bluuebunny/crossref_metadata_2025 huggingface dataset:
Discrete electron effects in lateral quantum islands
Authors: Sachrajda A. S. , Taylor R. P. , Dharma-Wardana C. , Adams J. A. , Zawadzki P. , Coleridge P. T. , Davies M.
Published: April 2011

The interplay between two different discrete electron effects has been studied in quantum dots. These are the Coulomb blockade effect and a novel resonant tunnelling process that occurs within the dot itself. We demonstrate that the two processes are governed by two different energy scales. We find that electron–electron interactions (not the single electron energy level spectrum) are required to explain both processes.
Leaky quantum wells: A basic theory and applications
Authors: Yang Rui Q. , Xu J. M.
Published: April 2011

In this work, we describe the basic properties of the leaky-quantum-well system and outline the progress made to date in various device applications. The key feature of the leaky quantum well, namely the existence of a leaky (tunneling) window near the bottom of the quantum well, and the associated quasi-bound states are analyzed in a two-band k∙P model. The unique and important properties of these types of quantum wells and their structure dependences are presented. A few representative electronic and photonic devices based on the leaky quantum wells are discussed.
Ferroelectrics for semiconductor devices
Authors: Sayer M. , Wu Z. , Kumar C. V. R. Vasant , Amm D. T. , Griswold E. M.
Published: April 2011

The integration of thin film ferroelectrics with silicon processing is being implemented for various types of devices. The technology is based on the sputtering or chemical deposition of lead-based perovskites such as lead zirconate titanate. Factors concerned with the integration of ferroelectric films with semiconductor processing are described. Major interests in Canada include nonvolatile ferroelectric random access memories for high-speed digital or long-term analog memory applications, high-density capacitors, electro-optic switches, and a wide range of sensors and actuators integrated into silicon.
Silicon micromachining technology for sub-nanogram discrete mass resonant biosensors
Authors: Prescesky S. , Parameswaran M. , Rawicz A. , Turner R. F. B. , Reichl U.
Published: April 2011

This paper describes a novel sub-nanogram discrete mass sensor with possible biological applications including potential biomass monitoring of living cells in culture. The sensing element is a micromachined silicon dioxide cantilever fabricated using a process compatible with commercial CMOS microelectronic processes. The 1 μm thick silicon dioxide device consists of two 8 × 150 μm cantilever arms supporting a 50 × 50 μm payload platform. Actuation of this first generation device is performed using a small vibration table. Resonant amplitude are measured by detecting the angular shift of a laser beam incident on the device. Finite element analysis is used to model resonant frequencies, mode shapes, and frequency response. Cantilever resonates in fundamental mode at 15.9 kHz and have an experimental mass sensitivity of 1.9 kHz ng−1.
Spectroscopic ellipsometry and photoluminescence from Si1−xGex alloys grown by atmospheric-pressure chemical vapor deposition
Authors: Hulse J. E. , Rowell N. L. , Wang A. , Namavar F. , Perry C. H.
Published: April 2011

Si1−xGex layers grown using atmospheric pressure chemical vapor deposition have been characterized using both room temperature spectroscopic ellipsometry (SE) and low-temperature photoluminescence (PL). Single layers of Si0.9Ge0.1, 30, 100, 1000, and 2000 nm thick on p+ Si(100) wafers were investigated to determine the effect of strain on the indirect and direct optical transitions. The thinner two layers were pseudomorphic and the thicker ones relaxed. The samples were examined by spectroscopic ellipsometry, which allowed the optical constants to be determined from the ultraviolet to near infrared (3.5–1.8 eV). Using optical constants available from the literature for cubic Si1−xGex, the thicknesses of the alloy layers were verified. From our optical constants and the published calibration curves for normal incidence reflectivity at 633 nm and for the energy of the E1 transition (both a function of x), we found that the average germanium concentration appeared to be significantly below the nominal 10% for these samples. Phonon-resolved PL spectra were observed at 2 K for the thicker three samples with the transition from strained to unstrained layers clearly visible in the shift of the Si1−xGex band gap as seen from the energy of the no-phonon line. Dislocation lines appeared only for the relaxed material and the no-phonon line widths were ~4 times smaller for the strained Si1−xGex material.
A versatile, low light level optical detection system: from time-integrated emission spectra to time-resolved, two-dimensional emission mapping
Authors: Allard L. B. , Charbonneau S. , Young Jeff F.
Published: April 2011

We describe a novel, low light level optical detection system that can be easily configured for various modes of operation. These include (i) time-integrated photoluminescence spectroscopy, (ii) transient, spectrally gated photoluminescence decay, (iii) time-windowed photoluminescence spectroscopy, (iv) two-dimensional, time-integrated photoluminescence mapping, and (v) time-resolved, two-dimensional photoluminescence mapping with a time resolution of ~100 ps. This new detection system is described technically and examples are given of how it can be used to study a variety of different recombination processes in semiconductors.
Nonlinear electrohydrodynamic Rayleigh–Taylor instability with mass and heat transfer: effect of a normal field
Authors: Mohamed Abou El Magd A. , Elhefnawy Abdel Raouf F. , Mahmoud Y. D.
Published: April 2011

The nonlinear electrohydrodynamic stability of two superposed dielectric fluids with interfacial transfer of mass and heat is presented for layers of finite thickness. The fluids are subjected to a normal electric field in the absence of surface charges. Using a technique based on the method of multiple scales it is shown that the evolution of the amplitude is governed by a Ginzburg–Landau equation. When the mass and heat transfer are neglected, the cubic nonlinear Schrödinger equation is obtained. Further, it is shown that, near the marginal state, a nonlinear diffusion equation is obtained in the presence of mass and heat transfer. The various stability criteria are discussed both analytically and numerically and the stability diagrams are obtained.
Depth distributions of heavy ions in LiNbO3 and KTiOPO4 at tilted angle implantation
Authors: Wang Ke-Ming , Shi Bo-Rong , Qu Bao-Dong , Zhai Hong-Ying , Ding Pei-Jun , Wang Wei , Liu Yao-Gang
Published: April 2011

200 keV Hg and 1000 keV Xe ions were implanted into LiNbO3 and KTiOPO4 at a tilted angle. The depth distributions of Hg and Xe ions in LiNbO3 and KTiOPO4 were measured by Rutherford backscattering of MeV He ions. The experimental ion distributions are compared with the transport of ions in matter (TRIM'89) simulation. The result shows a slight difference in the peak position between the experimental and theoretical ion distribution, but the form of the experimental ion distribution is found to be in agreement with the theoretical one predicted by TRIM'89 except for a different width in the case of LiNbO3. The FWHM (full width at half maximum) of the experimental ion distribution is larger than the theoretical one given by TRIM'89 in the case of KTiOPO4. The lateral spreads obtained are compared using different calculation procedures.
Modes supérieurs pour un résonateur électromagnétique à ondes de surface
Authors: Billaud M. , Wick A. , Ritoux G. , Zepp G.
Published: April 2011

The theory of an open electromagnetic surface-wave resonator with two dielectrics is developed. The analytical formulation of results allows easy study of resonant frequency and quality factor for the different transverse magnetic modes. The numerical study shows the interest of high order modes.
Introducing directionality to Anderson localization: The transport properties of quantum railroads
Authors: Barnes C. , Johnson B. L. , Kirczenow G.
Published: April 2011

We present a study of the transport properties of a general class of quantum mechanical wave guides: quantum railroads (QRR). These wave guides are characterized by having adiabatic modes that carry current along the wave guide in opposite directions; for example N forward modes and M reverse modes. Anderson localization and the integer quantum Hall effect are characteristic of the disordered N = M and M = 0 cases, respectively. We consider the general case of arbitrary N and M, and show that it can be understood in terms of directed localization. Thus, we unify the theories of Anderson localization and the integer quantum Hall effect and demonstrate how they fit into a broader conceptual framework. We find that in any QRR there are always [Formula: see text] perfectly transmitted effective adiabatic modes with the remainder being subject to multiple scattering and interference effects characteristic of the N = M case.
Elastic and gyrotropic anomalies and acoustic activity in lead germanate
Authors: Viswanathan K. S.
Published: April 2011

The paper discusses elastic and gyrotropic anomalies as well as the acoustic activity of lead germanate during the phase transition from the space group C3h to C3. Expressions are derived for all the anomalies of the elastic constants, and relations among them have been established. We show that the elastic constants C11, C33, C12, and C13 suffer a dip at the transition temperature, in agreement with experimental results. Several invariants involving the order parameter, strain variables, and their derivatives are constructed and from these the contribution to the coupling energy of the gyrotropic energy has been calculated. Expressions are given for the anomalies of the various gyrotropic constants. We show that the system will exhibit anomalies in acoustic activity in the low symmetry phase, and an expression for the polarization angle has been obtained.
Effect of a periodic acceleration on nonlinear modulation of interfacial gravity-capillary waves between two electrified fluids under the influence of a horizontal electric field
Authors: El-Dib Yusry O.
Published: April 2011

A theoretical analysis of the subharmonic response of two resonant modes of interfacial gravity-capillary waves between two electrified fluids of infinite depth under the influence of a constant horizontal electric field is investigated. The method of multiple scales is used to derive two parametrically nonlinear Schrödinger equations that describe the behavior of the disturbed system in the resonance case. One of them contains the first derivatives in space for a complex-conjugate type while the second contains a linear complex-conjugate term. A time-dependent solution of a traveling wave is obtained. Stability conditions are obtained analytically and are discussed numerically. It is found that the stability criteria are significantly affected by the amplitude of the temporal solution. The numerical calculations show that instability is produced in the system except for small stable areas due to the periodic forcing. It is observed that the acceleration frequency plays a dual role in the stability criterion. The results show that the horizontal electric field plays a dual role in the resonance case.
Path integral solution to a charged particle in an infinite square-well potential with a constant electric field
Authors: Yeon Kyu Hwang , Um Chung-In , George Thomas F. , Pandey Lakshmi N.
Published: April 2011

The propagator of a charged particle in an infinite square-well potential in one dimension with a constant electric field is solved exactly using Feynman path integration. The wave function of the system is evaluated by the propagator. This wave function depends on time in a way determined by the well size, particle mass and charge, and applied electric field. The uncertainty relation of the system is also obtained by this function.
Application of operator algebra technique to some problems in condensed matter physics
Authors: Choi Chang Ho , Sug Joung Young , Choi Sang Don , Yi Sam Nyung
Published: April 2011

We prove that the relaxation functions in condensed matter physics can be easily calculated by the operator algebra technique in the continued fraction representation scheme. The Dyson equation for the single particle Green's function in the Hubbard model obtained in this way is identical with the one derived by Fedro and Willson in the lowest order approximation. And the cyclotron transition absorption formula obtained by this technique is identical with that of Argyres and Sigel.
The first-order formalism for Yang–Mills theory
Authors: McKeon D. G. C.
Published: April 2011

It is possible to replace the second-order Yang–Mills Lagrangian [Formula: see text] with the first-order Lagrangian [Formula: see text]. In this form, the interaction term in the Lagrangian, [Formula: see text] is very simple; the only disadvantage is that now not only the vector [Formula: see text] but also the auxiliary field [Formula: see text] propagate. The gauge-fixing and ghost contributions to the effective Lagrangian can similarly be reduced to first-order form by the introduction of auxiliary fields. We demonstrate the procedure by computing the two-point function to one-loop order using background field quantization.
Low-frequency oscillations in semi-insulating GaAs and InP: deep level domain spectroscopy
Authors: Backhouse Christopher , Kato Hiroshi , Young Lawrence
Published: April 2011

Low-frequency-current oscillations in semi-insulating GaAs and InP, believed to be caused by the transit of high-field domains produced by the field-enhanced trapping of electrons, were investigated with the aim of finding the best method of extracting information on deep levels from the temperature dependence of the power spectra of the current, a procedure known as "deep-level domain spectroscopy." With low voltages applied to our specimens of undoped GaAs the conduction was "ohmic", the oscillations were fairly coherent over a range in temperature of about 300–350 K, and since the peaks in the spectra all had the same activation energy within experimental scatter, they were concluded to be harmonics determined by the shape of the high-field domains, with the activation energy corresponding to the trap causing the high-field domains. For higher voltages (above a "knee" in the l–V curve), the oscillations were much larger in amplitude and complex. As in our previous work with InP, the spectra after baseline removal contained peaks with differing activation energies. These peaks are believed to be due to deep levels other than those involved in the field-enhanced trapping process (EL2 in GaAs and an Fe-related level in InP:Fe). Rather than conclude that several traps independently cause high-field domains, it is suggested that the occupancy of such traps will be modulated by the passage of a high-field domain and this will produce the observed peaks. A simplified model is treated analytically. In practice, peaks were observed with differing activation energies only when the oscillations were incoherent and therefore had a broad spectral content. The oscillations occur at sufficiently low average fields to be a potentially serious problem for integrated circuits made on semi-insulating substrates. The maximum entropy (or "all poles") method was compared with windowed fast Fourier transform methods for use in deep-level domain spectroscopy. Results are presented for specimens of undoped and Cr-doped GaAs, and for Fe-doped InP.
On self-consistent thermal equilibrium structures in two–dimensional negative-temperature systems
Authors: Martinov Nikolay K. , Vitanov Nikolay K.
Published: April 2011

Two-dimensional negative-temperature systems described by means of analytical solutions of the Poisson–Boltzmann equation are investigated. It is shown that two kinds of doubly periodic self-consistent structures can exist. The structures obtained confirm the prediction of equilibrium statistical mechanics that no spatially homogeneous thermal equilibrium state for negative-temperature systems exists. The structures investigated are similar to the two kinds of structures in a two-dimensional, two-component positive-temperature Coulomb gas but the location of the elements of the system within the region of the elementary cell of the structure is different. By means of the approach developed in this paper the parameters of the structures, the self-consistent potential, the corresponding charge density, and the energy of the negative-temperature structures can be calculated.
Hamiltonian and BRST (Becchi–Rouet–Stora–Tyutin) formulations of a gauge-invariant chiral Schwinger model
Authors: Kulshreshtha Usha , Kulshreshtha D. S. , Müller-Kirsten H. J. W.
Published: April 2011

The Hamiltonian and BRST (Becchi–Rouet–Stora–Tyutin) formulations of a gauge-invariant chiral Schwinger model (in the standard regularization with the Jackiw–Rajaraman regularization parameter a = 1) obtained by the inclusion of the corresponding Stueckelberg term in the gauge-noninvariant Jackiw–Rajaraman model are investigated.
Focus wave modes in noncollisional plasma
Authors: Borisov Victor V. , Utkin Andrei B.
Published: April 2011

Using a special scalar potential, a set of Brittingham's electromagnetic focus wave modes is generalized to the cold noncollisional plasma system.
Light switching in a glass–Ag–polymer structure using attenuated total reflection (ATR)
Authors: Lévesque L. , Paton B. E.
Published: April 2011

Surface plasmon resonance (SPR) and a photon-tunneling modes were observed in the visible spectrum using a glass–silver–PVC–Mylar structure. This structure was used as an electro-optic device to modulate the reflected intensity of a He–Ne (632.8 nm) beam by more than a factor of 2. Low-frequency switching was achieved in the 0–40 Hz range with a driving amplitude in excess of 50 V. Results suggest the low-frequency response is governed mostly by ionic migration within the polymer and the perturbation of the evanescent field matching condition at the silver–PVC interface results in the modulation. As light-switching time is related to species mobility, the technique should be a good probe for studying ionic migration in polymers or biological membranes.
Radiation from a horizontal electric dipole in the presence of a lossy ground
Authors: Helaly A.
Published: April 2011

The radiation field of a horizontal electric dipole located above a lossy ground is formulated as a boundary-value problem employing dyadic Green's functions. The resulting field vectors are then split into the primary and secondary components. The primary components represent those that would exist in the absence of the ground, leaving the secondary components to represent the contribution of the ground. For different frequencies and various ground conductivities, the secondary components are determined and compared with the primary ones. The results indicate that in the vicinity of the air–ground interface the effect of the ground on the electric-field components is significant when the field point is inside the ground region while it becomes less significant as the field point moves in the air region away from the interface and toward the source location. On the other hand, the contribution of the ground to the magnetic-field components becomes appreciable only at high frequencies or when the ground conductivity becomes excessively large.
A phase transition in Ga2Se3 under high pressure
Authors: Savchenko K. V. , Shchennikov V. V.
Published: April 2011

Ga2Se3 crystals with an excess of Se were grown by the Bridgman–Stockbarger method and had a defect zinc blende structure with a0 = 5.42 Å [Formula: see text] (1 Å = 10−10 m). At room temperature the resistivity was equal to (4.5 ± 1.5) × 1011 Ω cm, the thermoelectric power was (−1.1 ± 0.1) mV K−1 and the Vickers microhardness was (357 ± 9) kg mm−2. The gamma-induced conductivity was measured in the gamma-emitting power range of 3–340 rad s−1. Pressure dependencies of electrical resistance and thermoelectric power at room temperature allowed us to determine a phase transition of the semiconductor–semiconductor type at 14.2 GPa.
Erratum: Contributions of density-dependent interactions in heavy-ion transfer reactions
Authors: Osman Ahmed , Farra A. A.
Published: April 2011

Heavy-ion reactions with particle transfer are studied using the distorted-wave Born-approximation formalism. Different forms of the distorting optical model potentials are introduced in the initial and final channels. The form factors of the reactions are presented explicitly using modified real Woods–Saxon potentials for the nucleus–nucleus bound-state interactions in the entrance and exit channels. The differential cross sections of heavy-ion transfer reations are calculated numerically using the double-folding model for distorting potentials with an effective density-dependent nucleon–nucleon interaction. The present analyses of the angular distributions introduce good descriptions of the experimental data. The observed backward oscillations are described successfully by the explicit inclusion of the density- and the parity-dependent terms in the distorting optical-model potential calculations. The values of the extracted spectroscopic factors and normalization coefficients obtained are reasonable.
Erratum: Angular distribution of optical photons from β decay of oriented nuclei
Authors: Al-Harkan A. M.
Published: April 2011

Internal and external optical bremsstrahlung accompanying the β decay of polarized nuclei were investigated. The features of angular distribution of light photons were analyzed taking into account multiple electron scattering. Monte-Carlo simulation was used to study the fate of β electrons and to calculate the intensity and angular distribution of the optical photons. It is shown that in pure water, the contribution of secondary electrons in the production of photons reaches 30–40%. We suggest using the angular distribution of optical photons to study the polarization states of β isotopes imbedded in transparent media.
Erratum: Vibration–rotation calculations for using a Morse-based discrete variable representation
Authors: Watson James K. G.
Published: April 2011

The Hamiltonian operator for the X3 symmetric triatomic molecule, using Eckart axes with the three internuclear distances as internal coordinates, is derived and applied to numerical calculations of the vibration–rotation spectrum of [Formula: see text]. A C2v-symmetrized discrete variable representation based on Morse functions is employed for the J = 0 vibrational problem. The unphysical points with ri < 0 or ri + rj < rk are avoided by giving them a large potential energy (106 cm−1). This procedure is not exact, but is adequate for vibrational levels well below the barrier to linearity. The matrices of the complete Hamiltonian in a D3h-symmetrized basis of products of the lowest 50 vibrational eigenvectors with the complete set of rotational functions for each J are set up and diagonalized, and the eigenvectors are used to calculate line strengths. Initially, the well-known potential surface of Meyer et al. (1986) was employed. Subsequently, 7 of the 30 coefficients in this potential were adjusted to give a least-squares fit to the published observed lines of [Formula: see text].
Erratum: 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.
Erratum: Focus wave modes in conducting media
Authors: Borisov Victor V. , Utkin Andrei B.
Published: April 2011

A family of localized solutions of the homogeneous equation for waves in lossy media is derived. These solutions are analogues to the set of Brittingham's focus wave modes (J. Appl. Phys. 54, 1179 (1988)).
Erratum: Reflection properties of conducting rectangular elements printed on a dielectric slab
Authors: Shaker J. , Shafai L.
Published: April 2011

The spectral domain dyadic Green's function that involves infinite spectral integrals is developed for stratified media. The Poisson summation formula is then used to transform these infinite spectral integrals into infinite double summations to make them applicable to infinite arrays of conductors printed on a dielectric slab. A Galerkin method is used to discretize and solve the electric-field integral equations for the geometry of a two-dimensional array of conducting patches. The method of analysis is then used to study the reflection and transmission properties of rectangular patch elements, for both co-polar and cross-polar components of the field. The variation of the resonance frequency with the aspect ratio of the rectangle is also investigated.
Reflector antenna pattern improvement using optimized array feed
Authors: Saka Birsen , Yazgan Erdem
Published: April 2011

A method to optimize the radiation pattern of rotationally symmetric reflector antennas with an array feed is introduced. The optimization of the radiation pattern for the purpose of beam scanning is achieved by adjusting the excitation condition of each element of the planar array. The radiation-pattern calculation is based on the physical optics radiation integral. The excitation conditions (amplitude and phase) of the array elements are calculated using the least-squares minimization method.
Maps with relative, apparently real periods
Authors: Chia T. T. , Tan B. L.
Published: April 2011

The discontinuous logistic map, with parameters a=1, [Formula: see text], and a sectional discontinuity at 0, has been shown to possess a truly infinite period but a fictitiously finite computationally determined period whose length is dependent on the precision of the computation. Besides this map, we demonstrate here that there exist many other such maps. These are all one dimensional with at least a discontinuity. We shall enumerate five types, demonstrate numerically and prove analytically that some examples of these types of maps possess truly infinite, but incorrect computationally determined precision-dependent periods. The method of construction of such maps is outlined from which it is clear that there should exist an infinite number of them. We give a signature for identifying these maps and explain why they do not undergo border-collision bifurcations.
Spin relaxation of polarized helium-3 in the magnetic field of a Helmholtz pair
Authors: Daniels J. M.
Published: April 2011

The fact that the motion of the atoms of helium-3 gas is so quick that the spins cannot follow the change of direction of an inhomogeneous magnetic field adiabatically is an important spin relaxation mechanism. The spin relaxation rates are calculated for a spherical sample situated at the centre of a Helmholtz pair. The calculation is made for the first two nonvanishing terms in the inhomogeneity, the fourth and sixth degree terms.
An ab initio and spectroscopic study of dimethyl ether: An analysis of the infrared and Raman spectra
Authors: Senent M. L. , Moule D. C. , Smeyers Y. G.
Published: April 2011

We have calculated the potential surface with respect to the torsional angles of the methyl groups, the torsional energy level structure, and the infrared and overtone Raman intensities by ab initio methods using extended basis sets. The conclusions that were drawn from a correlation to the observed spectra are the following, (i) The modest basis set MP2/6-31G(d, p) yields a value of 238.32 cm−1 for the v16 gearing mode in reasonable agreement with the observed value of 241.0 cm−1. Extension of the basis set to MP2/6-311G(df, p) marginally improves this value to 239.36 cm−1. A somewhat larger basis set, RHF/6-311+G(3df, 3p) was needed for a satisfactory description of the components of the polarizability and the dipole moment. (ii) The technique of full relaxation of the coordinates of the top and the frame was unable to account for the strong resonance between the 2v12 overtone gearing level and the v7 COC in-plane bending mode. (iii) The asymmetry along the diagonals of the potential surface that determines the sign and magnitude of the sine.sine gearing term was found to display complex behaviour and was positive in the region of the eclipsed-eclipsed equilibrium conformation and negative for the staggered–staggered conformation.
Cosmic-ray atmospheric electric field effects
Authors: Dorman L. I. , Dorman I. V.
Published: April 2011

Experimental data on the atmospheric electric field effect in the cosmic-ray muon component are discussed on the basis of the general theory of cosmic-ray meteorological effects. In this framework, we develop the theory of atmospheric electric field effects in the hard- and soft-muons of secondary cosmic rays and in the neutron-monitor counting rates as well. We show that the experimental results can be understood on the basis of this theory. We also show that a sufficient atmospheric electric field effect in the cosmic-ray neutron component is to be expected because the neutron monitors work as analyzers of soft muons and really detect only negative muons as well as neutrons.
Directivity enhancement by layered radomes
Authors: Zhou R. , Shafai L.
Published: April 2011

Single and layered radomes are often used to protect radiating elements. A two-dimensional analysis is used to investigate their influence on the antenna directivity and radiation patterns. Image-theory and integral-equation methods are used to formulate the problem for a line source and layered dielectrics of finite length over a ground plane, and solved numerically using the moment method. It is shown that, for a sufficiently long single-layer slab, the directivity enhancement is approximately equal to the square root of its relative permittivity. Properly truncating the slab can even improve the directivity. The optimum length of a finite dielectric slab is determined to maximize the directivity. For multilayer slabs, the directivity enhancement is much greater and they can be used with low-permittivity dielectrics to simulate a single-layer one of higher permittivity. Frequency dependency of the enhanced gain is also studied. Useful information for how to incorporate the radome parameters into the antenna design and use it for gain or directivity enhancement is provided.
Side-band spectroscopy in the visible with a tunable modulator
Authors: Häring U. , Kreiner W. A. , Magerl G. , Schupita W.
Published: April 2011

Phase modulation at microwave frequencies has been applied to the radiation of a cw (continuous wave) ring dye laser to observe side-band spectra. To do this, use was made of the electro-optic effect in a lithium tantalate bulk crystal. This spectroscopic technique allows relative frequencies to be determined instead of the more usual wavelengths. By scanning spectra simultaneously with two different side bands and superimposing them on the same recording, we were able to determine the frequency separation of hyperfine components for several iodine transitions. The design of the modulator allowed conservation of the phase-matching condition (equality between light velocity and phase velocity of the microwave in the crystal) at microwave frequencies between 11 and 17 GHz. Side-band power of the order of 1 mW was produced. We demonstrate the application of this spectroscopic technique to iodine. Examples of spectra around 576 and 633 nm are given. Further application of this source (e.g., double-resonance experiments, frequency standards) is discussed.
Lie subgroups of symmetry groups of fluid dynamics and magnetohydro-dynamics equations
Authors: Grundland A. M. , Lalague L.
Published: April 2011

We classify the subalgebras of the symmetry algebras of fluid dynamics and magnetohydrodynamics equations into conjugacy classes under their respective groups. Both systems of equations are invariant under a Galilean-similitude algebra. In the case of the fluid dynamics equations, when the adiabatic exponent γ = 5/3, the symmetry algebra widens to a Galilean-projective algebra. We extend our previous classification of the symmetry algebra in the case of a nonstationary and isentropic flow to the general case of fluid dynamics and magnetohydrodynamics equations in (3 + 1) dimensions. The representatives of these algebras are given in normalized lists and presented in tables. Examples of invariant and partially invariant solutions, for both systems, are computed from representatives of these classifications. The final part of this work contains an analysis of this classification in connection with a further classification of the symmetry algebras in the case of the equations describing the flow of perfect gases. An explicit solution, in terms of Riemann invariants, is constructed from infinite-dimensional subalgebras of the symmetry algebra of the magnetohydrodynamics equations in the (1 + 2)-dimensional case.
Coherent states, Schrödinger equation, and the Hamilton–Jacobi equation
Authors: Shoucri Rachad M.
Published: April 2011

The self-adjoint form of the classical equation of motion of the harmonic oscillator is used to derive a Hamiltonian-like equation and the Schrödinger equation in quantum mechanics. A phase variable ϕ(t) instead of time t is used as an independent variable. It is shown that the Hamilton–Jacobi solution in this case is identical with the solution obtained from the Schrödinger equation without the need to introduce the idea of hidden variables or quantum potential.
Hyperfine splittings and isotope shifts of (6s)21S0 → (6s6p) 1P1 transition in barium
Authors: Wijngaarden W. A. van , Li J.
Published: April 2011

The hyperfine and isotope shifts of the (6s)21S0 → (6s6p) 1P1 transition in barium were measured using a ring dye laser to excite an atomic beam. The laser frequency was scanned across the transition while fluorescence produced by the radiative decay of the excited state was detected. The frequency was calibrated using an acousto-optic modulator to frequency shift part of the laser beam. The spectrum therefore consisted of pairs of peaks separated by the modulation frequency. The shifts (MHz) relative to 138Ba are 137Ba F = 5/2, 63.43 ± 0.25; 135Ba F = 5/2, 120.55 ± 0.25; 136Ba, 128.02 ± 0.39; 137Ba F = 3/2, 274.56 ± 0.09; 135Ba, 323.44 ± 0.19; and 135,137Ba F = 1/2, 549.47 ± 0.12 where the transition in 138Ba occurs at the lower frequency. The magnetic dipole a and electric quadrupole b hyperfine coupling constants (MHz) of the (6s6p) 1P1 state were determined to be a135 = −98.16 ± 0.14, b135 = 34.01 ± 0.22, a137 = −109.50 ± 0.13, and b137 = 50.09 ± 0.21.
Energy bounds for the spiked harmonic oscillator
Authors: Hall Richard L. , Saad Nasser
Published: April 2011

A three-parameter variational trial function is used to determine an upper bound to the ground-state energy of the spiked harmonic-oscillator Hamiltonian [Formula: see text]. The entire parameter range λ > 0 and α ≥ 1 is treated in a single elementary formulation. The method of potential envelopes is also employed to derive a complementary energy lower bound formula valid for all the discrete eigenvalues.
Schemes for the calculation of the free energy and specific heat for marginal Fermi liquids in the normal and superconducting phase
Authors: Chaudhury Ranjan
Published: April 2011

Several schemes based on the fermionic many-body approach and the boson-exchange approach are developed to calculate the free energy and specific heat for a marginal Fermi liquid in the normal and superconducting phase. The merits and demerits of these schemes are analyzed and compared. The origin of the failure of the simple Bardeen–Stephen formula is also highlighted. The analysis is carried out in light of some experiments.
A BOXCARS investigation of the interspecies V–V energy transfer from highly excited SF6 to N2
Authors: Jones W. E. , Wang Lixin
Published: April 2011

The BOXCARS technique was used to investigate the V–V energy transfer between highly excited SF6 and N2. It was found that a Boltzmann population distribution among vibrational levels of N2 is present by 1 μs after laser excitation of SF6 and is maintained during the energy-transfer processes. The maximum energy transferred to N2 increases linearly with the increase of the average number of photons absorbed [Formula: see text] by SF6 in the range [Formula: see text]. The maximum energy transferred to the N2 vibrational levels increases with the partial pressure of SF6 and decreases with the partial pressure of N2. The apparent rate of energy increase in the N2 vibrational levels increases with both partial pressures of SF6 and N2. However, both the rate and the amount of energy transferred to N2 vibrational levels depend more strongly on the partial pressure of SF6 than on the partial pressure of N2. A model calculation produces a reasonable fit to the experimental data.
On Noether's formulation of her theorem
Authors: Flessas G. P. , Leach P. G. L. , Cotsakis S.
Published: April 2011

In some recent papers and comments there has been a difference of opinion as to what a Noetherian integral is. We settle the matter by referring to Noether's paper of 1918 which is a more reliable guide to her work than some textbooks.
Measurements of total and (or) positronium-formation cross sections for positrons scattered by alkali, magnesium, and hydrogen atoms
Authors: Stein T. S. , Jiang J. , Kauppila W. E. , Kwan C. K. , Li H. , Surdutovich A. , Zhou S.
Published: April 2011

Recent developments in measurements of total and (or) positronium-formation cross sections for positrons (in the range of 1 to 300 eV) scattered by alkali, magnesium, and hydrogen atoms are reviewed. Measurements of total and positronium (Ps)-formation cross sections for positrons scattered by sodium, potassium, and rubidium have revealed an interesting pattern of differences and similarities between these collision systems. These measurements, together with recent calculations, provide evidence that coupling effects between Ps formation and other scattering channels are very important at low energies for the alkali atoms. The calculations also indicate that formation of Ps in excited states in the cases of potassium and rubidium is more likely than formation in the ground state when the positron energy is greater than about 5 eV. Measurements of total and Ps-formation cross sections for positrons scattered by magnesium are in a preliminary stage, but the differences between them and the results of available theoretical calculations are providing a strong incentive to intensify experimental and theoretical investigations of this collision system. Refinements in a recently developed technique for measuring total cross sections for positrons and electrons scattered by atomic hydrogen have led to preliminary new results for these systems that are in very good agreement with recent theoretical calculations. These measurements, together with the calculations with which they agree, indicate that as the projectile energy is increased through the higher energy range (above 50 eV), the total cross sections for positrons approach the corresponding results for electrons from above, which is the reverse of the relationship between the positron and electron total cross sections in this energy range for all of the room-temperature gases that have been investigated, except possibly for molecular hydrogen.
Feshbach resonances in positron–hydrogen collisions
Authors: Gien T. T.
Published: April 2011

Sequences of Feshbach resonances, predicted by theory to exist below the n = 2 H threshold in positron–hydrogen scattering, that have long evaded all previous numerical searches, were finally located in our close-coupling calculation employing various coupling schemes. The energy ratios and width ratios of the successive resonances of these sequences were, furthermore, found to agree very well with the values predicted by theory.
Hyperspherical close-coupling calculation of D-wave positronium formation and excitation cross sections in positron–hydrogen scattering
Authors: Zhou Yan , Lin C. D.
Published: April 2011

The hyperspherical close-coupling method is used to calculate the elastic, positronium-formation, and excitation cross sections for positron collisions with atomic hydrogen at energies below the H (n = 4) threshold for the J = 2 partial wave. The resonances below each inelastic threshold were also analyzed. The adiabatic hyperspherical potential curves are used to identify the nature of these resonances.
Single ionization by positron impact
Authors: Moxom J. , Ashley P. , Laricchia G.
Published: April 2011

The single direct ionization cross sections for positron scattering [Formula: see text] on He, Ar, Kr, and H2 were measured and compared with existing data and the corresponding cross sections for electron impact [Formula: see text]. At most impact energies the present data for He and H2 are in reasonable accord with other measurements and, in the case of He, with some of the available calculations. At low energies, [Formula: see text] is found to increase more slowly than [Formula: see text], probably due to the importance of Ps formation in this energy range.
Multiple ionization of argon by positron impact
Authors: Hippler R. , Helms S. , Brinkmann U. , Deiwiks J. , Schneider H. , Segers D.
Published: April 2011

Recent experimental results for the multiple ionization of argon by positron impact have been reanalysed. Absolute cross sections for the double and triple ionization of argon were obtained from measured ratios of double-to-single and triple-to-single ionization, using known cross sections for single ionization and for positronium formation. Distinct differences compared to similar results for electron impact are noted.
K-, L-, and M-shell ionization of atoms by electron and positron impact
Authors: Khare S. P. , Wadehra J. M.
Published: April 2011

The plane-wave Born approximation with Coulomb, relativistic, and exchange corrections is employed to obtain the K-, L1-, L2-, L3-, and M-shell ionization cross sections of a number of atoms bombarded by electrons and positrons in the energy range varying from the threshold of ionization to 1 GeV. Transverse interaction of virtual photons with atoms is also included and it is found to be of great significance for impact energies greater than about 1 MeV. For K- and L-shell ionization, good agreement between the theoretical values and various experimental data for electron-impact cross sections is obtained. However, for the M shell, the theory overestimates the experimental cross sections. For positron impact the agreement between the present results and the limited experimental data is found to be quite satisfactory.
Formation of dtμ in muon-catalysed fusion: a resonant rearrangement process
Authors: Armour E. A. G.
Published: April 2011

A key process in the muon-catalysed fusion cycle is a low-energy collision of a tμ atom with a DA molecule, where A is H, D, or T, which leads at appropriate incident energies, to the formation of a resonant complex containing dtμ. In this paper, methods of calculating the resonant formation rate of dtμ are discussed. A description is given of a new approach that makes use of coupled equations for the rearrangement scattering process and elements of Feshbach's theory of resonances to obtain an expression for the cross section for resonant dtμ formation. The insights gained from this approach are discussed.
The ionization of organic molecules by slow positrons
Authors: Hulett Jr. L. D. , Xu Jun , McLuckey S. A. , Lewis T. A. , Schrader D. M.
Published: April 2011

The ionization of organic molecules by positrons having energies above and below their positronium-formation thresholds is reviewed. The sensitivity of sub-positronium-ionization yields to chemical and structural properties of the molecules is discussed, and possible mechanisms for ionization and fragmentation are suggested. Plans are presented for future experiments to further elucidate mechanisms and to search for evidence of positronium compound formation.
Positronium–atom scattering
Authors: McAlinden Mary T. , MacDonald F. G. R. S. , Walters H. R. J.
Published: April 2011

Calculations of total cross sections for Ps(1 s) scattering by atomic hydrogen, helium, and argon are reported for the energy range 0–150 eV. The results for atomic hydrogen have been evaluated exactly within the first Born approximation. For collisions with helium and argon in which the target remains in its initial state (so called target elastic collisions) it is assumed that the positronium scatters off a frozen target atom and a coupled positronium pseudostate approximation is then used to calculate the cross sections. For collisions in which the target atom is excited or ionized (target inelastic collisions) the first Born approximation is adopted. Here there is a significant problem in summing over all final states of the target and for this a scheme due to Hartley and Walters has been employed. It is found that for the light targets, hydrogen and helium, target inelastic collisions become dominant above 45 and 105 eV, respectively, while for the heavier argon atom, target elastic scattering is always more important. Except at the lowest energies, and for both target elastic and target inelastic collisions, positronium ionization is the main outcome of the collision for all three atoms. There is an encouraging degree of agreement at the higher energies with the total cross-section measurements of Zafar et al. and Laricchia et al. for helium and argon. The present approximations do not include electron exchange between the positronium and the atom which may be the main source of disagreement between theory and experiment elsewhere.
Positron scattering from Mg atoms
Authors: Gribakin G. F. , King W. A.
Published: April 2011

Scattering of low-energy positrons from Mg atoms is considered using many-body theory methods. For the first time the contribution of the Ps-formation channel is taken into account in addition to the polarization potential of the atom. At low energies the virtual Ps formation creates a strong attraction in the positron–atom channel, which produces positron–atom bound states in the s and p waves (0.985 and 0.159 eV are the corresponding binding energies), and a low-lying d-wave resonance at 1 eV. At higher energies this contribution gives rise to an effective positron–atom repulsion. On the whole, the inclusion of the Ps-formation contribution drastically changes the behaviour of the phase shifts and the shape of the partial-wave cross sections below 20 eV. Total and inelastic positron–Mg cross sections have been calculated. It appears that the present theory can explain the very recent results of the first experiment on positron–Mg scattering.
The ORELA positron-scattering program: Progress in the first energy-analysis experiments
Authors: Schrader D. M. , Hulett Jr. L. D. , Lewis T. A. , Xu Jun , Laricchia G. , Horsky T. N.
Published: April 2011

Implantation and fragmentation by positron impact have been extensively studied by the positron group at Oak Ridge National Laboratory, but the group has never attempted an energy analysis of the ions produced. In this contribution, we report on the status of the first energy-analysis experiments planned with the ORELA (Oak Ridge Electron Linear Accelerator) positron beam. These first experiments will be similar to the original ORELA ionization–fragmentation experiments in that the target molecules will be in a stagnant gas, and the positrons will be confined in a Penning trap. We plan (i) to measure binding energies of several positronium compounds, (ii) to measure the energies of fragment ions produced by positrons with energies below fragmentation thresholds, and (iii) to detect secondary electrons from processes following the formation of ions, and to measure their energies.