Showing papers on "Energy (signal processing) published in 1990"

On a simple algorithm to calculate the 'energy' of a signal

Abstract: A simple algorithm is derived that permits on-the-fly calculation of the energy required to generate, in a certain sense, a signal. The results of applying this algorithm to a number of well-known signals are shown. Some of the invariance and noise properties of the algorithm are derived and verified by simulation. The implementation of the algorithm and its application to speech processing are briefly discussed. >

Properties of boson-exchange superconductors

Abstract: The author reviews some of the important successes achieved by Eliashberg theory in describing the observed superconducting properties of many conventional superconductors. Functional derivative techniques are found to help greatly in understanding the observed deviations from BCS laws. Approximate analytic formulas with simple correction factors for strong-coupling corrections embodied in the single parameter $\frac{{T}_{c}}{{\ensuremath{\omega}}_{\mathrm{ln}}}$ are also found to be very helpful. Here ${T}_{c}$ is the critical temperature and ${\ensuremath{\omega}}_{\mathrm{ln}}$ is an average boson energy mediating the pairing potential in Eliashberg theory. In view of the discovery of high-${T}_{c}$ superconductivity in the copper oxides, results in the very strong coupling limit of $\frac{{T}_{c}}{{\ensuremath{\omega}}_{\mathrm{ln}}}\ensuremath{\sim}1$ are also considered, as is the asymptotic limit when $\frac{{T}_{c}}{{\ensuremath{\omega}}_{\mathrm{ln}}}\ensuremath{\rightarrow}\ensuremath{\infty}$. This case is of theoretical interest only, but it is nevertheless important because simple analytic results apply that give insight into the more realistic strong-coupling regime. A discussion more specific to the oxides is included in which it is concluded that some high-energy boson-exchange mechanism must be operative, with, possibly, some important phonon contribution in some cases. A more definitive application of boson-exchange models to the oxides awaits better experimental results.

Evaluation of seismic energy in structures

Abstract: Research engineers use two types of energy equations to study single-degree-of-freedom (SDOF) systems subject to earthquake induced ground motions. The first method uses an absolute energy formulation; the second method uses a relative energy formulation. While the relative energy formulation has been used in the majority of previous investigations, this study shows that the absolute energy equation is physically more meaningful. For a given ductility ratio, the input energy demands calculated by both methods are significantly different for both the short and long period ranges although the results are similar in the intermediate period range. A comparison between the analytically predicted absolute input energy of a SDOF system with the experimentally measured input energy of a six-storey braced steel frame shows good correlation.

Dissipative flux motion in high-temperature superconductors

Abstract: The dissipation below ${\mathit{T}}_{\mathit{c}}$ has been studied for representatives of all classes of cuprate high-temperature superconductors, including ${\mathrm{Ba}}_{2}$${\mathrm{YCu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$, and Bi and Tl compounds. The results are parametrized in the framework of flux creep, with the largest activation energies found in ${\mathrm{Ba}}_{2}$${\mathrm{YCu}}_{3}$${\mathrm{O}}_{7}$. It is argued that the magnitude of dissipative flux motion is more related to the electronic anisotropy of the material than the actual defect structure. The thermally activated flux creep model, whose parameters are extracted from dc measurements, consistently describes also dynamic measurements, including the irreversibility line and the melting transition. Finally, the similarities in dissipative behavior are emphasized between high-${\mathit{T}}_{\mathit{c}}$ materials, very thin films, and layered low-${\mathit{T}}_{\mathit{c}}$ superconductors.

Interactions between Light Waves in a Nonlinear Dielectric

Abstract: The induced nonlinear electric dipole and higher moments in an atomic system, irradiated simultaneously by two or three light waves, are calculated by quantum-mechanical perturbation theory. Terms quadratic and cubic in the field amplitudes are included. An important permutation symmetry relation for the nonlinear polarizability is derived and its frequency dependence is discussed. The nonlinear microscopic properties are related to an effective macroscopic nonlinear polarization, which may be incorporated into Maxwell's equations for an infinite, homogeneous, anisotropic, nonlinear, dielectric medium. Energy and power relationships are derived for the nonlinear dielectric which correspond to the Manley-Rowe relations in the theory of parametric amplifiers. Explicit solutions are obtained for the coupled amplitude equations, which describe the interaction between a plane light wave and its second harmonic or the interaction between three plane electromagnetic waves, which satisfy the energy relationship ${\ensuremath{\omega}}_{3}={\ensuremath{\omega}}_{1}+{\ensuremath{\omega}}_{2}$, and the approximate momentum relationship ${\mathrm{k}}_{3}={\mathrm{k}}_{1}+{\mathrm{k}}_{2}+\ensuremath{\Delta}\mathrm{k}$. Third-harmonic generation and interaction between more waves is mentioned. Applications of the theory to the dc and microwave Kerr effect, light modulation, harmonic generation, and parametric conversion are discussed.

Relaxation Effects in Nuclear Magnetic Resonance Absorption

Abstract: The exchange of energy between a system of nuclear spins immersed in a strong magnetic field, and the heat reservoir consisting of the other degrees of freedom (the "lattice") of the substance containing the magnetic nuclei, serves to bring the spin system into equilibrium at a finite temperature. In this condition the system can absorb energy from an applied radiofrequency field. With the absorption of energy, however, the spin temperature tends to rise and the rate of absorption to decrease. Through this "saturation" effect, and in some cases by a more direct method, the spin-lattice relaxation time ${T}_{1}$ can be measured. The interaction among the magnetic nuclei, with which a characteristic time $T_{2}^{}{}_{}{}^{\ensuremath{'}}$ is associated, contributes to the width of the absorption line. Both interactions have been studied in a variety of substances, but with the emphasis on liquids containing hydrogen.Magnetic resonance absorption is observed by means of a radiofrequency bridge; the magnetic field at the sample is modulated at a low frequency. A detailed analysis of the method by which ${T}_{1}$ is derived from saturation experiments is given. Relaxation times observed range from ${10}^{\ensuremath{-}4}$ to ${10}^{2}$ seconds. In liquids ${T}_{1}$ ordinarily decreases with increasing viscosity, in some cases reaching a minimum value after which it increases with further increase in viscosity. The line width meanwhile increases monotonically from an extremely small value toward a value determined by the spin-spin interaction in the rigid lattice. The effect of paramagnetic ions in solution upon the proton relaxation time and line width has been investigated. The relaxation time and line width in ice have been measured at various temperatures.The results can be explained by a theory which takes into account the effect of the thermal motion of the magnetic nuclei upon the spin-spin interaction. The local magnetic field produced at one nucleus by neighboring magnetic nuclei, or even by electronic magnetic moments of paramagnetic ions, is spread out into a spectrum extending to frequencies of the order of $\frac{1}{{\ensuremath{\tau}}_{c}}$, where ${\ensuremath{\tau}}_{c}$ is a correlation time associated with the local Brownian motion and closely related to the characteristic time which occurs in Debye's theory of polar liquids. If the nuclear Larmor frequency $\ensuremath{\omega}$ is much less than $\frac{1}{{\ensuremath{\tau}}_{c}}$, the perturbations caused by the local field nearly average out, ${T}_{1}$ is inversely proportional to ${\ensuremath{\tau}}_{c}$, and the width of the resonance line, in frequency, is about $\frac{1}{{T}_{1}}$. A similar situation is found in hydrogen gas where ${\ensuremath{\tau}}_{c}$ is the time between collisions. In very viscous liquids and in some solids where $\ensuremath{\omega}{\ensuremath{\tau}}_{c}g1$, a quite different behavior is predicted, and observed. Values of ${\ensuremath{\tau}}_{c}$ for ice, inferred from nuclear relaxation measurements, correlate well with dielectric dispersion data.Formulas useful in estimating the detectability of magnetic resonance absorption in various cases are derived in the appendix.

Lattice dynamics and origin of ferroelectricity in BaTiO3: Linearized-augmented-plane-wave total-energy calculations.

Abstract: Phonon frequencies and eigenvectors have been computed from first principles for the three optic ${\mathit{F}}_{1\mathit{u}}$ modes in ${\mathrm{BaTiO}}_{3}$ using the full-potential linearized-augmented-plane-wave method. We find that the ferroelectric instability in ${\mathrm{BaTiO}}_{3}$ can be understood from calculations for a perfect crystal with periodic boundary conditions. The energy wells for the soft-mode distortion are deeper for rhombohedral [111] displacements relative to tetragonal [001] displacements, but they are relatively shallow and comparable to the transition temperature. The nonrigid part of the charge-density distortion is centered around the Ti ion rather than the O, and the Ti charge is closer to 2.9+ than 4+. There is significant hybridization between the Ti and O, but the Ba is quite ionic and is well described as a ${\mathrm{Ba}}^{2+}$ ion. The Ti-O hybridization is essential to the ferroelectric instability.

Electron states in a GaAs quantum dot in a magnetic field.

Abstract: Self-consistent numerical solutions of the Poisson and Schr\"odinger equations have been obtained for electron states in a GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As heterostructure with confinement in all three spatial dimensions. The equations are solved in the Hartree approximation, omitting exchange and correlation effects. Potential profiles, energy levels, and the charge in the quantum dot are obtained as functions of the applied gate voltage and magnetic field. First, the zero-magnetic-field case is considered, and the quantum-dot charge is allowed to vary continuously as the gate voltage is swept. Then, in connection with the phenomenon of Coulomb blockade, the number of electrons in the quantum dot is constrained to integer values. Finally, the calculation is extended to examine the evolution of levels in a magnetic field applied perpendicular to the heterojunction. Our results indicate that the confining potential has nearly circular symmetry despite the square geometry of the gate, that the energy levels are quite insensitive to the charge in the quantum dot at a fixed gate voltage, and that the evolution of levels with increasing magnetic field is similar to that found for a parabolic potential.

Exciton versus band description of the absorption and luminescence spectra in poly(p-phenylenevinylene).

Abstract: The fluorescence spectra of poly(p-phenylenevinylene) have been investigated at 6 K employing site-selective laser spectroscopy. They delineate the existence of a well-defined energy in the tail of the absorption edge below which the fluorescence emission is quasiresonant with an excitation featuring a Stokes shift of 100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ only. Above this localization threshold ${\ensuremath{ u}}_{\mathrm{loc}}$ spectral diffusion is observed with the emission independent of excitation yet carrying a significant polarization memory for excitation energies up to 1.9 eV in excess of ${\ensuremath{ u}}_{\mathrm{loc}}$. This is incompatible with the band picture involving photogeneration of an uncorrelated electron-hole pair, yet consistent with the concept of random walks of neutral excitations through an inhomogeneously broadened density of states. The chromophores are associated with a distribution of segments of the polymer chains along which the excitation is delocalized. Published results on time-resolved fluorescence support the exciton picture.

Thermally activated phase slippage in high-Tc grain-boundary Josephson junctions.

Abstract: The effect of thermally activated phase slippage (TAPS) in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ grain-boundary Josephson junctions has been studied. TAPS has been found to be responsible for the dc noise voltage superimposed on the dc Josephson current near the transition temperature. Because of the reduced Josephson coupling energy of the grain-boundary junctions, which is caused by a reduced superconducting order parameter at the grain-boundary interface, TAPS is present over a considerable temperature range. The implications of TAPS on the applicability of high-${\mathrm{T}}_{\mathrm{c}}$ Josephson junctions are outlined.

Cubic ZnS under pressure: Optical-absorption edge, phase transition, and calculated equation of state

Abstract: We have measured the effect of pressure on the energy of the direct-optical-absorption edge (${\mathit{E}}_{0}$ gap, ${\mathrm{\ensuremath{\Gamma}}}_{15}^{\mathit{v}}$\ensuremath{\rightarrow}${\mathrm{\ensuremath{\Gamma}}}_{1}^{\mathit{c}}$) of cubic ZnS, covering the full stability range (0\char21{}15 GPa) of the tetrahedral phase. The ${\mathit{E}}_{0}$ gap exhibits a sublinear increase under pressure, with the corresponding (linear) gap deformation potential being -5.0(2) eV. X-ray diffraction shows the high-pressure phase of ZnS to have the NaCl-type structure for pressures up to at least 27 GPa. In contrast to earlier reports, this phase is not metallic but shows a broad optical-absorption edge with onset near 2 eV, which is characteristic of an indirect-gap material. Experimental results for the equation of state and band-gap deformation potential are compared with ab initio calculations based on local-density theory and the relativistic linear-muffin-tin-orbital method.

Digital speech coder having optimized signal energy parameters

Abstract: A speech coder and decoder methodology wherein pitch excitation and codebook excitation source energies (100) are represented by parameters that are readily transmissible with minimal transmission capacity requirements. The parameters are the long term energy value, a short term correction factor which is applied to the long term energy value to match the short term energy, and proportionality factor(s) that specify the relative energy contribution of the excitation sources to the short term energy value (101).

Solution of the solar-neutrino problem

Abstract: Comparison of the results from the Kamiokande neutrino-electron scattering experiment with those from the chlorine experiment and with solar models shows that the explanation of the solar-neutrino problem probably requires physics beyond the standard electroweak model with zero neutrino masses. The experimental results, including the shape of the electron-recoil energy spectrum measured by Kamiokande, are in excellent agreement with a nonadiabatic solution of the Mikheyev-Smirnov-Wulfenstein effect, yielding a neutrino mass difference of \ensuremath{\Delta}${\mathit{m}}^{2}$=1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}8}$ ${\mathrm{sin}}^{\mathrm{\ensuremath{-}}2}$${\mathrm{\ensuremath{\Theta}}}_{\mathit{V}}$ ${\mathrm{eV}}^{2}$.

Analytic representation of multi-ion interatomic potentials in transition metals.

Abstract: The first-principles, density-functional version of the generalized pseudopotential theory (GPT), previously developed for empty- and filled-d-band metals, recently has been extended to pure transition metals with partially filled d bands [Phys. Rev. B 38, 3199 (1988)]. Within this formalism, a rigorous real-space expansion of the bulk total energy has been obtained in terms of widely transferable, structure-independent interatomic potentials, including both central-force pair interactions and angular-force triplet and quadruplet interactions. In the central transition metals, the three- and four-ion potentials, ${\mathit{v}}_{3}$ and ${\mathit{v}}_{4}$, are essential to a proper description of materials properties, but are necessarily multidimensional functions which cannot be easily tabulated for application purposes.We develop here a simplified version of the theory, the model GPT, in which these potentials can be expressed analytically while retaining the most important physics of the full first-principles treatment. The analytic treatment of ${\mathit{v}}_{3}$ and ${\mathit{v}}_{4}$ is made possible because of three simplifying features in the central transition metals. First, due to the nonspherical nature of the Fermi surface in such metals, the long-range sp-d hybridization tails of the first-principles potentials destructively interfere in total-energy calculations and thus can be dropped at the outset without major consequences. Second, the direct d-d contributions to the potentials are short ranged and need only be retained to fourth order in interatomic d-state matrix elements to obtain a good representation of the d-band-structure energy. Third, the d bands are canonical in nature, with the interatomic matrix elements well approximated by simple forms, so that all remaining low-order d-state contributions can be evaluated analytically.This leads to a description of ${\mathit{v}}_{3}$ and ${\mathit{v}}_{4}$ in terms of universal short-range radial and angular functions. The model GPT is made quantitatively accurate for real materials by allowing the coefficient of each d-state contribution to be adjusted to match first-principles calculations and/or experimental data. In this manner, one can achieve a set of potentials which simultaneously yield a good description of cohesion, vacancy formation, structural phase stability, elastic constants, and phonons, as is demonstrated for the representative case of molybdenum. More generally, the analytic potentials are suitable for widescale applications and permit for the first time the use of the transition-metal GPT in molecular-dynamics and Monte Carlo simulations.

Equation of state of the one-component plasma derived from precision Monte Carlo calculations

Abstract: Analytical fits to the one-component plasma (OCP) equation of state have been derived for internal energies that have reduced all {ital N}-dependent effects to within the statistical uncertainties introduced by the Monte Carlo computational process, which themselves are very small. Values of {ital N}{approx gt}500 adequately represent the thermodynamic limit. Using the fluid internal energies for only {ital N}=686, various analytical fits are generated, compared, and discussed. The thermal energy is accurately represented by a simple power-series fit with the leading term given by {Gamma}{sup 1/3}, but also requires a small correction to the bcc Madelung term that brings that coefficient down to nearly {minus}0.9, the value derived for hypernetted-chain theory. The fluid thermal energy data are reproduced to better than 0.2% over all {Gamma} by our fit(s). The solid phase requires {ital both} anharmonic terms to be included in the fit, implying that the previous justification for dropping the first anharmonic correction is unwarranted. The location of the fluid-solid phase transition utilizing these new fits yields {Gamma}{sub bcc}=178 and {Gamma}{sub fcc}=192.

Infrared studies of the superconducting energy gap and normal-state dynamics of the high-Tc superconductor YBa2Cu3O7.

Abstract: A detailed study of infrared properties (reflectivity, conductivity, and dielectric response), emphasizing reproducible results from fully oxygenated ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ crystals (${\mathit{T}}_{\mathit{c}}$\ensuremath{\simeq}93 K) and films, is presented. The extrapolated values of ${\mathrm{\ensuremath{\sigma}}}_{1}$(\ensuremath{\omega}) at low frequency are roughly consistent with the measured temperature-dependent dc resistivity. Although not well understood, this infrared conductivity can be interpreted in terms of a frequency-dependent scattering rate of \ensuremath{\sim}kT+\ensuremath{\Elzxh}\ensuremath{\omega}, with a low-frequency mass enhancement of roughly 2 to 4 associated with a carrier-spin related interaction. Infrared measurements polarized along the c axis suggest a conductivity anisotropy of roughly 40:1 near ${\mathit{T}}_{\mathit{c}}$ in the normal state. In the superconducting state an energy scale of 2${\mathrm{\ensuremath{\Delta}}}_{\mathit{c}}$\ensuremath{\simeq}3${\mathit{kT}}_{\mathit{c}}$ is suggested by c-axis polarized measurements, while a much larger characteristic energy of 2${\mathrm{\ensuremath{\Delta}}}_{\mathit{a}\mathrm{\ensuremath{-}}\mathit{b}}$\ensuremath{\simeq}8${\mathit{kT}}_{\mathit{c}}$ is evident in the (a-b)-plane conductivity. From the area missing from the conductivity up to this very large gap, a reasonable estimate (\ensuremath{\simeq}1700 \AA{}) for the (a-b)-plane penetration depth is obtained. Evidence for non-BCS temperature dependence, strong pair breaking scattering, and possible fluctuation effects is discussed. A comparison to infrared data from ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8\mathrm{\ensuremath{-}}\mathit{y}}$ shows a similarly large energy scale, 2${\mathrm{\ensuremath{\Delta}}}_{\mathit{a}\mathrm{\ensuremath{-}}\mathit{b}}$\ensuremath{\simeq}8${\mathit{kT}}_{\mathit{c}}$; for the cubic ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_{3}$ superconductor a more conventional energy scale, 2\ensuremath{\Delta}\ensuremath{\simeq}4${\mathit{kT}}_{\mathit{c}}$ is observed. The unusually large energy scale obtained from the (a-b)-plane measurements of the layered cuprates lies far beyond the range of previously studied superconducting energy gaps (2\ensuremath{\Delta}\ensuremath{\simeq}3 to 5${\mathit{kT}}_{\mathit{c}}$).

Influence of substrate composition and crystallographic orientation on the band structure of pseudomorphic Si-Ge alloy films.

Abstract: An analysis of the pseudomorphic ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ge}}_{\mathrm{x}}$ band-structure variation with substrate composition and crystallographic orientation is reported. A method is presented for determining all six independent elements of the strain tensor in a strained epitaxial film grown on a substrate of arbitrary orientation. The substrate orientation is found to be an important factor in determining the band-structure properties of the epitaxial film. The strain-dependent band-structure properties investigated are the following: (1) The conduction band ${\mathrm{\ensuremath{\Gamma}}}_{2}^{\ensuremath{'}}$, ${\mathrm{\ensuremath{\Delta}}}_{1}$, and ${\mathrm{L}}_{1}$ valleys' shifts and degeneracy splittings, (2) the k=0 valence-band energy levels' shifts and degeneracy splittings, (3) the valence-band-state mixing, (4) the variation in the conduction- and valence-band-edge effective densities of state, (5) the variation in the intrinsic Fermi energy, and (6) the variation of the intrinsic-carrier concentration. It is shown that many aspects of the band structure\char22{}including the band gap, the density of states, and the position of the ${\mathrm{\ensuremath{\Delta}}}_{1}$-${\mathrm{L}}_{1}$ conduction-band-edge crossover\char22{}are each controllable through proper selection of film and substrate composition and crystallographic orientation.

The equivalence theorem.

Abstract: The equivalence theorem states that, at an energy $E$ much larger than the vector-boson mass $M$, the leading order of the amplitude with longitudinally polarized vector bosons on mass shell is given by the amplitude in which these vector bosons are replaced by the corresponding Higgs ghosts. We prove the equivalence theorem and show its validity in every order in perturbation theory. We first derive the renormalized Ward identities by using the diagrammatic method. Only the Feynman-'t Hooft gauge is discussed. The last step of the proof includes the power-counting method evaluated in the large-Higgs-boson-mass limit, needed to estimate the leading energy behavior of the amplitudes involved. We derive expressions for the amplitudes involving longitudinally polarized vector bosons for all orders in perturbation theory. The fermion mass has not been neglected and everything is evaluated in the region ${m}_{f}\ensuremath{\approx}MllEll{m}_{\mathrm{Higgs}}$.

Extending the collocation method to multidimensional molecular dynamics: direct determination of the intermolecular potential of argon-water from tunable far-infrared laser spectroscopy

Abstract: The authors present an extension of the collocation method developed by Peet and Yang for calculating the bound states of rotating atom-diatom systems to atom-polyatom complexes. The method is shown to be general, accurate, efficient, and straightforward to implement. The collocation algorithm is incorporated into a nonlinear least-squares program, which is used in a direct fit of far-infrared vibration-rotation spectra of the Ar-H{sub 2}O complex to a detailed analytical model for the anisotropic intermolecular potential energy surface. The surface denoted AW1 was obtained without any dynamical approximations. The minimum (D{sub e} = 174.7 cm{sup {minus}1}, R{sub e} = 3.598 {angstrom}) in the intermolecular potential surface occurs for the argon located in the plane of the H{sub 2}O, nearly perpendicular to the symmetry axis.

Landau-Ginzburg model of interphase boundaries in improper ferroelastic Perovskites of D4h18 symmetry.

Abstract: The Landau model of Slonczewski and Thomas (1970) for the improper ferroelastic ${\mathit{O}}_{\mathit{h}}^{1}$-${\mathit{D}}_{4\mathit{h}}^{18}$ phase transition in perovskite-structure compounds has been extended by including spatial gradient terms of the three-component primary order parameter (OP) and applied to calculate the OP profile and the strain distribution for antiphase and for twin boundaries in the tetragonal phase. In order to obtain quasi-one-dimensional kink-type solitary-wave solutions for which the OP and the strain depend only on the coordinate normal to the interface plane, lateral surface forces are required which allow for the shape change associated with ferroelastic interfaces, but which prevent expansion or contraction within the boundary plane. Numerical application to ${\mathrm{SrTiO}}_{3}$, including calculation of the thickness and energy of both types of interphase boundaries versus temperature, is also presented.

A frequency-domain procedure for accurate real-time signal parameter measurement

Abstract: A method for real-time measurement of multifrequency waveforms based on the evaluation of certain energy parameters is discussed. Very low processing effort is achieved by means of a frequency-domain approach, whereas amplitude, frequency, and phase of each spectral component are obtained with high accuracy. Unlike other frequency-domain methods, the proposed procedure provides accurate results even with a small number of analyzed samples and with measurements taken over a short time interval; therefore, real-time tracking of time-varying signals can be achieved. >

Optical studies of gap, exchange, and hopping energies in the insulating cuprates

Abstract: We have measured the insulating energy gap \ensuremath{\Delta} and the exchange interaction J in a series of cuprate crystals, including T'-phase ${\mathit{M}}_{2}$${\mathrm{CuO}}_{4}$ (M=Pr, Nd, Sm, Eu, and Gd), ${\mathit{T}}^{\mathrm{*}}$-phase La,Tb,${\mathrm{Sr}}_{2}$${\mathrm{CuO}}_{4}$, and T-phase ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$. We find that the energy gap scales predominantly with the in-plane Cu-O distance, scaling as \ensuremath{\delta} log\ensuremath{\Delta}/ \ensuremath{\delta} logd\ensuremath{\sim}-6. Furthermore, contrary to simple expectations, the energy gap increases with decreasing Cu-O distance, suggesting that Coulomb and other repulsive energies dominate the effects of band hybridization. Using a three-band Hubbard-model expression, our studies of \ensuremath{\Delta} and J in the cuprates allow us to estimate that the hopping energy t scales with Cu-O distance as \ensuremath{\delta} logt/\ensuremath{\delta} logd\ensuremath{\sim}-4.

Vibrationally resolved core-level photoelectron spectroscopy: Si 2p levels of SiH4 and SiF4 molecules.

Abstract: High-resolution photoelectron spectra (total instrumental width \ensuremath{\le}0.1 eV) of the Si 2p core levels of ${\mathrm{SiH}}_{4}$ and ${\mathrm{SiF}}_{4}$ were obtained using the Aladdin undulator source. The vibrationally resolved Si 2p photoelectron spectrum of ${\mathrm{SiH}}_{4}$ yields a Si-H stretching energy of 0.295\ifmmode\pm\else\textpm\fi{}0.002 eV for the core-hole ion and a lifetime width of \ensuremath{\sim}45 meV. Numerous vibrational lines are observed in the ${\mathrm{SiF}}_{4}$ Si 2p photoelectron spectrum. The Z+1 core-equivalent model is assessed and changes in the molecular geometries and bonding properties of the molecules upon ionization are discussed.

Energy gaps measured by scanning tunneling microscopy

Abstract: A scanning tunneling microscope (STM) has been used to measure energy gaps in the charge-density-wave (CDW) phases of the layer-structure dichalcogenides and in the high-temperature superconductor ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$. Measured values of ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{CDW}}$ at 4.2 K for 2H-${\mathrm{TaSe}}_{2}$, 2H-${\mathrm{TaS}}_{2}$, and 2H-${\mathrm{NbSe}}_{2}$ are 80, 50, and 34 meV giving values of 2${\mathrm{\ensuremath{\Delta}}}_{\mathrm{CDW}}$/${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{c}}$ equal to 15.2, 15.4, and 23.9, indicating strong coupling in these CDW systems. Measured values of ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{CDW}}$ at 4.2 K in 1T-${\mathrm{TaSe}}_{2}$ and 1T-${\mathrm{TaS}}_{2}$ are \ensuremath{\sim}150 meV for both materials giving 2${\mathrm{\ensuremath{\Delta}}}_{\mathrm{CDW}}$/${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{c}}$\ensuremath{\approxeq}5.8. STM scans of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ at 4.2 K resolve atoms on the ${\mathrm{BiO}}_{\mathit{x}}$ layer and show possible variations in electronic structure. The energy gap determined from I versus V and dI/dV versus V curves is in the range 30--35 meV giving values of 2\ensuremath{\Delta}/${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{c}}$\ensuremath{\approxeq}8. Spectroscopy measurements with the STM can exhibit large zero-bias anomalies which complicate the analysis of the energy-gap structure, but adequate separation has been accomplished.

Picosecond stimulated photon echo due to intrinsic excitations in semiconductor mixed crystals.

Abstract: Time-resolved demonstration of picosecond photon echoes due to intrinsic optical excitations of a semiconductor is presented. Phase relaxation times up to several hundred picoseconds are found for excitons localized by chemical disorder in ${\mathrm{CdS}}_{\mathrm{x}}$${\mathrm{Se}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ mixed crystals. These long dephasing times are attributed to reduced scattering of the localized excitons as compared to free excitons. Dephasing is identified as due to energy relaxation of the excitons.

Ultrasonic level detector

Abstract: An ultrasonic level detector includes a transducer for generating and receiving bursts of sonic energy at a surface to locate the position of the surface. In response to receiving a burst of energy reflected from the surface, the transducer generates an electrical signal, which is then supplied to a variable gain amplifier. After amplification, the electrical signal is supplied to a comparator and a peak detector. The comparator generates a timing signal upon the electrical signal exceeding a threshold. A window generator circuit generates a receive window that controls whether the electrical signal is supplied to the comparator and the magnitude of the threshold on the comparator. The peak detector determines the maximum amplitude of the electrical signal, which may be used to vary the amplifier gain and the number of pulses included in an excitation signal which drives the transducer. Upon receipt of the timing signal, a microprocessor determines the distance between the transducer and the detected surface.

Rayleigh-Taylor and Richtmyer-Meshkov instabilities in multilayer fluids with surface tension

Abstract: Surface tension modifies the evolution of the Rayleigh-Taylor and the Richtmyer-Meshkov instabilities in fluids undergoing a constant acceleration or a shock, respectively. We analyze the general case of N fluids with arbitrary densities and surface tensions and derive the eigenvalue equation determining the growth rate of the perturbations. For N=2 we recover the classical case of two semi-infinite fluids and extend it to the case of two finite-thickness fluids between fixed boundaries. The N=3 case is studied in detail; we find universal modes that are independent of the thickness of the intermediate fluid, and we find how surface tension modifies Taylor's modes for a single fluid with free boundaries. We also analyze in detail recent and future two- or three-fluid experiments. Representing a shock as an impulsive acceleration we find that post-shock oscillations have frequencies and amplitudes that depend on the wave number k, leading to a nontrivial evolution for the spectrum of perturbations. Finally, we study turbulence at the interface between two fluids with surface tension and present specific predictions for the turbulent energy ${\mathit{E}}_{\mathrm{turbulent}}$ as function of the surface tension ${\mathit{T}}^{(\mathit{s})}$. We propose new experiments, physical and/or numerical, to test our predictions.

An analysis of Kohonen's self-organizing maps using a system of energy functions

Abstract: In this paper a new method for analyzing Kohonen's self-organizing feature maps is presented. The method makes use of a system of energy functions, one energy function for each processing unit. It is shown that the training process is equivalent to minimizing each energy function subject to constraints. The analysis is used to prove the formation of topologically correct maps when the inherent dimensionality of the input patterns matches that of the network. The energy equations can be used to compute the steady-state weight values of the network. In addition, the analysis allows bounds on the training parameters to be determined. Finally, examples of energy landscapes are presented to graphically show the behavior of the network.

The dependence of MHD turbulence spectra on the inner solar wind stream structure near solar minimum

Abstract: The variation of the power spectra (e{sup {plus minus}}) of the fluctuating Elsaesser variables {delta}Z{sup {plus minus}} with the solar wind velocity has been examined by either individual case study or statistical analysis with Helios data obtained near solar minimum. It is found, when going from high-speed to low-speed wind, that: (1) The slopes of both energy spectra systematically increase toward the value given by Kolmogorov's law. (2) The power density of {delta}Z{sup +} in the Alfvenic regime above f {approx} 2 {times} 10{sup {minus}4} Hz strongly decreases while the power density of {delta}Z{sup {minus}} does not change a lot. (3) A somewhat higher level of e{sup {minus}} is found in the leading edges of high-speed streams near the regions of large velocity shear. The authors also found that density fluctuations might make some contribution to e{sup {minus}} in the low-frequency domain. The relevance of these findings with respect to the evolution of the solar wind MHD turbulence is discussed.

Interaction of longitudinal-optic phonons with free holes as evidenced in Raman spectra from Be-doped p-type GaAs

Abstract: The interaction between small-wave-vector, longitudinal-optic (LO) lattice vibrations and free-hole plasmas in Be-doped p-type GaAs is studied with use of nonresonant, allowed Raman scattering. In contrast to the coupled LO-phonon--plasmon modes, ${\mathrm{\ensuremath{\omega}}}_{+}$ and ${\mathrm{\ensuremath{\omega}}}_{\mathrm{\ensuremath{-}}}$ typically observed in n-type zinc-blende-structure semiconductors, only one mode is observed for hole densities between 1\ifmmode\times\else\texttimes\fi{}${10}^{18}$ and 1.6\ifmmode\times\else\texttimes\fi{}${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. With increasing hole density, this single mode shifts first to higher energies, then back to lower energies, between that of the LO and transverse-optic (TO) phonons, and finally asymptotes at the TO-phonon energy. The observed Raman spectra are accurately fitted with calculated coupled-mode spectra which take into account wave-vector-dependent intra- and inter-valence-band transitions within the heavy- and light-hole bands. Intra-light-hole and inter-heavy- to light-hole transitions are shown to make very significant contributions to the spectra, although they alone cannot account for the novel density dependence of the coupled-mode energy. A detailed analysis of the coupled-mode dependence on wave vector and phenomenological damping reveals that the observed density dependence can, in principle, occur even in a single-component plasma due to two distinctly different physical mechanisms. In the case of the p-type GaAs studied here, it is shown that the novel density dependence is primarily due to the overdamped nature of intra-heavy-hole transitions.