation, where s is known for each order of n. The numerical application to the standard Lennard–Jones model potential shows that good results are obtained for Dν, Hν, Lν,…,Oν, Pν, for ν = 0 to 22, which is only at 2 × 10−4 of the well depth. The program uses one routine (the integration of the equation y″ + fy = s) repeated for different s; it is quite simple and gives no difficulties at the boundaries and there is no need to use any mathematical or numerical artifices.
Radiation characteristics of a slot antenna on a conducting prolate spheroid
Authors: Zhang M. , Sebak A. A.
Published: April 2011

In this paper, the radiation characteristics of an asymmetrical slot antenna on a conducting prolate spheroid are considered. The radiated fields are expanded in terms of prolate-spheroidal vector wave functions. The unknown expansion coefficients are determined by a system of equations derived from the boundary conditions. Numerical results for radiation patterns and conductance are obtained and compare well with published data for a slotted sphere. The effect of the slot length and the shape of the spheroid on the magnitude of the radiated field and radiation conductance is also presented.
COMMENT ON: Two-photon absorption series of calcium
Authors: Krumrine J. R. , Musgrove A. , Lucatorto T. B.
Published: April 2011

Using two-photon absorption laser spectroscopy of calcium, Makdisi and Bhatia presented a well-resolved series that they tentatively assigned as 4s21S0 – 4snf1F3 (n = 33 – 59). We find this assignment to be erroneous. Calculations of quantum defects and a comparison of their observed energy levels with previously determined values reveal that the spectra actually represent the 4s21S0 – 4snp1P1 series (n = 34 – 60).
Oscillator strengths for highly stripped ions of carbon isoelectronic sequence
Authors: Ghosh T. K. , Ray D. , Mukherjee P. K.
Published: April 2011

Time-dependent coupled Hartree–Fock theory was applied to estimate the allowed oscillator strengths of the astrophysically important highly stripped ions of the carbon isoelectronic sequence Ne4+, Mg6+, Si8+, S10+, Ar12+, Ca14+, and Ti16+ from their ground and valence-excited states up to principal quantum number n = 6. Oscillator-strength values compare favourably with the existing available data and many of the results are reported for the first time.
Description of reflection-generated polytopes using decorated Coxeter diagrams
Authors: Champagne B. , Kjiri M. , Patera J. , Sharp R. T.
Published: April 2011

A new method of explicit description of n-dimensional polytopes generated by reflections of a single point (D-polytopes) or a face of maximal dimension (V-polytopes) is used to provide a comprehensive uniform description of all such polytopes in dimensions three and four. In addition a class of six-dimensional polytopes is also described; its symmetry group is the Weyl group of the simple Lie group D6.
Calcul analytique du transport des charges dans le a-Si :H
Authors: Chen Wen Chao , Hamel Louis-André
Published: April 2011

Multitrapping transport of carriers through exponential band tails is studied for carriers generated at an arbitrary position in the device in presence of a linearly varying electric field, thus generalizing the usual treatment of time of flight experiments. An analytical expression is found for the free carrier density distribution n(x,t) for values of the dispersion parameter 0 < α < 1. In the case α = 1/2, analytical solutions are given for the transient current I(t) and the induced charge Q(t). Comparison with previous calculations is made for the special case of uniform electric field and carriers initially generated at one interface.
The effect of a time harmonic-magnetic field on the stability of cylindrical ferrofluids in the presence of heat and mass transfer
Authors: Moatimid Galal M.
Published: April 2011

The stability of two ferrofluids separated by a cylindrical interface and pervaded by a time harmonic-magnetic field is considered. The magnetic fluids are sandwiched between two rigid cylindrical edges. The cylindrical interface allows heat and mass transfer. A general dispersion equation is obtained. Some previous studies are compared using appropriate data. The case of a uniform field has been studied before, the attention is thus focussed to the case of the periodic field. The transition curves are obtained by means of Whittaker's technique. The analytical results are confirmed numerically. It is found that heat and mass transfer and the frequency of the magnetic field have a destabilizing influence.
The Lorentz force and Newton's third principle
Authors: Cornille Patrick
Published: April 2011

The Lorentz forces exerted by freely moving charges upon one another are not equal and opposite because the Lorentz force does not obey Newton's third principle, contrary to the Ampère force, which follows this principle. As a result, a pair of charged particles in relative motion can change its state of motion without external help. We review past experiments that show this effect.
Formation of the interaction matrix in pion–nucleon scattering in optimal conditions and amplitude determination at 471, 547, and 625 MeV c−1
Authors: Ghahramany N. , Falinejad H.
Published: April 2011

The most recent measurements of the polarization parameters were used to determine the magnitudes and relative phase of spin amplitudes in charged pion – proton scattering at 471, 547, and 625 MeV c−1. In addition to optimal conditions, invariance laws were imposed to reduce the number of independent amplitudes to two. In the transverse frame we obtained two amplitudes both being nonflip. In the helicity frame also we obtained two amplitudes, one spin flip and the other being spin nonflip. The magnitudes of the transverse amplitudes were also determined at 2.39 GeV c−1 in terms of square momentum transfer. In all the calculations, the differential cross section is normalized to unity.
On the long-wavelength limit of the structure factor
Authors: Apostol M.
Published: April 2011

The behaviour of the structure factor is discussed in the long-wavelength limit for some many-body systems.
Magnetic and powder neutron-diffraction studies of hexagonal β-NaCoPO4: A possible geometrically frustrated magnet
Authors: Raju N. P. , Greedan J. E.
Published: April 2011

Magnetic and powder neutron-diffraction studies were performed on a polycrystalline hexagonal compound β-NaCoPO4 with space group P65 and lattice constants, a = 10.167 Å and c = 23.785 Å (1Å = 10−10 m). The susceptibility data display a magnetic transition below about 3.2 K and follow the Curie–Weiss law in the temperature range 5–300 K with a paramagnetic Curie temperature, θp, of −16.6 K indicating that the spins are strongly frustrated. The Co2+ ions form a helix about the c direction but due to the topology of the structure the helix can be decomposed into sets of six roughly triangular layers per unit cell. Thus, nearest-neighbor antiferromagnetic interactions can give rise to frustration within the pseudolayers. Low-temperature neutron-diffraction data reveal magnetic Bragg peaks below the ordering temperature. A Rietveld refinement of the data suggests a 120° magnetic structure of Co2+ spins within each pseudolayer.
Membranes studied using neutron scattering and NMR
Authors: Bloom Myer , Bayerl Thomas M.
Published: April 2011

After reviewing some of the basic measurements that characterize the study of physical properties of matter using neutron scattering and nuclear magnetic resonance (NMR), connections between information obtained in current research on fluid membranes using these two complementary techniques are explored in two major chapters. In the first, the type of information on the structure of fluid membranes obtained from coherent elastic neutron scattering is compared with that from NMR spectral characteristics. Then, the type of information obtained on dynamical properties from NMR relaxation (T1 and T2) measurements is compared with that from quasi-elastic neutron scattering. Examples of such connections are given with an emphasis on relationships between the time and distance scales intrinsic to neutron scattering and NMR.
RITA: The reinvented triple axis spectrometer
Authors: Mason T. E. , Mason T. E. , Clausen K. N. , Aeppli G. , McMorrow D. F. , Kjems J. K. , Aeppli G.
Published: April 2011

Since the pioneering work of Brockhouse on inelastic neutron scattering the triple-axis spectrometer has been the instrument of choice for studies of dynamics, both magnetic and structural, in single crystals. In the years since the late fifties improvements in source brightness, beam optics, and detector technology have resulted in increased sensitivity and capabilities of the basic triple-axis design. Work has begun at Risø National Laboratory on a new spectrometer design, RITA, the reinvented triple axis, which attempts to incorporate more recent innovations such as multilayer supermirrors and microstrip proportional counters into a rethinking of the triple-axis spectrometer. By optimizing the beam optics using supermirrors and extending the analyser to map regions of (Q, ω) space using an array of independently controllable pyrolytic graphite crystals focussed on an area detector, we hope to increase the efficiency of single-crystal inelastic experiments by about a factor of 20.
Phonon density of states in vanadium
Authors: Sears V. F. , Svensson E. C. , Powell B. M.
Published: April 2011

The normalized phonon density of states g(v) of vanadium is accurately determined at room temperature (294 K) from the analysis of neutron inelastic-scattering data obtained using a triple-axis crystal spectrometer with a constant momentum transfer Q = 6.5 Å−1, (1 Å = 10−10 m) a constant scattered-neutron energy of 8.0 THz, and a variable incident-neutron energy. The energy transfer in the experiment varies from −1.9 to 10.0 THz, and the energy resolution (FWHM) is 0.35 THz at the elastic position. Necessary corrections are made for background scattering, multiple scattering, multiphonon scattering, absorption and self-shielding, and for the spatial inhomogeneity of the incident beam. The resulting g(v) distribution has an average statistical precision of about 3% and is characterized by peaks at 4.9 and 6.9 THz, which we attribute to transverse and longitudinal phonons, respectively, and by a cutoff at about 8.1 THz. The peaks in our g(v) distribution are much more clearly resolved than in any previous work on vanadium. We also see a small shoulder in g(v) in the region 2–3 THz, but it is far less pronounced than in some of the earlier experiments on vanadium. Below 2 THz we find that g(v) = av2, and the observed value of a leads to a Debye temperature that is in excellent agreement with that obtained from the measured elastic cosntants of vanadium at room temperature. A theoretical g(v) distribution calculated by Clark on the basis of a nearest- and next-nearest-neighbor central force model is in generally good agreement with our results although it differs in some details. In particular, Clark's theory predicts that the transverse peak should be slightly more intense than the longitudinal peak, whereas our experimental results indicate the opposite.
The isotope effect in the electronic structure factors of liquid NH3 and ND3
Authors: Takeda S. , Egelstaff P. A.
Published: April 2011

Gamma rays from a radioisotope source, 241Am, were scattered from samples of liquid light ammonia (NH3) and heavy ammonia (ND3) at temperatures of 294.5 and 284.5 K under their normal vapour pressures to determine the electronic liquid structure factors. In addition the densities of these samples were checked by the gamma ray absorption technique. From the difference between the total electronic structure factors of liquid NH3 and ND3, the difference of the r-space electronic correlation functions of the two liquids (at the same temperature) was evaluated. The observable difference is due (mainly) to quantum effects in the intermolecular electron-density correlation functions of liquid ammonia, which will be discussed.