Showing papers on "Energy (signal processing) published in 1994"

Chaos and quantum thermalization

Abstract: We show that a bounded, isolated quantum system of many particles in a specific initial state will approach thermal equilibrium if the energy eigenfunctions which are superposed to form that state obey Berry's conjecture. Berry's conjecture is expected to hold only if the corresponding classical system is chaotic, and essentially states that the energy eigenfunctions behave as if they were Gaussian random variables. We review the existing evidence, and show that previously neglected effects substantially strengthen the case for Berry's conjecture. We study a rarefied hard-sphere gas as an explicit example of a many-body system which is known to be classically chaotic, and show that an energy eigenstate which obeys Berry's conjecture predicts a Maxwell-Boltzmann, Bose-Einstein, or Fermi-Dirac distribution for the momentum of each constituent particle, depending on whether the wave functions are taken to be nonsymmetric, completely symmetric, or completely antisymmetric functions of the positions of the particles. We call this phenomenon eigenstate thermalization. We show that a generic initial state will approach thermal equilibrium at least as fast as O(\ensuremath{\Elzxh}/\ensuremath{\Delta})${\mathit{t}}^{\mathrm{\ensuremath{-}}1}$, where \ensuremath{\Delta} is the uncertainty in the total energy of the gas. This result holds for an individual initial state; in contrast to the classical theory, no averaging over an ensemble of initial states is needed. We argue that these results constitute a sound foundation for quantum statistical mechanics.

Minimal Discrete Energy on the Sphere

Abstract: We investigate the energy of arrangements of N points on the surface of a sphere in R3, interacting through a power law potential V = rα, −2 < α < 2, where r is Euclidean distance. For α = 0, we take V = log(1/r). An area-regular partitioning scheme of the sphere is devised for the purpose of obtaining bounds for the extremal (equilibrium) energy for such points. For α = 0, finer estimates are obtained for the dominant terms in the minimal energy by considering stereographical projections on the plane and analyzing certain logarithmic potentials. A general conjecture on the asymptotic form (as N → ∞) of the extremal energy, along with its supporting numerical evidence, is presented. Also we introduce explicit sets of points, called “generalized spiral points”, that yield good estimates for the extremal energy. At least for N ≤ 12, 000 these points provide a reasonable solution to a problem of M. Shub and S. Smale arising in complexity theory.

Leptoproduction of heavy quarks. II. A unified QCD formulation of charged and neutral current processes from fixed-target to collider energies.

Abstract: A unified QCD formulation of leptoproduction of massive quarks in charged current and neutral current processes is described. This involves adopting consistent factorization and renormalization schemes which encompass both vector-boson-gluon-fusion (``flavor creation'') and vector-boson-massive-quark-scattering (``flavor excitation'') production mechanisms. It provides a framework which is valid from the threshold for producing the massive quark (where gluon fusion is dominant) to the very high energy regime when the typical energy scale \ensuremath{\mu} is much larger than the quark mass ${\mathit{m}}_{\mathit{Q}}$ (where the quark scattering should be prevalent). This approach effectively resums all large logarithms of the type [${\mathrm{\ensuremath{\alpha}}}_{\mathit{s}}$(\ensuremath{\mu})ln(${\mathrm{\ensuremath{\mu}}}^{2}$/${\mathit{m}}_{\mathit{Q}}^{2}$)${]}^{\mathit{n}}$ which limit the validity of existing fixed-order calculations to the region \ensuremath{\mu}\ensuremath{\sim}O(${\mathit{m}}_{\mathit{Q}}$). We show that the (massive) quark-scattering contribution (after subtraction of overlaps) is important in most parts of the (x,Q) plane except near the threshold region. We demonstrate that the factorization scale dependence of the structure functions calculated in this approach is substantially less than those obtained in the fixed-order calculations, as one would expect from a more consistent formulation.

Resolving the 180-degree ambiguity in vector magnetic field measurements: The 'minimum' energy solution

Abstract: I present a robust algorithm that resolves the 180-deg ambiguity in measurements of the solar vector magnetic field. The technique simultaneously minimizes both the divergence of the magnetic field and the electric current density using a simulated annealing algorithm. This results in the field orientation with approximately minimum free energy. The technique is well-founded physically and is simple to implement.

Baseband-pulse-antenna techniques

Abstract: Frequency-domain concepts and terminology are commonly used to describe antennas. These are very satisfactory for a CW or narrowband application. However, their validity is questionable for an instantaneous wideband excitation. Time-domain and/or wideband analyses can provide more insight and more effective terminology. Two approaches for this time-domain analysis have been described. The more complete one uses the transfer function, a function which describes the amplitude and phase of the response over the entire frequency spectrum. While this is useful for evaluating the overall response of a system, it may not be practical when trying to characterize an antenna's performance, and trying to compare it with that of other antennas. A more convenient and descriptive approach uses time-domain parameters, such as efficiency, energy pattern, receiving area, etc., with the constraint that the reference or excitation signal is known. The utility of both approaches, for describing the time-domain performance, was demonstrated for antennas which are both small and large, in comparison to the length of the reference signal. The approaches have also been used for other antennas, such as arrays, where they also could be applied to measure the effects of mutual impedance, for a wide-bandwidth signal. The time-domain ground-plane antenna range, on which these measurements were made, is suitable for symmetric antennas. However, the approach can be readily adapted to asymmetric antennas, without a ground plane, by using suitable reference antennas. >

Theory of high-field electron transport and impact ionization in silicon dioxide.

Abstract: A detailed theoretical study of impact-ionization-related transport phenomena in ${\mathrm{SiO}}_{2}$ thin films is presented. The Boltzmann transport equation is integrated by the Monte Carlo method using acoustic-phonon-scattering rates derived from photoinduced electron transmission experiments. It is shown that these empirical scattering rates necessitate the inclusion of impact ionization at fields Fg${\mathit{F}}_{\mathrm{th}}^{\mathrm{ii}}$=7 MV/cm because phonon scattering alone can no longer stabilize the electron energy distribution below the ionization energy of 9 eV. However, even above ${\mathit{F}}_{\mathrm{th}}^{\mathrm{ii}}$, acoustic-phonon scattering is found to considerably delay the heating of electrons, leading to a wide dark space in which impact ionization cannot take place or is strongly reduced. Therefore, the electron multiplication factors m(F,${\mathit{t}}_{\mathrm{ox}}$) decrease rapidly with decreasing oxide thickness, ${\mathit{t}}_{\mathrm{ox}}$, for ${\mathit{t}}_{\mathrm{ox}}$30 nm. These predictions are shown to be consistent with results of several high-field transport experiments in silicon--silicon-dioxide device structures. The calculated electron energy distributions develop high-energy tails which extend beyond the band-gap energy at fields larger than ${\mathit{F}}_{\mathrm{th}}^{\mathrm{ii}}$, as observed by vacuum emission experiments. The calculated impact-ionization coefficients are found to be in good agreement with values derived from experiments.The hole generation factors m-1 quantitatively agree with substrate hole-current to channel-current ratios measured by the carrier separation technique in n-channel field effect transistors with gate oxide thicknesses ${\mathit{t}}_{\mathrm{ox}}$\ensuremath{\ge}25 nm. The field and thickness dependence of the measured positive charge buildup (hole trapping) near the Si/${\mathrm{SiO}}_{2}$ interface can be quantified in terms of impact ionization in the oxide film. The calculated carrier multiplication, however, cannot fully account for the substrate hole currents and the hole trapping measured in thinner oxides, ${\mathit{t}}_{\mathrm{ox}}$\ensuremath{\le}20 nm, indicating that another mechanism, likely related to hole injection from the anode, becomes the dominant source for hole currents in thin oxides. Dielectric breakdown of thin ${\mathrm{SiO}}_{2}$ films on silicon is reevaluated on the bases of all of these findings. It is proposed that time-dependent breakdown is the result of cumulative degradation of the oxide near its interfaces caused by impact ionization and by hot-electron-induced hydrogen release together. This ansatz is shown to yield a good understanding for the oxide field and thickness dependence of the interface-state generation and of the charge to breakdown. Since impact ionization is strongly suppressed in thin films, ${\mathit{t}}_{\mathrm{ox}}$25 nm, degradation and time-dependent breakdown appear to be largely caused by hydrogen release and its subsequent secondary reactions in these thin films.

Evidence for mean free path fluctuation induced pinning in yba2cu3o7 and yba2cu4o8 films

Abstract: The critical current ${\mathit{j}}_{\mathit{c}}$ and the pinning energy ${\mathit{U}}_{\mathit{c}}$ have been determined for three types of yttrium-based superconducting films from current ${\mathit{j}}_{\mathit{s}}$ and dynamic relaxation Q=d ln${\mathit{j}}_{\mathit{s}}$/d ln(dB/dt) data by means of the generalized inversion scheme. For B2 T and T80 K the temperature dependence of ${\mathit{j}}_{\mathit{c}}$ and ${\mathit{U}}_{\mathit{c}}$ for all films is found to be in excellent agreement with a model of single vortices pinned by randomly distributed weak pinning centers via spatial fluctuations of the charge carrier mean free path. Pinning due to spatial fluctuations of ${\mathit{T}}_{\mathit{c}}$ is not observed.

Semiconductor surface roughness: Dependence on sign and magnitude of bulk strain.

Abstract: Changes in surface roughness have been studied as a function of bulk compressive and tensile strains (biaxial in the plane of the sample surface) in thin films of compositionally uniform and dislocation-free ${\mathrm{Ge}}_{0.5}$${\mathrm{Si}}_{0.5}$. A pronounced surface roughness is observed only for films under compressive strains exceeding 1.4%. Molecular dynamics simulations show that this striking result has its origin in the strain-induced lowering of surface step free energies.

QCD-based interpretation of the lepton spectrum in inclusive B-bar-->Xul nu -bar decays.

Abstract: We present a QCD-based approach to the end-point region of the lepton spectrum in $\overline{B}\ensuremath{\rightarrow}{X}_{u}\ensuremath{\ell}\overline{\ensuremath{ u}}$ decays. A genuinely nonperturbative form factor, the shape function, describes the falloff of the spectrum close to the end point. The moments of this function are related to forward scattering matrix elements of local, higher-dimension operators. We find that nonperturbative effects are dominant over a finite region in the lepton energy spectrum, the width of which is related to the kinetic energy of the $b$ quark inside the $B$ meson. In this region, a resummation of the most singular terms in the operator product expansion is performed. Applications of our method to the extraction of fundamental standard model parameters, among them ${V}_{\mathrm{ub}}$, are discussed.

Size dependence of the ferroelectric transition of small BaTiO3 particles: Effect of depolarization.

Abstract: A theory has been developed to examine the depolarization effect on the ferroelectric transition of small ${\mathrm{BaTiO}}_{3}$ particles. To reduce the depolarization energy, a crystal would break up into domains of different polarization. In this study, we consider cubic particles with alternating domains separated by 180\ifmmode^\circ\else\textdegree\fi{} domain walls. The depolarization energy and the domain-wall energy were incorporated into the Landau-Ginzburg free-energy density. Assuming a hyperbolic tangent polarization profile across the domain wall, the domain-wall energy \ensuremath{\gamma} and the domain-wall half thickness \ensuremath{\xi} can be obtained by minimizing \ensuremath{\gamma} with respect to \ensuremath{\xi}. To account for ${\mathrm{BaTiO}}_{3}$ not being a perfect insulator, a Schottky space charge layer beneath the particle surface that shields the interior of the crystal from the depolarization field was considered. The equilibrium polarization P and domain width D can be obtained by minimizing the total free-energy density with respect to both P and D. The results of the calculations show that the ferroelectric transition temperature of small particles can be substantially lower than that of the bulk transition temperature as a result of the depolarization effect. Consequently, at a temperature below the bulk transition temperature, the dielectric constant \ensuremath{\epsilon} can peak at a certain cube size L. These results agree with the existing experimental observations. Finally, the theory can also be applied to other ferroelectric materials such as ${\mathrm{KH}}_{2}$${\mathrm{PO}}_{4}$ or ${\mathrm{PbTiO}}_{3}$.

Tunneling delay times in one and two dimensions.

Abstract: We demonstrate that although the well-known analogy between the time-independent solutions for two-dimensional tunneling (e.g., frustrated total internal reflection) and tunneling through a one-dimensional potential barrier cannot, in general, be extended to the time domain, there are certain limits in which the delay times for the two problems obey a simple relationship. In particular, when an effective mass is chosen such that ${\mathit{mc}}^{2}$=\ensuremath{\Elzxh}\ensuremath{\omega}, the ``classical'' traversal times for allowed transmission become identical for a photon of energy \ensuremath{\Elzxh}\ensuremath{\omega} traversing an air gap between regions of index n and for a particle of mass m traversing the analogous square barrier of height ${\mathit{V}}_{0}$ in one dimension. The quantum-mechanical group delays are also identical, given this effective mass, both for E\ensuremath{\approxeq}${\mathit{V}}_{0}$ (\ensuremath{\theta}\ensuremath{\approxeq}${\mathrm{\ensuremath{\theta}}}_{\mathit{c}}$) and for E\ensuremath{\gg}${\mathit{V}}_{0}$ (\ensuremath{\theta}\ensuremath{\ll}${\mathrm{\ensuremath{\theta}}}_{\mathit{c}}$). (For a smoothly varying potential or index of refraction, the agreement persists for all values of E where the WKB approximation applies.) The same relation serves to equate the quantum-mechanical ``dwell'' times for any values of E and ${\mathit{V}}_{0}$. On the other hand, in the ``deep tunneling'' limit, E\ensuremath{\ll}${\mathit{V}}_{0}$ (\ensuremath{\theta}\ensuremath{\approxeq}\ensuremath{\pi}/2), one must choose ${\mathit{mc}}^{2}$=${\mathit{n}}^{2}$\ensuremath{\Elzxh}\ensuremath{\omega} in order to make the group delays equal for the two problems. These equivalences simplify certain calculations, and the two-dimensional analogy may also be useful for geometrically visualizing the tunneling process and the anomalously small group delays known to occur in the opaque limit. We also demonstrate that the equality of the group delays for transmission and reflection for lossless barriers follows from a simple intuitive argument based on time-reversal invariance, and discuss the extension of the result to the case of lossy barriers.

Spectrum and binding of an off-center donor in a spherical quantum dot.

Abstract: The energy levels and binding energies of an off-center donor in a GaAs-${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As spherical quantum dot are calculated by a linear variational method. The results clearly show the quantum size effect. The location effects of a donor ion on splitting, level ordering, and binding of the donor states are studied. It is found that the alteration of the position of a single Coulomb center can largely change the single-electron spectrum in a quantum dot with a larger radius.

Temperature dependence of the near-infrared refractive index of silicon, gallium arsenide, and indium phosphide

Abstract: Infrared laser interferometry was used to measure the temperature dependence, \ensuremath{\beta}(T), of the refractive index of Si, GaAs, and InP at \ensuremath{\lambda}=1.15, 1.31, 1.53, and 2.39 \ensuremath{\mu}m. Semiconductor wafer samples that had been polished on both sides were either heated or cooled while measuring the sample temperature and the transmitted or reflected intensity of an infrared laser beam. The changing optical path length within the material causes alternating constructive and destructive interference between reflections off the front and back surfaces of the wafer. By subtracting the contribution of thermal expansion, \ensuremath{\alpha}(T), which is small and accurately known, \ensuremath{\beta}(T) was obtained. Representative values of \ensuremath{\beta} (293 K) at 1.53 \ensuremath{\mu}m are 5.15\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ ${\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$, 6.65\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ ${\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$, and 5.95\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ ${\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$ for Si, GaAs, and InP. Polynomial expressions are presented for Si, GaAs, and InP, yielding values of \ensuremath{\beta}(T) that are accurate to within \ifmmode\pm\else\textpm\fi{}5%. $beta (T)--- increases with increasing temperature and decreases with increasing wavelength. There is a large resonance enhancement of \ensuremath{\beta}(T) in direct-gap semiconductors as the photon energy ${\mathit{E}}_{\mathit{h}\ensuremath{ u}}$ approaches the band-gap energy ${\mathit{E}}_{\mathit{g}}$. Absolute values and temperature dependences of \ensuremath{\beta} calculated from published theory agree reasonably well with the measurements. The extreme accuracy in \ensuremath{\beta} needed for interferometric thermometry, however, cannot be met by these theoretical calculations, and so requires the experimental measurements.

Inverse energy cascade in stationary two-dimensional homogeneous turbulence.

Abstract: Direct numerical simulations with up to ${1024}^{2}$ resolution are performed to study statistical properties of the inverse energy cascade in stationary homogeneous two-dimensional turbulence driven by small-scale Gaussian white-in-time noise. The energy spectra for the inverse energy cascade deviate strongly from the expected ${\mathit{k}}^{\mathrm{\ensuremath{-}}5/3}$ law and are close (somewhat flatter) to ${\mathit{k}}^{\mathrm{\ensuremath{-}}3}$. The reason for the deviation is traced to the emergence of strong vortices distributed over all scales. Statistical properties of the vortices are explored.

Optical properties of zinc-blende CdSe and ZnxCd1-xSe films grown on GaAs.

Abstract: We present room-temperature ellipsometric measurements of the dielectric function of ${\mathrm{Zn}}_{\mathit{x}}$${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$Se single-crystal films grown on (001)GaAs in the 1.5--6.0-eV energy region x ranging from 0 to 1. We identify the ${\mathit{E}}_{0}$, ${\mathit{E}}_{0}$+${\mathrm{\ensuremath{\Delta}}}_{0}$, ${\mathit{E}}_{1}$, ${\mathit{E}}_{1}$+${\mathrm{\ensuremath{\Delta}}}_{1}$, and ${\mathit{E}}_{2}$ threshold energies using the CdSe band structure calculated with a nonlocal empirical pseudopotential method. We find that the contact exciton effect has to be included in the calculated dielectric function of CdSe in order to obtain good agreement with our measurements. A compositon-dependent critical-point analysis of the ${\mathit{E}}_{1}$ and ${\mathit{E}}_{1}$+${\mathrm{\ensuremath{\Delta}}}_{1}$ structures has been performed. We also find that the spin-orbit-splitting band gap ${\mathrm{\ensuremath{\Delta}}}_{1}$(x), the linewidths, and the excitonic angles are about maximum at x=0.5, which we attribute to the statistical fluctuation of the alloy composition. Finally, the ${\mathit{E}}_{1}$ and ${\mathit{E}}_{1}$+${\mathrm{\ensuremath{\Delta}}}_{1}$ critical-point amplitudes cannot be understood by the one-electron approximation, confirming the existence of strong excitonic effects.

Testing scalar-tensor gravity with gravitational-wave observations of inspiralling compact binaries.

Abstract: Observations of gravitational waves from inspiralling compact binaries using laser-interferometric detectors can provide accurate measures of parameters of the source. They can also constrain alternative gravitation theories. We analyze inspiralling compact binaries in the context of the scalar-tensor theory of Jordan, Fierz, Brans, and Dicke, focusing on the effect on the inspiral of energy lost to dipole gravitational radiation, whose source is the gravitational self-binding energy of the inspiralling bodies. Using a matched-filter analysis we obtain a bound on the coupling constant ${\mathrm{\ensuremath{\omega}}}_{\mathrm{BD}}$ of Brans-Dicke theory. For a neutron-star--black-hole binary, we find that the bound could exceed the current bound of ${\mathrm{\ensuremath{\omega}}}_{\mathrm{BD}}$g500 from solar-system experiments, for sufficiently low-mass systems. For a 0.7${\mathit{M}}_{\mathrm{\ensuremath{\bigodot}}}$ neutron star and a 3${\mathit{M}}_{\mathrm{\ensuremath{\bigodot}}}$ black hole we find that a bound ${\mathrm{\ensuremath{\omega}}}_{\mathrm{BD}}$\ensuremath{\approxeq}2000 is achievable. The bound decreases with increasing black-hole mass. For binaries consisting of two neutron stars, the bound is less than 500 unless the stars' masses differ by more than about 0.5${\mathit{M}}_{\mathrm{\ensuremath{\bigodot}}}$. For two black holes, the behavior of the inspiralling binary is observationally indistinguishable from its behavior in general relativity. These bounds assume reasonable neutron-star equations of state and a detector signal-to-noise ratio of 10.

Theory of levy matrices

Abstract: We investigate the statistical properties of the spectrum of large symmetrical matrices with each element ${\mathit{H}}_{\mathit{i}\mathit{j}}$ chosen according to a broad distribution \ensuremath{\rho}(H) decaying for large H as ${\mathit{H}}^{\mathrm{\ensuremath{-}}1\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\mu}}}$. For \ensuremath{\mu}g2, 〈${\mathit{H}}^{2}$〉 is finite and the well known Gaussian orthogonal ensemble (GOE) results are recovered. When \ensuremath{\mu}2, the semicircular law is replaced by a density which extends over the whole energy axis. Furthermore, while all states are extended in the case of GOE matrices, we show numerically and analytically that two mobility edges appear, separating extended from localized states, with an intermediate ``mixed'' phase in between. The unusual nature of these localized states is discussed.

An excited state of Th 229 at 3.5 eV

Abstract: It has been known for many years that the first excited state of $^{229}\mathrm{Th}$ lies close to the ground state. Originally this energy was given as 0.1 keV; later, the authors reported a value of -1\ifmmode\pm\else\textpm\fi{}4 eV. In an attempt to improve the value for this level energy, we have remeasured the energies of a number of \ensuremath{\gamma} rays from $^{223}\mathrm{U}$ whose positions in the $^{229}\mathrm{Th}$ level scheme can be used to establish it. Compared with our earlier study, we have considered more \ensuremath{\gamma} rays in $^{229}\mathrm{Th}$, used more well-measured energy calibration and reference lines, used more detectors, used detectors with better low-energy resolution, more closely matched the counting rates in the \ensuremath{\gamma}-ray peaks whose relative energy is measured, and specifically considered certain systematic errors. More than 111 \ensuremath{\gamma}-ray spectra have been measured. From this large set of measurements we have deduced a value of 3.5\ifmmode\pm\else\textpm\fi{}1.0 eV for the energy of this level.

Optical spectroscopy and scintillation mechanisms of CexLa1-xF3.

Abstract: In this paper we present spectroscopic and scintillation studies of mixed cerium lanthanum trifluoride crystals ${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{F}}_{3}$. A scintillation mechanism is proposed in which the light output of the ${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{F}}_{3}$ scintillator is determined by three processes: a direct excitation of ${\mathrm{Ce}}^{3+}$ ions by secondary electrons and x rays, an ionization of ${\mathrm{Ce}}^{3+}$ ions followed by the capture of electrons and formation of Ce bound excitons and, eventually, a transfer of the energy from the electronic-lattice excitations to ${\mathrm{Ce}}^{3+}$ ions. These three processes occur in various degrees in all inorganic Ce scintillators, and the mixed (Ce,La) trifluorides provide, therefore, an excellent example of their relative importance. The peculiarity of fluorides is that ${\mathrm{Ce}}^{3+}$ ions occur in regular and ``perturbed'' sites. The lack of a fast energy migration between the Ce ions and, at the same time, an efficient energy transfer to ``perturbed'' Ce ions lead to nonexponential decays of the Ce emission. Thermal quenching is moderate and radiation trapping can be minimized, and there is no evidence of luminescence concentration quenching. The light output under \ensuremath{\gamma} excitation has a maximum value of about 4500 photons per MeV, which is significantly lower than the estimated conversion-limited value of about 25 000 photons per MeV. It is suggested that the stable ${\mathrm{Ce}}^{2+}$ provides electron traps, competing for electrons with holes localized on ${\mathrm{F}}_{2}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{Ce}}^{4+}$ ions. Therefore, mostly one process, namely the direct excitation of ${\mathrm{Ce}}^{3+}$ ions by secondary electrons and photons, contributes to the light output of ${\mathrm{CeF}}_{3}$. The deomonstrated feasibility of reducing perturbed Ce makes it a strong contender in those applications where high speed, not high light output, is of prime concern.

Design of filters to detect a noisy target in nonoverlapping background noise

Abstract: Two types of filter are proposed to detect a noisy target embedded in nonoverlapping background noise by optimization of two proposed criteria that are used in the assessment of filter design and performance. Criterion 1 is defined as the ratio of the square of the expected value of the correlation-peak amplitude to the expected value of the output-signal energy. Criterion 2 is defined as the ratio of the square of the expected value of the correlation-peak amplitude to the average output-signal variance. It is shown that, for the nonoverlapping target and scene noise models, the target window and the scene noise window affect the filter functions significantly. Computer-simulation tests of the generalized optimum filter for various kinds of noisy input image are provided to investigate filter performance in terms of peak-to-output-energy ratio, discrimination against undesired objects, and tolerance to target distortion (for example, target rotation and scaling). We compare the results with those of other filters to verify the performance of the optimum filters.

Energy-Momentum Complex in M\o ller's Tetrad Theory of Gravitation

Abstract: M\o ller's Tetrad Theory of Gravitation is examined with regard to the energy-momentum complex. The energy-momentum complex as well as the superpotential associated with M\o ller's theory are derived. M\o ller's field equations are solved in the case of spherical symmetry. Two different solutions, giving rise to the same metric, are obtained. The energy associated with one solution is found to be twice the energy associated with the other. Some suggestions to get out of this inconsistency are discussed at the end of the paper.

Type-II band alignment in Si/ Si 1 − x Ge x quantum wells from photoluminescence line shifts due to optically induced band-bending effects: Experiment and theory

Abstract: We compare measured energy shifts of photoluminescence lines in Si/${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ quantum wells as a function of excitation power with theoretical calculations to conclude that the band alignment of the heterointerface is type II. Experimentally, we study molecular-beam epitaxy-grown fully strained Si/${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ single quantum wells with Ge fractions from 10% to 36%. These show significant blueshifts of the luminescence at increasing excitation density. Theoretically, we calculate self-consistently transition energy changes taking into account band bending due to the photoinduced charge carriers. Only for type-II band alignment are the experimental and theoretical results compatible.

Generalized-gradient-approximation description of band splittings in transition-metal oxides and fluorides.

Abstract: The local-spin-density approximation yields a metallic ground state for the antiferromagnetic insulators CoO, FeO, ${\mathrm{FeF}}_{2}$, and ${\mathrm{CoF}}_{2}$ By using the generalized gradient approximation (GGA) we find the difluorides to be insulators, while in CoO the GGA splits the bands near ${\mathit{E}}_{\mathit{F}}$ and opens a direct gap, but a small indirect band overlap remains For FeO a significant improvement is found but not quite enough to make it an insulator The magnetic moments and gaps increase The GGA improves angular (and also in-out) correlations and thus affects the energy bands, a feature in addition to the total-energy improvement

Very High Energy Gamma Rays from PSR1706-44

Abstract: We have obtained evidence of gamma-ray emission above 1 TeV from PSR1706-44, using a ground-based telescope of the atmospheric \v{C}erenkov imaging type located near Woomera, South Australia. This object, a $\gamma$-ray source discovered by the COS B satellite (2CG342-02), was identified with the radio pulsar through the discovery of a 102 ms pulsed signal with the EGRET instrument of the Compton Gamma Ray Observatory. The flux of the present observation above a threshold of 1 TeV is $\bf \sim $ 1 $\cdot$ 10$^{-11}$ photons cm$^{-2}$ s$^{-1}$, which is two orders of magnitude smaller than the extrapolation from GeV energies. The analysis is not restricted to a search for emission modulated with the 102 ms period, and the reported flux is for all $\gamma$-rays from PSR1706-44, pulsed and unpulsed. The energy output in the TeV region corresponds to about 10$^{-3}$ of the spin down energy loss rate of the neutron star.

Spectroscopic ellipsometry study of the diluted magnetic semiconductor system Zn(Mn,Fe,Co)Se.

Abstract: Single-crystal films of (001)${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathit{A}}_{\mathit{x}}$Se (A=Mn, Fe, Co) (0\ensuremath{\le}x0.14) grown by molecular-beam epitaxy on (001)GaAs have been studied by spectroscopic ellipsometry in the 3.5--5.5 eV photon-energy range. Using fits of the ${\mathit{E}}_{1}$ and ${\mathit{E}}_{1}$+${\mathrm{\ensuremath{\Delta}}}_{1}$ peaks with a standard analytic expression, we find that the linewidths increase with x for all samples, the energies increase with x for ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Fe}}_{\mathit{x}}$Se and ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Co}}_{\mathit{x}}$Se, and the energies decrease with x for ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se. A model describing the effects of the sp-d exchange interaction on the L point band-gap energy is developed and applied. We find that the strength of the energy correction due to this interaction, which is proportional to the product of the square of the exchange integrals and the magnetic susceptibility of the material, is largest in Mn-doped and smallest in Co-doped ZnSe. While the sp-d exchange interaction model is consistent with the composition dependence of the ${\mathit{E}}_{1}$ and ${\mathit{E}}_{1}$+${\mathrm{\ensuremath{\Delta}}}_{1}$ band-gap energies in ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se, it does not describe the behavior observed in ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Fe}}_{\mathit{x}}$Se and ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Co}}_{\mathit{x}}$Se. We show that an sp-d hybridization model, which includes the location of the energy levels of the magnetic impurity d levels, can account for the composition dependence of ${\mathit{E}}_{1}$ and ${\mathit{E}}_{1}$+${\mathrm{\ensuremath{\Delta}}}_{1}$ band-gap energies of all three materials.

Shape of large single- and multiple-shell fullerenes

Abstract: The morphology of multiple-shell fullerenes is investigated by ab initio calculations using Yang's O(N) method. It is found that for large single-shell fullerenes with ${\mathit{I}}_{\mathit{h}}$ symmetry, the spherical morphology has lower energy than that of polyhedrons. The formation energy per atom follows a simple scaling law. Including an estimate of intershell van de Waals interactions leads to the conclusion that spherical multiple-shell fullerenes are likely the most stable structure of large carbon clusters. These results are in good agreement with recent experiments.

Reducing vortex motion in YBa2Cu3O7 crystals with splay in columnar defects.

Abstract: A strategy to boost the current-carrying capacity of cuprate superconductors beyond the levels attainable with parallel columnar defects was recently proposed. It consists of the enforcement of vortex entanglement by controlled splay of columns. We demonstrate the validity of this suggestion in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{2}$ single crystals using the difference in splay naturally occurring in irradiations with two ions differing in mass and energy. The terminal dispersion of the columns produced by 0.58-GeV $^{116}\mathrm{Sn}^{30+}$ is about 10\ifmmode^\circ\else\textdegree\fi{}, as compared with 1\ifmmode^\circ\else\textdegree\fi{} for 1.08-GeV $^{197}\mathrm{Au}^{23+}$. At high temperatures, this large splay results in a persistent current density one order of magnitude larger and a creep rate one order of magnitude smaller.

Dynamics of infrared-to-visible upconversion in Cs 3 Lu 2 Br 9 :1%Er 3+

Abstract: Single crystals of ${\mathrm{Cs}}_{3}$${\mathrm{Lu}}_{2}$${\mathrm{Br}}_{9}$:1%${\mathrm{Er}}^{3+}$ were grown using the Bridgman technique. The low phonon energy of \ensuremath{\le}190 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ strongly suppresses multiphonon relaxation in this host. For a series of excited states radiative relaxation is found to be dominant. As a consequence, ${\mathrm{Cs}}_{3}$${\mathrm{Lu}}_{2}$${\mathrm{Br}}_{9}$:1%${\mathrm{Er}}^{3+}$ shows strong visible upconversion luminescence from $^{2}$${\mathit{H}}_{9/2}$ and $^{4}$${\mathit{F}}_{7/2}$ when excited into the $^{4}$${\mathit{I}}_{9/2}$ and $^{4}$${\mathit{I}}_{11/2}$ multiplets in the near infrared, respectively. The upconversion dynamics for both excitations was studied in detail using time-resolved spectroscopy. Both excited-state absorption (ESA) and energy-transfer upconversion (ETU) were found to be active and could be identified on the basis of the upconversion transients. For the population of the $^{4}$${\mathit{F}}_{7/2}$ and $^{4}$${\mathit{S}}_{3/2}$ states both ESA and ETU can be observed simultaneously. The relative efficiency of ESA and ETU is determined by the energetic resonance of the respective processes. Whereas nonresonant ESA during the 10 ns laser pulse is strongly suppressed for energy mismatches g12 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, ETU is still efficient. An analysis of the upconversion transients shows that energy migration is dominant even at 1% ${\mathrm{Er}}^{3+}$ dotation. The excitations have excitonic character leading to a characteristic prolongation of the decays in the upconversion transients.

Monitoring installation for containers and trucks

Abstract: A monitoring installation for containers and trucks is provided making detection of large items, particularly trucks, at minimal expense possible. An X-ray monitoring installation having low X-ray energy, permits detection of stolen passenger cars, is arranged on a monitoring path.