Detailed Theoretical Results Research Articles

Sr2FeMoO6: A Prototype to Understand a New Class of Magnetic Materials

We propose a new mechanism to explain the magnetic structure of a recently discovered magnetoresistive double perovskite oxide system, Sr2FeMoO6, with the help of detailed experimental and theoretical results. This model, based on a strong antiferromagnetic coupling between the local moment and the charge carriers arising from local hopping interactions, can give rise to ferromagnetic metallic as well as ferromagnetic insulating ground states. The relevance of this mechanism in understanding the magnetism in dilute magnetic semiconductors such as Ga1−xMnxAs, is also discussed.

Separation of the bulk lifetime and surface recombination velocities in semiconductor wafer by a single laser microwave photoconductance

In this paper, we present a new approach for separating the bulk lifetime and surface recombination velocities via a single laser microwave photoconductance. When a pulsed laser pumps on the probed sample, with the illumination of first the front surface and then the back surface with the same light wavelength, the bulk lifetime and the surface recombination velocities are obtained. Detailed theoretical analysis and preliminary experimental results are shown to demonstrate the applicability of the method.

A sensorless switched reluctance drive system using a self-inductance estimating technique

This paper proposes a new method for estimating the shaft position of a switched reluctance motor (SRM). The shaft position of the SRM is obtained by on-line estimation of the self-inductances of the motor, and a closed-loop drive system can thus be achieved. The drive system performs well in both the pulse-width-modulated (PWM) region and the single pulse region. The adjustable speed range of the system is from 20 rev/min to 2000 rev/min. In addition, the drive system is automatically started from standstill to a required speed and exhibits good transient and load disturbance responses. The detailed theoretical analysis and several experimental results are presented in the paper. First, the basic principle of the proposed method is explained. The self-inductance estimating technique is derived by using the mathematical model of the SRM. Then, the relationship between the self-inductance and the shaft position of the motor is discussed. Next, a starting technique is presented. It is then shown how a 32-bit microprocessor system is used to estimate the position and speed, speed-loop control, and the generation of three-phase current commands. Several simulated and experimental results validate the theoretical analysis.

Sezawa and love modes in Co/Cu superlattices

AbstractIn this paper detailed theoretical study and experimental results concerning acoustic surface and undersurface propagation modes in multilayered Co/Cu superlattices are provided. The theoretical approach was adopted from that of Adler and Farnel. Experimental results were obtained in Brillouin light-scattering measurements. The Sezawa and Love acoustic modes were observed as a result of boundary conditions at the interface between hexagonal and cubic structures.

Spectral switching of Gaussian-Schell model beams passing through an aperture lens

Starting from the propagation equation of partially coherent light, the spectral switching of Gaussian-Schell model (GSM) beams passing through an aperture lens is studied. Our detailed theoretical and numerical results have shown that the spectral switching of GSM beams is mainly dependent on the truncation parameter, coherence parameter and Fresnel number. By a suitable choice of the system parameters, the position, number, and transition amplitude of the spectral switch can be varied.

Three-dimensional optical profilometry for artwork inspection

A special fibre-optic interferometer is proposed for 3D profilometry. Sinusoidal fringes are produced by the interference of two spherical wavefronts emitted from the output fibres of a directional coupler. The fringe pattern is projected on an object surface and the deformed grating image is captured by a CCD camera. Subsequently, the captured image is analysed, and the relationship between the fringe phase and the height distribution of the illuminated surface is derived. The proposed method is simple and accurate. It is suitable for automatic measurements. Detailed theoretical and experimental results are presented. Tests show that the system can detect indents and outdents with a maximum depth of less than 10 µm.

Development of a modified diesel engine cycle

This paper presents an investigation of a novel type of diesel engine cycle, an approximate constant pressure cycle. The prototype engine is called the diesel engine with oil cushioned piston (DEOCP). In contrast to conventional engines, the DEOCP has a variable cylinder volume which is controlled automatically by the engine's cylinder pressure. The variable cylinder pressure is achieved by inserting a hydraulic cylinder into the engine's piston rod. The engine cylinder pressure is thus self-controlled and an ideal constant pressure cycle can theoretically be achieved. Computer simulations and engine tests have shown that the DEOCP distinguishes itself with high cycle efficiency, improved low-load performance and good starting ability. A detailed theoretical analysis and test results are presented in the paper, as well as a discussion of existing problems and possible further developments. Development of the diesel engine with oil cushioned piston could break the limit of cycle efficiency of conventional engines, hence opening a new avenue for diesel engine development.

A new control strategy to achieve sinusoidal line current in a cascade buck-boost converter

This work presents a detailed theoretical analysis and experimental results of a novel means of obtaining sinusoidal input current and unity power factor (UPF) via a cascade buck-boost power converter. Using the new configuration, sinusoidal line current in phase with the bus voltage is achieved, thanks to a new and simple to implement control strategy. Comparison between the input and output voltages is used to select the instantaneous operating mode of the converter. Offline references are calculated and stored in two EPROM circuits and then compared to measured currents to generate the gating signals of the appropriate switches. Complete theoretical analysis, simulation results and experimental data on a 500 W power converter are presented, to demonstrate the superiority of the new control strategy. Low order harmonics in the input current are eliminated and the input power factor is found to be over 0.99.

Simultaneous implementation of Leendertz and Duffy's methods for in-plane displacement measurement

Leendertz two-beam illumination and Duffy's two-aperture arrangements are two well established optical configurations in speckle interferometry for in-plane displacement measurement. However, the measurement sensitivities of both these techniques are quite different. In Leendertz method the interbeam angle between the two illumination beams determines the sensitivity, while in Duffy's arrangement, it is governed by the subtense formed by aperture separation at the object. In the present paper, we implement both these configurations simultaneously for in-plane displacement measurement. The proposed optical arrangement extends the range of in-plane displacement measurement considerably. Detailed theoretical and experimental results are presented.

Eddy viscosity of three-dimensional flow

Detailed theoretical and numerical results are presented for the eddy viscosity of three-dimensional forced spatially periodic incompressible flow.As shown by Dubrulle & Frisch (1991), the eddy viscosity, which is in general a fourth-order anisotropic tensor, is expressible in terms of the solution of auxiliary problems. These are, essentially, three-dimensional linearized Navier–Stokes equations which must be solved numerically.The dynamics of weak large-scale perturbations of wavevector k is determined by the eigenvalues – called here ‘eddy viscosities’ – of a two by two matrix, obtained by contracting the eddy viscosity tensor with two k-vectors and projecting onto the plane transverse to k to ensure incompressibility. As a consequence, eddy viscosities in three dimensions, but not in two, can become complex. It is shown that this is ruled out for flow with cubic symmetry, the eddy viscosities of which may, however, become negative.An instance is the equilateral ABC-flow (A = B = C = 1). When the wavevector k is in any of the three coordinate planes, at least one of the eddy viscosities becomes negative for R = 1/v > Rc [bsime ] 1.92. This leads to a large-scale instability occurring for a value of the Reynolds number about seven times smaller than instabilities having the same spatial periodicity as the basic flow.

Comparative TV holography for vibration analysis

A comparative time-averaged TV holographic technique that allows fringe compensation is presented. The technique employs two identical objects, the test and the master, that are subjected to sinusoidal vibration to detect the difference in either out-of-plane vibration amplitude or phase distributions. In addition, the present optical configuration offers simultaneous visualization of vibration amplitudes of the test object. Detailed theoretical and experimental results are presented.

Adiabatic population transfer with frequency-swept laser pulses

We present detailed experimental and theoretical results on population transfer with frequency-swept picosecond laser pulses. Here, we demonstrate that intense frequency-swept pulses, when applied in the adiabatic limit, lead to both more efficient and more selective excitation than do unmodulated laser pulses. The experimental work is performed on quasi-two-level systems (pentacene/p-terphenyl crystal and Na vapor), quasi-three-level systems (Na vapor), and on more complex multilevel systems (I2 vapor). We discuss the different characteristics of adiabatic population transfer in both few-level, and multilevel cases, and, in particular, present computer calculations to explore the effects of molecular rotations in multilevel adiabatic population transfer.

Downshifted maximum features in stimulated electromagnetic emission spectra

New and detailed experimental and theoretical results concerning the prominent downshifted maximum (DM) feature in spectra of stimulated electromagnetic emissions are reported. The experimental results were obtained at the Sura ionospheric modification facility in Russia by transmitting a powerful high‐frequency ordinary mode pump wave into the ionospheric F region. We present detailed experimental results of the dependence of the DM on the pump frequency. Different frequency components of the DM have slightly different growth times after pump turn‐on and are suppressed in different pump frequency ranges at harmonics of the electron cyclotron frequency. The pump frequency range near the cyclotron harmonics in which the DM does not develop, decreases significantly with increasing harmonic, from several kilohertz at the fourth harmonic to an extremely narrow range of 0.2 kHz or less at the seventh harmonic. We discuss conditions for plasma wave propagation to explain this dependence on the cyclotron harmonics.

Separation of the influence of in-plane displacement in multiaperture speckle shear interferometry

An imaging system with its aperture fully open or with a multiaperture mask in front has been used for carrying out speckle shear interferometry. The shearing is achieved by installing shear elements in front of the imaging system. The multiaperture configuration has built-in in-plane displacement sensitivity, and hence shear fringes obtained from multiaperture shear interferometer always carry the in-plane displacement contribution. This problem does not arise when the full aperture of the lens is used. A three-aperture speckle shear interferometric technique is introduced here to separate the in-plane displacement and slope contributions from the shear interferogram. Detailed theoretical and experimental results are presented.

Frame Sequential Color Thin Film Transistor/Liquid Crystal Displays: Display Characteristics and Grayscale Consideration

We report a theoretical study on the optical characteristics of a frame sequential color thin film transistor/liquid crystal display (TFT/LCD) operating with the TN LCD. Important liquid crystal (LC) and display parameters (cell gap, LC surface pretilt angle, applied voltage, LC viscosity, frame rate, and operation sequence) are considered and optimized to improve the display performances. Both the modeling and experimental results for a TN LCD using an EM Merck ZLI-3651 LC material are presented. We also present detailed theoretical results for a TFT/LCD operating at frame rates of 30 Hz and 71 Hz. Sufficiently fast switching-on and switching-off dynamic responses can be obtained by raising the temperature to 40-60°C to reduce the LC rotational viscosity. A display operating at the 71 Hz frame rate, can obtain a good bi-level display, but it is difficult to obtain a good display with 512 colors.

Neural Networks and Their Applications for the Oil Industry

Neural Networks can be used in many different areas of problems related to Petroleum Exploration and Production. There already exist well defined classes of applications, together with appropriate Neural Networks architectures. Detailed theoretical results allow to monitor and evaluate the results obtained by Neural Networks. Sophisticated applications will certainly require the use of multi-modular architectures.

Theory of angular distributions in resonant transfer and excitation

It is shown that angular distributions of Auger electrons and X-rays from deexcitation of doubly excited states produced in the resonant transfer and excitation of projectiles in collisions with light targets are, in general, not isotropic. The X-ray emission is nonisotropic with respect to the projectile beam direction because the doubly excited states produced are collisionally aligned. The Auger electron emission to the ground state shows nonisotropic behavior due to the alignment, and the interference with the elastic scattering channel. The detailed theoretical results for the 2p 21D resonance in F 8+ + H 2 are presented at a projectile energy of 20 MeV. The angular distribution of the Auger electron is strongly peaked in the forward direction in the laboratory, and the Auger electrons exhibit significant deviations from the Lorentzian profile, in particular for large angles in the laboratory frame.

Angular distribution of Auger electrons and photons in resonant transfer and excitation in collisions of ions with light targets.

It is shown that angular distributions of Auger electrons and x rays from deexcitation of doubly excited states produced in the resonant transfer and excitation of projectiles in collisions with light targets are, in general, not isotropic. The detailed theoretical results for the 2${\mathit{p}}^{2}$ $^{1}\mathrm{D}$ resonance in ${\mathrm{F}}^{8+}$+e/r${\mathit{H}}_{2}$ are presented at a projectile energy of 20 MeV.

Doubly differential detachment cross sections for 0.5-MeV H- on He including projectile excitation to H(n=2).

Detailed theoretical results are presented for the electron-detachment cross section, doubly differential in both the electron momentum and angle, for the process 0.5-MeV ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$+He\ensuremath{\rightarrow}H(n=2)+${\mathit{e}}^{\mathrm{\ensuremath{-}}}$+${\mathrm{He}}^{\mathrm{*}}$. As discussed briefly elsewhere [C. R. Liu and A. F. Starace, Phys. Rev. Lett. 62, 407 (1989)], the laboratory-frame doubly differential cross sections (DDCS's) for electron detachment in the forward direction are shown to depend sensitively on the low-energy states of the H(n=2)-${e}^{\mathrm{\ensuremath{-}}}$ three-body system. In particular, the angular dependence of characteristic cusp and shape resonance features is presented. We find that the projectile frame DDCS for detached electron energies in the vicinity of the shape resonance peak is nearly isotropic. This is due in part to the $^{1}\mathrm{P}$ symmetry of the resonance feature, which limits the angular distribution to constant and ${\mathrm{cos}}^{2}$\ensuremath{\theta} terms, and in part to cancellation in the integral over momentum transfer on which the coefficient of the ${\mathrm{cos}}^{2}$\ensuremath{\theta} term depends.We also find that the rapid variation of these cusp and shape resonance features with angle in the laboratory frame requires that experimental angular and energy resolutions be accounted for in order to obtain good agreement with the measured results of M. G. Menendez and M. M. Duncan [Phys. Rev. A 36, 1653 (1987)] on the energy spectrum of detached electrons coincident with the formation of H(2p). The DDCS's for 0.5-MeV ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$+He\ensuremath{\rightarrow}H(1s)+${e}^{\mathrm{\ensuremath{-}}}$+${\mathrm{He}}^{\mathrm{*}}$ are also presented. When these latter results are added to those for producing H(n=2), the sum gives good agreement with the experimental data of M. G. Menendez and M. M. Duncan [Phys. Rev. 20, 2327 (1979)], which include all final states of the H atom, thereby confirming the important contribution the H(n=2) states make to the total-detachment cross section. We present evidence on the sensitivity of these results to the choice of the average excitation energy of the helium target. Finally, we pinpoint the origin of our predicted Gailitis-Damburg oscillations in the DDCS near threshold as stemming from a rapid decrease of an analytically known phase appropriate for long-range dipole fields.

A New Microwave Discriminator

A new form of microwave FM discriminator using the technique of injection synchronization is described. Detailed theoretical and experimental results on its performance have been presented. The merits of the discriminator are: (i) tracking but with the snag of false locking, (ii) low distortion at the output, and (iii) needing only few off-the-shelve microwave components.