Showing papers on "Energy (signal processing) published in 1988"

Feature Detection in Human Vision: A Phase-Dependent Energy Model

Abstract: This paper presents a simple and biologically plausible model of how mammalian visual systems could detect and identify features in an image. We suggest that the points in a waveform that have unique perceptual significance as 'lines' and 'edges' are the points where the Fourier components of the waveform come into phase with each other. At these points 'local energy' is maximal. Local energy is defined as the square root of the sum of the squared response of sets of matched filters, of identical amplitude spectrum but differing in phase spectrum by 90 degrees: one filter type has an even-symmetric line-spread function, the other an odd-symmetric line-spread function. For a line the main contribution to the local energy peak is in the output of the even-symmetric filters, whereas for edges it is in the output of the odd-symmetric filters. If both filter types respond at the peak of local energy, both edges and lines are seen, either simultaneously or alternating in time. The model was tested with a series of images, and shown to predict well the position of perceived features and the organization of the images.

Static and dynamic spin correlations in pure and doped La 2 CuO 4

Abstract: We report elastic, quasielastic (F dE), and inelastic neutron-scattering studies of the instantaneous and dynamic spin fluctuations in as-grown and doped ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$. Four samples have been studied: (A) as-grown ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ with ${T}_{N}$=195 K, (B) oxygenated ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ with ${\mathit{T}}_{\mathit{N}}$\ensuremath{\simeq}100 K, (C) ${\mathrm{La}}_{2}$${\mathrm{Cu}}_{0.95}$${\mathrm{Li}}_{0.05}$${\mathrm{O}}_{4}$, and (D) ${\mathrm{La}}_{1.97}$${\mathrm{Sr}}_{0.03}$${\mathrm{Cu}}_{0.95}$${\mathrm{Li}}_{0.05}$${\mathrm{O}}_{4}$. All crystals exhibit variable-range-hopping conductivity behavior. At room temperature each sample exhibits two-dimensional (2D) antiferromagnetic instantaneous correlations in the ${\mathrm{CuO}}_{2}$ sheets with correlation length varying from \ensuremath{\sim}200 A\r{} in crystal A to \ensuremath{\sim}14 A\r{} in crystal D. The integrated intensity and therefore the effective moment is, however, constant to within the experimental error. In samples A and B the 2D correlation length becomes sufficiently large with decreasing temperature that the interplanar coupling is able to drive a transition to 3D long-range order. The spin dynamics have been studied in detail in crystals A and B and quite unusual behavior is observed. In contrast to previously studied planar antiferromagnets, there is no significant E\ensuremath{\simeq}0 component for temperatures \ensuremath{\ge}${T}_{N}$ and instead the 2D response function is highly inelastic. The effective dispersion of the spin excitations is \ensuremath{\ge}0.4 eV A\r{}. This large energy scale for the spin fluctuations gives credence to models of the superconductivity in doped ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ in which the pairing is magnetic in origin.

Dichotomy of the hydrogen-atom in superintense, high-frequency laser fields

Abstract: We study the behavior of atomic hydrogen in a monochromatic radiation field of high frequency $\ensuremath{\omega}$ and high intensity $I$, when its structure depends only on the parameter ${\ensuremath{\alpha}}_{0}={I}^{\frac{1}{2}}{\ensuremath{\omega}}^{\ensuremath{-}2}$ a.u., and when multiphoton ionization is quenched. At large ${\ensuremath{\alpha}}_{0}$ the ground-state binding energy undergoes a drastic reduction. This is coupled to an unprecedented stretching of the (oscillating) electron wave function, culminating in its separation into two parts (dichotomy) for ${\ensuremath{\alpha}}_{0}g50$ a.u.

Effect of electron-electron scattering on nonequilibrium transport in quantum-well systems

Abstract: We model nonequilibrium transport in a GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As quantum-well structure using an ensemble Monte Carlo simulation of the full multisubband system in which we include electron-electron (e-e) scattering explicitly into the calculation. The e-e scattering cross section is calculated using the Born approximation and introduced into the transient Monte Carlo simulation via a self-scattering technique. This interaction is found to be especially effective in transferring energy between different subbands, thus thermalizing the carriers within a picosecond. The model described herein is applied to the study of laser-excited carriers in a quantum-well system and to the response of such a system to high parallel electric fields. In the case of laser excitation, e-e interaction may dominate the initial evolution, reducing the cascade of carriers via optical-phonon emission.

Nonadiabatic coupled-rearrangement-channel approach to muonic molecules.

Abstract: A new variational approach to muonic molecules is proposed. The total three-body wave function is expanded in terms of basis functions spanned over the three rearrangement channels in the Jacobian coordinate system. Energy levels of the dt\ensuremath{\mu} molecule are calculated with a high accuracy and a short computation time. For the weakly bound state with J=v=1, which is a key to the muon-catalyzed d-t fusion, the calculated energy ${\ensuremath{\varepsilon}}_{11}$ is better than the literature data. With the use of the most up-to-date, 1986 CODATA--recommended [E. R. Cohen and B. N. Taylor, CODATA Bull. 63 (1986)] values of physical constants, we obtained ${\ensuremath{\varepsilon}}_{11}$=-0.660 264 eV with 2662 basis functions and ${\ensuremath{\varepsilon}}_{11}$(\ensuremath{\infty})=-0.66030\ifmmode\pm\else\textpm\fi{}000 02 eV by extrapolation.

Ferromagnetic strip domains in an atomic monolayer.

Abstract: We determine the range of values of the uniaxial surface anisotropy ${K}_{s}$ that leads to the formation of domains in an atomic monolayer of ferromagnetically coupled spins. If the ratio f of ${K}_{s}$ to the dipolar energy is larger than a minimum value ${f}_{\mathrm{min}}$ determined by the ratio of exchange to dipolar energies, and if the easy direction is normal to the layer plane, then a domain pattern is energetically favorable compared to any uniformly magnetized configuration. The maximum component of magnetization normal to the layer, ${M}_{\ensuremath{\perp}}$, increases continuously from zero as f increases from its threshold ${f}_{\mathrm{min}}$ and tends to the saturation magnetization value for large f. Above threshold (f=${f}_{\mathrm{min}}$), the width of the domains is very sensitive to the value of f, increasing very rapidly with f and reaching the macroscopic value of the order of 1 cm for a value f=1.4. Contrary to the usual assumption of thin domain walls determined by the ratio of exchange to anisotropy energy, in monolayers it is necessary to treat the domain structure as a whole and to include explicitly the dipolar energy. A variational treatment gives, at the threshold of ${K}_{s}$, a simple cosine dependence on distance for the magnetization normal to the layer. The calculation is extended to layers consisting of a few atomic planes.

Inelastic scattering and pair breaking in anisotropic and isotropic superconductors.

Abstract: The pair-breaking effect of inelastic scattering of electrons off boson fluctuations is examined for anisotropic and isotropic superconductors. We show that the ratio g of the couplings of the boson fluctuations to the pairing and normal electron self-energies is an important parameter. Phonon-mediated s-wave superconductivity corresponds to a value of g=1, and spin-fluctuation-mediated d-wave superconductivity to gl1. For gl1, there is a critical frequency ${\ensuremath{\omega}}_{c}$\ensuremath{\sim}${T}_{c}$${e}^{1/g}$ in the boson spectrum: bosons at \ensuremath{\omega}${\ensuremath{\omega}}_{c}$ are pair breaking. We give an approximate expression for the pair-breaking effect of low-lying bosons. We also study the dependence of ${T}_{c}$ upon g in an Einstein model in which the boson spectral weight is concentrated at a frequency ${\ensuremath{\omega}}_{E}$. We show that for fixed electron-boson coupling, ${T}_{c}$/${\ensuremath{\omega}}_{E}$ decreases rapidly as g decreases from 1, and for g\ensuremath{ e}1 ${T}_{c}$/${\ensuremath{\omega}}_{E}$ saturates at a low value as the coupling tends to infinity. Applications to heavy-fermion superconductors and to the new high-${T}_{c}$ materials are discussed.

Energy gap, excitonic, and "internal" Mn2+ optical transition in Mn-based II-VI diluted magnetic semiconductors.

Abstract: The piezomodulated and the photomodulated reflectivity spectra of Mn-based diluted magnetic semiconductors (${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te,${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$ Te,${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se,${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se) reveal sharp signatures associated with the excitonic transitions near the band gap. The dependence of the excitonic energy on the manganese concentration (x), on temperature (T), and, in the wurtzite structure, on the polarization with respect to the optic axis, are determined from the modulation experiments. The excitonic energy in the ``hypothetical'' tetrahedrally coordinated MnTe and MnSe, viewed as the end members of the alloy systems, are deduced from the x dependence of the excitonic transition. A signature associated with ${\mathrm{Mn}}^{2+}$ is observed in piezomodulated ``pseudoreflection,'' when the energy gap is sufficiently large; its energy is insensitive to x. The absence of the Zeeman effect for the ${\mathrm{Mn}}^{2+}$ signature, in contrast to the large Zeeman effect of the excitonic feature, demonstrates that the former is an optical transition associated with the 3${d}^{5}$ shell of ${\mathrm{Mn}}^{2+}$.

High-energy unitarity of gravitation and strings

Abstract: It is known that the behavior of a four-point string amplitude at large center-of-mass energy \ensuremath{\surd}s and fixed momentum transfer q= \ensuremath{\surd}-t is not perturbative. We study this region of phase space by summing multiple Reggeized graviton exchange in the eikonal approximation in D space-time dimensions. It is argued that the eikonal sum is at least representative of the summation of the leading powers of s in a string theory. The masslessness and high spin of the (Reggeized) graviton determine the character of the result. For ${\ensuremath{\kappa}}^{2}$${\mathrm{sq}}^{D\mathrm{\ensuremath{-}}4}$\ensuremath{\lesssim}1 (\ensuremath{\kappa} is the gravitational coupling), the eikonal amplitude is dominated by single Reggeized graviton exchange. The amplitude in the region ${\ensuremath{\kappa}}^{2}$${\mathrm{sq}}^{D\mathrm{\ensuremath{-}}4}$\ensuremath{\gg}1 is quite nonperturbative in character: simple Regge behavior and the Froissart bound are violated, and the amplitude does not satisfy a fixed-momentum-transfer dispersion relation. Although order by order the amplitude exhibits in ${q}^{2}$ the exponential decrease of Regge behavior, the final amplitude has only power-law falloff dependent on the number of space-time dimensions but independent of the Regge slope. The unitarity of the partial-wave projections of the eikonal amplitude is also studied. It is demonstrated that for D\ensuremath{\ge}4 noncompact dimensions, the partial-wave amplitudes are bounded as s\ensuremath{\rightarrow}\ensuremath{\infty} only for large values of angular momentum, l\ensuremath{\gtrsim}${x}_{0}$ \ensuremath{\surd}s , where ${x}_{0}$ is the dominant value of the impact parameter. A heuristic argument is presented that the eikonal approximation is successful in unitarizing Reggeized graviton exchange as t/s\ensuremath{\rightarrow}0 in four dimensions but not in higher dimensions.

Energetics of charged metallic particles: From atom to bulk solid.

Abstract: The energy of a spherical metallic particle of radius R, charged with Z excess electrons, is simply ${E}_{Z}$=${E}_{0}$-ZW+${Z}^{2}$${e}^{2}$/2(R+a), where W is the bulk work function, e is the charge of one electron, and R+a is the radial centroid of the excess charge. Consequently, the ionization energy is I=W+${e}^{2}$/2(R+a), and the electron affinity is A=W-${e}^{2}$/2(R+a). These formulas apply even to the smallest microparticle, a single monovalent atom. Thus they may be used to estimate the bulk work function W=(I+A)/2 and density parameter (Wigner-Seitz radius) ${r}_{s}$ from atomic values for I and A; ${r}_{s}$ is the solution of the equation ${r}_{s}$+a(${r}_{s}$)=${e}^{2}$/(I-A). The link between microcosm and macrocosm is further shown by the relationship ${\ensuremath{\varepsilon}}_{\mathrm{coh}\mathrm{\ensuremath{\approxeq}}\mathrm{\ensuremath{\sigma}}4\mathrm{\ensuremath{\pi}}{r}_{s}^{2}}$ between the cohesive energy ${\ensuremath{\varepsilon}}_{\mathrm{coh}}$ and the surface tension \ensuremath{\sigma}. These relationships are illustrated for atoms and small jellium spheres.

Wave-breaking amplitude of relativistic oscillations in a thermal plasma.

Abstract: The maximum amplitude of relativistic plasma oscillations (${\ensuremath{\upsilon}}_{\mathrm{ph}}$ near $c$) is obtained with a combined one-dimensional waterbag and warm-fluid model. The waterbag description is used to obtain expressions for the pressure and internal energy as functions of the proper density. A relativistic Euler's equation that is valid for arbitrarily large amplitudes and an analytic expression for the wave-breaking amplitude in the limit ${\ensuremath{\upsilon}}_{\mathrm{ph}}\ensuremath{\cong}c$ are obtained. Even a small amount of thermal energy can significantly reduce the maximum plasma-wave amplitude relative to the cold wave-breaking value. The significance of the results for recent accelerator schemes is discussed.

Direct-energy-gap dependence on Al concentration in Al x Ga 1 − x As

Abstract: The direct exciton energy at 2 K in ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As epitaxial layers (0.10lxl0.75) has been determined by optical transmission measurements. Microprobe analysis has been employed for the evaluation of the aluminum concentration. From these data, the direct-energy-gap dependence on x has been obtained; its comparison with the literature clearly shows that the relation ${E}_{\mathrm{gap}{}^{\ensuremath{\Gamma}}(\mathrm{x}}$) commonly used in the optical determination of the alloy concentration must be revised. Contrary to older evaluations, our determination of the nonlinear contribution to ${E}_{\mathrm{gap}{}^{\ensuremath{\Gamma}}(\mathrm{x}}$) agrees with the theoretically predicted values.

Central charge and universal combinations of amplitudes in two-dimensional theories away from criticality.

Abstract: For a general isotropic two-dimensional theory near criticality, the universal singular free energy per correlation volume ${f}_{s}{\ensuremath{\xi}}^{2}$ is equal to $\ensuremath{-}(\frac{c}{12}\ensuremath{\pi})(2\ensuremath{-}\ensuremath{\alpha}){(1\ensuremath{-}\ensuremath{\alpha})}^{\ensuremath{-}1}$, where $c$ is the central charge of the theory at criticality, $\ensuremath{\alpha}$ is the usual specific-heat exponent, and $\ensuremath{\xi}$ is defined (for $\ensuremath{\alpha}g0$) in terms of the second moment of the energy correlations. Some generalizations of this result are also noted.

Far-infrared response of one-dimensional electronic systems in single- and two-layered quantum wires.

Abstract: The far-infrared (FIR) response of arrays of periodic single- and two-layered quantum wires has been investigated. The wire structures have been prepared by ultrafine deep-mesa etching of modulation-doped $\frac{{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}}{\mathrm{GaAs}}$ heterostructures and two-layered quantum-well systems. Due to narrow geometrical dimensions (500 nm), quantum confinement arises and leads to the formation of one-dimensional electronic subbands with a typical energy separation of 1-3 meV. The FIR transmission spectra of the single- and two-layered quantum wire structures show one and two resonances, respectively. The resonance frequencies are observed at significantly higher energies compared to the one-dimensional subband separation. This implies that collective interactions have a strong influence on the excited transitions and leads to the conclusion that the FIR resonances in the one-dimensional electronic systems have predominantly the character of layer-coupled local plasmon modes.

The CDF central electromagnetic calorimeter

Abstract: The central electromagnetic calorimeter for the Collider Detector at Fermilab uses a hybrid design with scintillator and wavelength shifter for energy measurement and an embedded strip chamber for position determination and longitudinal shower development. Complementary calibration systems are incorporated in the design. Calorimeter characteristics and performance are summarized. An average energy resolution, σ(E) E , of 13.5%√E sin θ (with E in GeV), and a position resolution of ±2 mm at 50 GeV are measured.

A method for recording multi-phase flows through a transport system.

Abstract: A method for determining the flow situation in a transport system where the flow medium occurs in multi-phase and is conducted through a structural detail such as a choke valve, which acts on the flow of the medium and produces turbulence causing acoustic energy signals which are generated in and immediately close to the structural detail. The acoustic energy signals are sensed by one or a plurality of acoustic sensors with an acoustic broad-band connection with the structural detail. The sensor converts the energy signal to transmittable signals to a signal processing unit utilizing known signal analysis principles to provide interpretable information on the actual flow situation and the flowing medium which is influenced by the structural detail. The structural detail to which the acoustic sensor is connected may be incorporated in a subsea production system.

Neutron scattering study of the heavy-fermion compound CeRu 2 Si 2

Abstract: We have performed inelastic-neutron-scattering experiments on single crystals of the heavy-fermion compound ${\mathrm{CeRu}}_{2}$${\mathrm{Si}}_{2}$. At intermediate temperatures (25 KlTl90 K) the magnetic response, Im(\ensuremath{\chi}/\ensuremath{\omega}), is well described by a quasielastic Lorentzian as a function of energy. The half-width \ensuremath{\Gamma} of the Lorentzian follows a \ensuremath{\surd}T law. At low temperatures (Tl20 K), the magnetic response, Im(\ensuremath{\chi}/\ensuremath{\omega}), seems to be better described by an inelastic peak. The half-width \ensuremath{\Gamma} remains temperature independent down to the lowest temperature (T\ensuremath{\simeq}1.7 K) and amounts about 1.2 meV yielding a Kondo temperature ${T}_{K}$=14 K. The other important result we are presenting is the existence of magnetic correlations with a modulation characterized by two incommensurate wave vectors ${\mathrm{k}}_{1}$=(0.3,0,0) and ${\mathrm{k}}_{2}$=(0.3,0.3,0). These magnetic correlations start to develop already at rather high temperatures (\ensuremath{\sim}60 K) but they saturate at T\ensuremath{\simeq}15 K when the magnetic response becomes inelastic. So the hybridation of 4f electrons with conduction electrons seems to prevent the divergence of the magnetic correlations which otherwise would give rise to a long-range magnetic ordering.

Energy field sensor using summing means

Abstract: An energy field disturbance sensor comprises an energy field emission means such as a light emitting diode; the emitted field may contain a time-varying component An energy sensing device such as a photodiode receives the field energy to produce a sensing signal related to the intensity of the emitted field and any disturbance therein A second signal complementary to the sensed signal is added in a summing junction to the sensing signal to create a summation signal If the emitted energy field contains a time-varying component, a detection circuit is used to sample the summation signal to convert the summation signal to a proportional detection signal An analog to digital converter converts the detection signal to a digital form Digital processing circuitry modulates the amplitude of the complementary signal or the intensity of the energy field according to the desired mode of operation, and in accordance with the behavior of the digital detection signal and external control signals The circuitry is capable of creating a null condition in the detection signal when the received amplitude of the energy field is such that when added to the complementary signal a cancellation effect occurs; the null condition may be transient or continuous in nature depending on the mode of operation and the nature of the field disturbance

Test of Wannier threshold laws: Double-photoionization cross section in helium.

Abstract: An extensive study of the threshold law for the cross section of double photoionization in helium is presented. It provides quantitative information about the Wannier exponent \ensuremath{\alpha}, which agrees with the theoretical prediction. In addition, the hitherto unknown constant of proportionality ${\ensuremath{\sigma}}_{0}$ for the cross section ${\ensuremath{\sigma}}^{++}$ was determined as well as the energy range of validity, which was found to be smaller than expected.

Gap anisotropy and phonon self-energy effects in single-crystal YBa2Cu3O7- delta.

Abstract: We report a Raman scattering investigation of superconducting gap anisotropy in single-crystal $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$. Gap anisotropy is investigated by studying the peak in the low-temperature Raman continuum in various symmetries. Roughly a 35% difference in the energy of this peak is observed between different symmetries, suggesting the presence of substantial gap anisotropy. Anisotropy is further evidenced by the damping behavior of the 340-${\mathrm{cm}}^{\ensuremath{-}1}$ Raman-active phonon below ${T}_{c}$, which displays increased attenuation due to phonon-induced pair breaking. The temperature dependence of this attenuation is consistent with a $T=0$ pair-breaking peak which is much larger than the 340-${\mathrm{cm}}^{\ensuremath{-}1}$ phonon in certain regions of the Fermi surface.

Laboratory drilling performance of PDC bits

Abstract: A laboratory study of Polycrystalline Diamond Compact(PDC) bit designs has generated data that gives an insight into PDC bit performance in the field. The tests reviewed in this paper include those for rate of penetration, torque response, hydraulic energy sensitivity, balling tendency, dull bit performance, and bit performance after the removal of selected cutters. A total of four bit designs were tested. The designs included flat-faced profiles and parabolic profiles.

Effects of isolated atomic collision cascades on SiO 2 /Si interfaces studied by scanning tunneling microscopy

Abstract: Si(100) wafers with oxide thicknesses of 40 and 200 A\r{} were implanted with ${\mathrm{Ge}}^{+}$ and ${\mathrm{As}}^{+}$ ions with energies from 20 keV to 1 MeV. Low ion doses, \ensuremath{\sim}${10}^{11}$ ions/${\mathrm{cm}}^{2}$, were used to avoid overlap of collision cascades. The extent of cascade damage at the ${\mathrm{SiO}}_{2}$/Si interface due to single ion impacts was examined by scanning tunneling microscopy after the oxide layer was removed by HF. The surface topography in the form of craters was found to relate to the cascade damage rather than to the removal of atoms by sputtering. There is a 1:1 correlation between the number of ion impacts and the number of craters. The crater diameter as a function of implant-ion energy agrees well with theoretical estimates of the lateral extent of the cascade due to nuclear energy deposition at the interface.

Two-magnon Raman scattering in ( La 1 − x Sr x ) 2 CuO 4

Abstract: The two-dimensional two-magnon peak in (${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${)}_{2}$${\mathrm{CuO}}_{4}$ decreases rapidly from x=0 to x=0.01. For xg0.03 scattering is observed over a very wide energy region from low energy to over 4000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. This indicates that the correlation length of the two-dimensional spin order decreases rapidly from x=0 to x=0.01. This decrease can be related to the rapid decrease of the three-dimensional antiferromagnetic transition temperature (${T}_{N}$), while at xg0.03 the correlation still retains a short length and a short time. At xl0.01 the scattering is in a resonant state with narrow electronic levels of which transition energy is about 18 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The narrowness of these energy levels, and the simultaneous extinction of resonance and the two-magnon scattering, suggest that these are localized levels relating to the Cu spins.

Apparatus and method for controlling pulse energy in antitachyarrhythmia and bradycardia pacing device

Abstract: An apparatus and method for controlling pulse energy in an arrythmia control device wherein bradycardia pacing pulses are normally delivered at a first energy, and after antitachycardia therapy, bradycardia pulses are delivered at a second energy, the second energy being greater than the first energy. The antitachycardia therapy may be at least one of antitachycardia pacing, cardioversion and defibrillation; and is delivered at a third energy level greater than said second energy level. The second energy level is maintained for a predetermined period of time and then returned to the normal bradycardia pacing energy.

Theory of resonant tunneling in heterostructures

Abstract: For a parallel-plane barrier heterostructure of any form or composition, it is shown by considering the electron quantum states as functions of complex values of the energy that the full linewidth of a resonant peak in the coherent transmission probability, 2 \ensuremath{\Delta}E, and the tunneling lifetime of the quasilevel giving rise to the resonance, \ensuremath{\tau}, are necessarily related by (2 \ensuremath{\Delta}E)\ensuremath{\tau}=\ensuremath{\Elzxh}, and the conventional wave-packet transit time is 2\ensuremath{\tau} at the resonance energy. A formula is obtained for the peak transmission probability in terms of the ratio of the two outward currents for an electron escaping from the quasilevel.

Phase shift analysis of 0-30 MeV pp scattering data

Abstract: A multienergy phase shift analysis of all published proton-proton (pp) scattering data in the energy range ${T}_{\mathrm{lab}\mathrm{\ensuremath{\le}}30}$ MeV is presented. In the description of all partial waves the well-known long-range interaction is included: the improved Coulomb, the vacuum polarization, and the one-pion-exchange potential. In the lower partial waves the energy-dependent analysis uses a P-matrix parametrization for the short-range interaction. Special attention is paid to the electric interaction, the definition of the phase shifts, and the selection of the data. The fit to the final data set comprising 360 scattering observables results in ${\ensuremath{\chi}}^{2}$/${N}_{\mathrm{df}=1.0}$, where ${N}_{\mathrm{df}}$ is the number of degrees of freedom. The pp${\ensuremath{\pi}}^{0}$ coupling constant is determined to be ${\mathit{g}}_{\mathrm{pp}}$${\mathrm{\ensuremath{\pi}}}_{^{2}}^{0}$/4\ensuremath{\pi}=14.5\ifmmode\pm\else\textpm\fi{}1.2, but there are several indications for a lower value. The optimum value for the P-matrix radius b\ensuremath{\approxeq}1.4 fm is satisfying. Single-energy phase shifts with second derivative matrices, and effective range parameters are given.

Multiresolution representations and wavelets

Abstract: Multiresolution representations are very effective for analyzing the information in images. In this dissertation we develop such a representation for general purpose low-level processing in computer vision. We first study the properties of the operator which approximates a signal at a finite resolution. We show that the difference of information between the approximation of a signal at the resolutions 2$\sp{j+1}$ and 2$\sp{j}$ can be extracted by decomposing this signal on a wavelet orthonormal basis of ${\bf L}({\bf R}\sp{n}$). In ${\bf L}\sp2({\bf R})$, a wavelet orthonormal basis is a family of functions $\left\lbrack\sqrt{2\sp{j}}\ \psi(2\sp{j}x+n)\right\rbrack\sb{(j,n)\in{\rm Z}\sp2}$, which is built by dilating and translating a unique function $\psi(x)$, called a wavelet. This decomposition defines an orthogonal multiresolution representation called a wavelet representation. It is computed with a pyramidal algorithm of complexity n log(n). We study the application of this signal representation to data compression in image coding, texture discrimination and fractal analysis. The multiresolution approach to wavelets enables us to characterize the functions $\psi(x) \in {\bf L}\sp2({\bf R})$ which generate an orthonormal basis. The inconvenience of a linear multiresolution decomposition is that it does not provide a signal representation which translates when the signal translates. It is therefore difficult to develop pattern recognition algorithms from such representations. In the second part of the dissertation we introduce a nonlinear multiscale transform which translates when the signal is translated. This representation is based upon the zero-crossings and local energies of a multiscale transform called the dyadic wavelet transform. We experimentally show that this representation is complete and that we can reconstruct the original signal with an iterative algorithm. We study the mathematical properties of this decomposition and show that it is well adapted to computer vision. To illustrate the efficiency of this Energy Zero-Crossings representation, we have developed a coarse to find matching algorithm on stereo epipolar scan lines. While we stress the applications towards computer vision, wavelets are useful to analyze other types of signal such as speech and seismic-waves.

Effect of buffer-layer composition on new optical transitions in Si/Ge short-period superlattices

Abstract: Local empirical pseudopotentials with spin-orbit coupling have been used to calculate transition energies and transition probabilities for the Si/Ge (4:4) superlattice grown on (001) ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ge}}_{\mathrm{x}}$ (0\ensuremath{\le}x\ensuremath{\le}1) buffer layers. The characters of superlattice states close to the band edges are shown in terms of their real-space charge densities and their origin in wave-vector space. Influences of heterojunction-interface bond length and band offset are examined and the individual contributions of compositional modulation and atomic relaxation to the enhancement of matrix elements for cross-gap quasidirect transitions are established. A strain-induced reversal of \ensuremath{\Vert}${m}_{J}$\ensuremath{\Vert}=(3/2 and \ensuremath{\Vert}${m}_{J}$\ensuremath{\Vert}=1/2 valence states is demonstrated in terms of the effects on subband energy levels and polarization-dependence of cross-gap transition probabilities. In the case of the Si/Ge (4:4) superlattice grown on Si, a direct comparison is made between theoretical results and recent electroreflectance data of Pearsall et al. [Phys. Rev. Lett. 58, 729 (1987)]. Comparison is also made between the results of the present empirical-pseudopotential calculations and results of recent local-density, quasiparticle, tight-binding, and effective-mass--type calculations. Predictions are made which can be used to discriminate between different transition assignments which have been given to the same structure in the electroreflectance spectra for the (4:4) superlattice grown on (001) Si.

Quantum statistical mechanics of an array of resistively shunted Josephson junctions.

Abstract: We have constructed a fully quantum-mechanical model of an ordered array of resistively shunted Josephson junctions, and have determined the nature of the phase diagram as a function of the Josephson coupling, V, the capacitance, C (or, equivalently, the charging energy ${E}_{0}$=4${e}^{2}$/C), the shunt resistance, R, and the temperature, T. In order to treat the dissipative element (R) in a quantum system, we have modeled it by a heat bath with spectral weight chosen to reproduce Ohmic resistance in the classical limit. Among other results, we find that in the extreme quantum limit, ${E}_{0}$\ensuremath{\gg}V\ensuremath{\gg}${k}_{B}$T, the onset of global phase coherence (superconductivity) in the array occurs only if R is less than a critical value ${R}_{c}$=Ah/${e}^{2}$, where A is a number of order 1 which depends on the dimension and the lattice structure. The fact that the dissipation enters the thermodynamics at all is a consequence of the quantum nature of the transition. This transition is reminiscent of the results of recent experiments on thin films of granular superconductors.

Programmable sensor apparatus

Abstract: A user-programmable sensor system for generating an alarm signal upon detection of an alarm event is disclosed. The system is particularly suited for use in vehicle security systems. The system employs a three axes accelerometer as a shock/motion detector, and the transducer outputs are summed and the composite signal filtered to provide several channels, each covering a specific frequency or frequency band. The outputs of the channels are digitized and processed by a microprocessor. An event is characterized by seveal signal parameters with an alarm event triggered by the simultaneous occurrence of several predetermined conditions, and not just one condition. The limit values for the signal parameters which are required to qualify a shock/motion event as an alarm event may be readily programmed by the user at any time or place. The system further includes a glass breakage transducer and analyzes higher frequency components of the transducer signal to sense a glass breakage event when predetermined signal parameters are exceeded and when energy from the shock detector correlates with the glass breakage signal. The correlation with several parameters substantially reduces false alarm signals resulting from ambient high frequency noises, such as caused by passing siren-sounding vehicles or low-flying jets or helicopters.