Showing papers on "Energy (signal processing) published in 1996"

Mass estimates of X-ray clusters

Abstract: We use cosmological gasdynamic simulations to investigate the accuracy of galaxy cluster mass estimates based on X-ray observations. The experiments follow the formation of clusters in different cosmological models and include the effects of gravity, pressure gradients, and hydrodynamical shocks. A subset of our ensemble also allows for feedback of mass and energy from galactic winds into the intracluster medium. We find that mass estimates based on the hydrostatic, isothermal {beta}-model are remarkably accurate when evaluated at radii where the cluster mean density is between 500 and 2500 times the critical density. At lower densities, radial temperature information becomes important. In the quoted radial regime, the distribution of the estimated-to-true mass ratio, derived from 174 artificial images constructed from the simulations, is nearly unbiased and has a standard deviation of 14{percent}{endash}29{percent}. The scatter can be considerably reduced (to 8{percent}{endash}15{percent}) by using an alternative mass estimator that exploits the tightness of the mass-temperature relation found in the simulations. The improvement over {beta}-model estimates is due to the elimination of the variance contributed by the gas outer slope parameter. We discuss these findings and their implications for recent measurements of cluster baryon fractions. {copyright} {ital 1996 The American Astronomical Society.}

Gravitational energy in spherical symmetry.

Abstract: Various properties of the Misner-Sharp spherically symmetric gravitational energy E are established or reviewed. In the Newtonian limit of a perfect fluid, E yields the Newtonian mass to leading order and the Newtonian kinetic and potential energy to the next order. For test particles, the corresponding H\'aj\'{\i}\ifmmode \check{c}\else \v{c}\fi{}ek energy is conserved and has the behavior appropriate to energy in the Newtonian and special-relativistic limits. In the small-sphere limit, the leading term in E is the product of volume and the energy density of the matter. In vacuo, E reduces to the Schwarzschild energy. At null and spatial infinity, E reduces to the Bondi-Sachs and Arnowitt-Deser-Misner energies, respectively. The conserved Kodama current has charge E. A sphere is trapped if Eg1/2r, marginal if E=1/2r, and untrapped if E1/2r, where r is the areal radius. A central singularity is spatial and trapped if Eg0, and temporal and untrapped if E0. On an untrapped sphere, E is nondecreasing in any outgoing spatial or null direction, assuming the dominant energy condition. It follows that E\ensuremath{\ge}0 on an untrapped spatial hypersurface with a regular center, and E\ensuremath{\ge}1/2${\mathit{r}}_{0}$ on an untrapped spatial hypersurface bounded at the inward end by a marginal sphere of radius ${\mathit{r}}_{0}$. All these inequalities extend to the asymptotic energies, recovering the Bondi-Sachs energy loss and the positivity of the asymptotic energies, as well as proving the conjectured Penrose inequality for black or white holes. Implications for the cosmic censorship hypothesis and for general definitions of gravitational energy are discussed. \textcopyright{} 1996 The American Physical Society.

Observation of Spin-Charge Separation in One-Dimensional SrCuO2.

Abstract: Angle-resolved photoemission data from one-dimensional SrCu${\mathrm{O}}_{2}$ compounds are found to be qualitatively different from that of two-dimensional S${\mathrm{r}}_{2}$Cu${\mathrm{O}}_{2}$C${\mathrm{l}}_{2}$. The data can be quantitatively accounted for by the exact diagonalization calculation based on the t-J model. We identify the two underlying bands having approximately 1.2 and 0.3 eV energy dispersion as that of holon and spinon, with their energy scaled by t and J, respectively.

Rapid carrier relaxation in self-assembled inxga1-xas/gaas quantum dots

Abstract: Carrier capture and relaxation processes in self-assembled 15-nm ${\mathrm{In}}_{0.5}$${\mathrm{Ga}}_{0.5}$As/GaAs quantum dots are investigated by means of time-resolved photoluminescence spectroscopy. In a systematic study of photoluminescence rise times and barrier decay times (variation of temperature, excitation energy, and excitation density) we aim to identify the physical mechanisms responsible for fast carrier capture and relaxation in quantum dots. Both processes are separated by using appropriate excitation energies. Carrier capture and relaxation are shown to proceed with rates as high as \ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}${10}^{10}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ at low temperature even if less than one electron-hole pair per dot and excitation pulse is created. We interpret our results in terms of multiphonon processes at low excitation densities and in terms of Auger processes at high excitation densities. \textcopyright{} 1996 The American Physical Society.

Undulations and Dynamic Structure Factor of Membranes.

Abstract: We investigate the dynamic structure factor $S(q,t)$ of membrane phases at large wave numbers $q$ which are sensitive to single membrane dynamics Considering an ensemble of membrane plaquettes at random orientations, the effect of membrane thermal undulations is calculated Their statistics is modeled by bending energy and standard hydrodynamic dispersion law We predict a stretched exponential relaxation $S(q,t)\ensuremath{\sim}{e}^{\ensuremath{-}({\ensuremath{\gamma}}_{q}t{)}^{2/3}}$ with a relaxation rate ${\ensuremath{\gamma}}_{q}\ensuremath{\propto}{\ensuremath{\kappa}}^{\ensuremath{-}1/2}{q}^{3}$ where $\ensuremath{\kappa}$ is the bending modulus Our results are in good agreement with recent dynamic light scattering spectra from dilute sponge phases obtained by Freyssingeas, Roux, and Nallet [(unpublished)]

Experimental test of the quantum shot noise reduction theory.

Abstract: The quantum suppression of shot noise predicted for mesoscopic conductors is observed using absolute measurements of the equilibrium and nonequilibrium electrical fluctuations of a quantum point contact. The small energy ( $30\char21{}600\mathrm{mK}$) and low frequency ( $1\char21{}10\mathrm{kHz}$) used for measurements allow for a reliable quantitative experimental confirmation of the quantum noise theory. The noise reduction factor is found to be in excellent agreement with theoretical expectations, evolving from nearly unity at low electronic wave transmission to nearly zero on a conductance plateau.

On the analytic signal, the Teager-Kaiser energy algorithm, and other methods for defining amplitude and frequency

Abstract: This paper compares the Teager-Kaiser algorithm (TKA) and other similar local methods to the analytic signal (AS) procedure. The general concepts of the instantaneous amplitude and frequency are discussed. It is shown that only AS meets certain physical conditions for the amplitude, phase, and frequency (APF). The advantage of accuracy and simplicity of the AS is also demonstrated.

Nonlinear saturation of the turbulent alpha effect.

Abstract: We study the saturation of the turbulent $\ensuremath{\alpha}$ effect in the nonlinear regime. A numerical experiment is constructed based on the full nonlinear magnetohydrodynamics equations that allows the $\ensuremath{\alpha}$ effect to be measured for different values of the mean magnetic field. The object is to distinguish between two possible theories of nonlinear saturation. It is found that the results are in close agreement with the theories that predict strong suppression and are incompatible with those that predict that the turbulent $\ensuremath{\alpha}$ effect persists up to mean fields of order of the equipartition energy.

Dimer reconstruction and electronic surface states on clean and hydrogenated diamond (100) surfaces

Abstract: We present ab initio investigations of the structural and electronic properties of clean and hydrogen-covered diamond (100) surfaces within local-density-functional theory. Our calculations are based on a variational solution of the Kohn-Sham equations using a preconditioned conjugate-gradient approach and on the optimization of the atomic structure via a quasi-Newton quench based on the exact Hellmann-Feynman forces. The computations are performed in a plane-wave basis, the electron-ion interaction is described by optimized ultrasoft pseudopotentials. We find that the clean and the monolayer-covered surfaces reconstruct in a (2\ifmmode\times\else\texttimes\fi{}1) cell via the formation of rows of symmetric \ensuremath{\pi}-bonded dimers. Further hydrogenation to a coverage of 1.5 ML stabilizes a surface with a (1\ifmmode\times\else\texttimes\fi{}1) periodicity in the C layers, albeit with a low H-desorption energy for the formation of the reconstructed monohydride surface. The two-step desorption process is in good agreement with experimental observations. Electronic surface states within the bulk gap are predicted for the clean surface, but not for the monohydride case. The detailed analysis of the layer-resolved local densities of states and of the dispersion of the surface states demonstrates that the results are in good agreement with recent photoemission experiments. A negative electron affinity is predicted for the monohydride surface, but not for the clean surface. \textcopyright{} 1996 The American Physical Society.

Local density of states in a dirty normal metal connected to a superconductor

Abstract: A superconductor in contact with a normal metal not only induces superconducting correlations, known as the proximity effect, but also modifies the density of states at some distance from the interface. These modifications can be resolved experimentally in microstructured systems. We therefore study the local density of states N(E,x) of a superconductor--normal-metal heterostructure. We find a suppression of N(E,x) at small energies, which persists to large distances. If the normal metal forms a thin layer of thickness ${\mathit{L}}_{\mathit{n}}$, a minigap in the density of states appears which is of the order of the Thouless energy \ensuremath{\sim}\ensuremath{\Elzxh}D/${\mathit{L}}_{\mathit{n}}^{2}$. A magnetic field suppresses the features. We find good agreement with recent experiments of Gu\'eron et al. \textcopyright{} 1996 The American Physical Society.

Decaying vacuum energy and deflationary cosmology in open and closed universes

Abstract: We consider a nonsingular deflationary cosmological model with a decaying vacuum energy density in universes of arbitrary spatial curvature. Irrespective of the value of k, the models are characterized by an arbitrary time scale ${\mathit{H}}_{\mathit{I}}^{\mathrm{\ensuremath{-}}1}$ which determines the initial temperature of the universe and the largest value of the vacuum energy density, the slow decay of which generates all the presently observed matter energy of the Universe. If ${\mathit{H}}_{\mathit{I}}^{\mathrm{\ensuremath{-}}1}$ is of the order of the Planck time, the models begin with the Planck temperature and the present day value of the cosmological constant satisfies ${\mathrm{\ensuremath{\Lambda}}}_{\mathit{I}}$/${\mathrm{\ensuremath{\Lambda}}}_{0}$\ensuremath{\simeq}${10}^{118}$ as theoretically suggested. It is also shown that all models allow a density parameter ${\mathrm{\ensuremath{\Omega}}}_{0}$2/3 and that the age of the Universe is large enough to agree with observations even with the high value of ${\mathit{H}}_{0}$ suggested by recent measurements. \textcopyright{} 1996 The American Physical Society.

Conclusive evidence for the influence of nuclear orientation on quasifission.

Abstract: Fission fragment anisotropies and mass distributions have been measured to high accuracy, over a wide range of angles, for the $^{16}\mathrm{O}$ + $^{238}\mathrm{U}$ reaction. The bombarding energies spanned the fusion barrier distribution, in steps of 1 MeV. Fission following transfer reactions was rejected by making use of the deduced velocity vectors of the fissioning nuclei. The resulting mass distributions for full momentum transfer fission show a small but significant skewness, which increases as the beam energy falls through the fusion barrier region, displaying a similar energy dependence as the fission fragment angular anisotropies. This is conclusive evidence for the interpretation that collisions with the tips of the deformed $^{238}\mathrm{U}$ target nuclei lead to quasifission, while collisions with the sides result in fusion-fission. \textcopyright{} 1996 The American Physical Society.

Two-electron quantum disks.

Abstract: The energy levels of a quantum disk containing one or two electrons are calculated as a function of an external magnetic field. The confinement potential is a hard wall of finite height. The cyclotron transition energies are investigated and the effect of the finite width of the disk on the Coulomb energy is studied. Our results are applied to the ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs dots and agree well with experiment. \textcopyright{} 1996 The American Physical Society.

Transcutaneous energy and information transmission apparatus

Abstract: An apparatus for transcutaneously transmitting power and communication signals to an implantable device. The apparatus can include generators for power and information signals; receivers; and a coupler for independently coupling the power signal and a first information signal. An internal unit can have receivers for power and information signals; a second signal generator; and an independent coupler. The first and second information signal can be transmitted at a frequency greater than the power frequency. The external unit can also include a signal conditioner for symmetrically transceiving the first and second information signals; and an external data controller for symmetrically controlling the information signals. The internal unit can also include a voltage regulator converter; an internal signal conditioner for symmetrically transceiving the information signals; and a data controller. Symmetrically controlling can include ASK modulation of a data signal upon an RF carrier signal.

Data driven signal processing: an approach for energy efficient computing

Abstract: The computational switching activity of digital CMOS circuits can be dynamically minimized by designing algorithms that exploit signal statistics. This results in processors that have time-varying power requirements and perform computation on demand. An approach is presented to minimize the energy dissipation per data sample in variable-load DSP systems by adaptively minimizing the power supply voltage for each sample using a variable switching speed processor. In general, using buffering and filtering, the computation can be spread over multiple samples averaging the workload and lowering energy further. It is also shown that four levels of voltage quantization combined with dithering is sufficient to closely emulate arbitrary voltage levels.

Conduction in LaCoO3 by small-polaron hopping below room temperature.

Abstract: In order to study the electrical transport of holes in LaCo${\mathrm{O}}_{3}$, the bulk conductivities and dielectric properties in a polycrystalline specimen were investigated. The bulk conduction was separated from others by complex-plane impedance analysis. At high temperatures, i.e., $Tg170$ K, the activation energy required for bulk conduction is 0.34 eV, while that required for dielectric relaxation is 0.25 eV, and the spectral intensity of the relaxation peak is thermally activated with an energy of 0.097 eV. This is consistent with a polaronic picture in which the bulk conduction involves the excitation of carriers through the band gap, ${E}_{g}$, between the two energy states participating in the electrical transport; in such a picture the hopping process of the holes in the lower-energy states has a hopping energy ${W}_{H}$, and the temperature dependence of the bulk conduction is proportional to $\frac{\mathrm{exp}[\ensuremath{-}\frac{({W}_{H}+\frac{{E}_{g}}{2})}{{k}_{B}T}]}{T}$. The present experiment obtained these energies independently, i.e., $({W}_{H}+\frac{{E}_{g}}{2})\ensuremath{\cong}0.35$ eV, $\frac{{E}_{g}}{2}\ensuremath{\cong}0.097$ eV, and ${W}_{H}\ensuremath{\cong}0.25$ eV. At low temperatures, i.e., $Tl170$ K, the non-Arrhenius regime is observed, which might be interpreted as a usual crossover from a thermally activated process to a tunnel-hopping process.

Quantum dot self-consistent electronic structure and the Coulomb blockade.

Abstract: We employ density-functional theory to calculate the self-consistent electronic structure, free energy, and linear source-drain conductance of a lateral semiconductor quantum dot patterned via surface gates on the two-dimensional electron gas formed at the interface of a GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As heterostructure. The Schr\"odinger equation is reduced from 3D to multicomponent 2D and solved via an eigenfunction expansion in the dot. This permits the solution of the electronic structure for dot electron number N\ensuremath{\sim}100. We present details of our derivation of the total dot-lead-gates interacting free energy in terms of the electronic structure results, which is free of capacitance parameters. Statistical properties of the dot level spacings and connection coefficients to the leads are computed in the presence of varying degrees of order in the donor layer. Based on the self-consistently computed free energy as a function of gate voltages, ${\mathit{V}}_{\mathit{i}}$, and N, we modify the semiclassical expression for the tunneling conductance as a function of gate voltage through the dot in the linear source-drain, Coulomb blockade regime. Among the many results presented, we demonstrate the existence of a shell structure in the dot levels which (a) results in envelope modulation of Coulomb oscillation peak heights, (b) influences the dot capacitances and should be observable in terms of variations in the activation energy for conductance in a Coulomb oscillation minimum, and (c) possibly contributes to departure of recent experimental results from the predictions of random-matrix theory. \textcopyright{} 1996 The American Physical Society.

Effects of carbon on Fe-grain-boundary cohesion: First-principles determination.

Abstract: The cohesive properties of the C/Fe\ensuremath{\Sigma}3(111) grain boundary are investigated by means of the direct determination of the difference in binding energies of C in grain-boundary and free-surface environments. The atomic force approach based on the full-potential linearized augmented plane-wave method is used to optimize the atomic structure for the clean and C-segregated grain-boundary and free-surface systems. The \ensuremath{\omega} phase structure obtained in a previous grain-boundary cluster calculation is found to be only a metastable state that is 0.72 eV/cell (0.81 J/${\mathrm{m}}^{2}$) higher in energy than the distorted bcc ground state. The calculated binding-energy difference (i.e., \ensuremath{\Delta}${\mathit{E}}_{\mathit{b}}$-\ensuremath{\Delta}${\mathit{E}}_{\mathit{s}}$) is -0.61 eV/adatom, which is a theoretical demonstration that C is a cohesion enhancer in the Fe grain boundary. Comparisons with earlier results obtained for B, S, and P show that the number of hybridized p electrons and the resulting spatial anisotropy of bonding with the surrounding Fe atoms is the key factor determining the relative embrittling or cohesion enhancing behavior of a metalloid impurity. \textcopyright{} 1996 The American Physical Society.

Low bit-rate speech coding system and method using voicing probability determination

Abstract: A modular system and method is provided for low bit rate encoding and decoding of speech signals using voicing probability determination. The continuous input speech is divided into time segments of a predetermined length. For each segment the encoder of the system computes a model signal and subtracts the model signal from the original signal in the segment to obtain a residual excitation signal. Using the excitation signal the system computes the signal pitch and a parameter which is related to the relative content of voiced and unvoiced portions in the spectrum of the excitation signal, which is expressed as a ratio Pv, defined as a voicing probability. The voiced and the unvoiced portions of the excitation spectrum, as determined by the parameter Pv, are encoded using one or more parameters related to the energy of the excitation signal in a predetermined set of frequency bands. In the decoder, speech is synthesized from the transmitted parameters representing the model speech, the signal pitch, voicing probability and excitation levels in a reverse order. Boundary conditions between voiced and unvoiced segments are established to ensure amplitude and phase continuity for improved output speech quality. Perceptually smooth transition between frames is ensured by using an overlap and add method of synthesis. LPC interpolation and post-filtering is used to obtain output speech with improved perceptual quality.

Technique for reducing rectifier reverse-recovery-related losses in high-voltage high power converters

Abstract: A circuit technique that substantially reduces the boost-converter losses caused by the reverse-recovery characteristics of the rectifier is described The losses are reduced by inserting an inductor in the series path of the boost switch and the rectifier to control the di/dt rate of the rectifier during its turn-off The energy from the inductor after the boost switch turn-off is returned to the input or delivered to the output via an active snubber The same technique can be extended to any member of the PWM-converter family

Comparison of the high-frequency magnetic fluctuations in insulating and superconducting La2-xSrxCuO4.

Abstract: Inelastic neutron scattering performed at a spallation source is used to make absolute measurements of the dynamic susceptibility of insulating La{sub 2}CuO{sub 4} and superconducting La{sub 1.86}Sr{sub 0.14}CuO{sub 4} over the energy range 15{le}{h_bar}{omega}{le}350 meV. The effect of Sr doping on the magnetic excitations is to cause a large broadening in wave vector and a substantial change in the spectrum of the local spin fluctuations. Comparison of the two compositions reveals a new energy scale {sq_bullet}{Gamma}=22{plus_minus}5 meV in La{sub 1.86}Sr{sub 0.14}CuO{sub 4}. {copyright} {ital 1996 The American Physical Society.}

Effective single-band models for the high- T c cuprates. I. Coulomb interactions

Abstract: Starting with the three-band extended Hubbard model (or d-p model) widely used to represent the ${\mathrm{CuO}}_{2}$ planes in the high-${\mathit{T}}_{\mathit{c}}$ cuprates, we make a systematic reduction to an effective single-band model using a previously developed cell-perturbation method The range of parameters for which this mapping is a good approximation is explored in the full Zaanen-Sawatzky-Allen diagram (copper Coulomb repulsion ${\mathit{U}}_{\mathit{d}}$ versus charge-transfer energy \ensuremath{\varepsilon}), together with an investigation of the validity of a further mapping to an effective charge-spin (t-J-V) model The variation of the effective single-band parameters with the parameters of the underlying multi-band model is investigated in detail, and the parameter regime where the model represents the high-${\mathit{T}}_{\mathit{c}}$ cuprates is examined for specific features that might distinguish it from the general case In particular, we consider the effect of Coulomb repulsions on oxygen (${\mathit{U}}_{\mathit{p}}$) and between copper and oxygen (${\mathit{V}}_{\mathit{pd}}$) We find that the reduction to an effective single-band model is generally valid for describing the low-energy physics, and that ${\mathit{V}}_{\mathit{pd}}$ and ${\mathit{U}}_{\mathit{p}}$ (unless unrealistically large) actually slightly improve the convergence of the cell-perturbation method Unlike in the usual single-band Hubbard model, the effective intercell hopping and Coulomb interactions are different for electrons and holesWe find that this asymmetry, which vanishes in the extreme Mott-Hubbard regime (${\mathit{U}}_{\mathit{d}}$\ensuremath{\ll}\ensuremath{\varepsilon}), is quite appreciable in the charge-transfer regime (${\mathit{U}}_{\mathit{d}}$\ensuremath{\gtrsim}\ensuremath{\varepsilon}), particularly for the effective Coulomb interactions We show that for doped holes (forming Zhang-Rice singlets) on neighboring cells the interaction induced by ${\mathit{V}}_{\mathit{pd}}$ can even be attractive due to locally enhanced pd hybridization, while this cannot occur for electrons The Coulomb interaction induced by ${\mathit{U}}_{\mathit{p}}$ is always repulsive; in addition ${\mathit{U}}_{\mathit{p}}$ gives rise to a ferromagnetic spin-spin interaction which opposes antiferromagnetic superexchange We show that for hole-doped systems this leads to a subtle cancellation of attractive and repulsive contributions, due to antiferromagnetic and charge-polarization effects, to the net static interaction in a charge-spin (t-J-V) model, and we discuss the significance of this result The asymmetry in the ee, hh, and eh effective hopping parameters can be particularly large for next-nearest neighbors Specializing to cuprate parameters, we find that the asymmetry in the nearest-neighbor hopping parameters almost vanishes (accidentally), while the next-nearest-neighbor hopping parameter ${\mathit{t}}^{\ensuremath{'}}$ is close to zero for electrons but is appreciable for holes (${\mathit{t}}^{\ensuremath{'}}$\ensuremath{\approxeq}-006 eV) The effective Coulomb interaction between doped holes is found to be repulsive, and even slightly larger than for electrons All the underlying d-p parameters make significant contributions to the effective interactions and it is shown that certain approximations, such as ${\mathit{U}}_{\mathit{d}}$=\ensuremath{\infty} and ${\mathit{t}}_{\mathit{pp}}$=0, can be qualitatively incorrect \textcopyright{} 1996 The American Physical Society

System and method for simulating interference received by subscriber units in a spread spectrum communication network

Abstract: A method and apparatus for simulating signal interference in one communication channel within a communication system, such as cellular or wireless subscriber telephone and/or data systems. The communication system may be of a cellular type in which users from a plurality of cells communicate information signals between one another using at least one base station and code division multiple access (CDMA) spread spectrum type communication signals. The base station is included in a first of the cells and has a transmitter from which information is transmitted to subscriber units over at least one communication channel. The disclosed method includes the step of determining a first composite signal energy associated with signal transmission from the base station transmitter over a first set of simulated communication channels. The simulation method further contemplates estimating a first average data rate for the first composite signal energy. Signal power transmitted over the one communication channel is adjusted in accordance with a first interference signal which is based on the first composite signal energy and first average data rate. In a preferred implementation, a determination is also made of a second composite signal energy associated with signal transmissions from base station transmitters in other cells over a second set of simulated communication channels. An estimate is also made of a second average data rate for the second composite signal energy to enable generation of a second interference signal.

Exact solution for a hydrogen atom in a magnetic field of arbitrary strength

Abstract: An exact solution describing the quantum states of a hydrogen atom in a homogeneous magnetic field of arbitrary strength is obtained in the form of a power series in the radial variable with coefficients being polynomials in the sine of the polar angle. Energy levels and wave functions for the ground state and for several excited states are calculated exactly for the magnetic field varying in the range 0B/(${\mathit{m}}^{2}$${\mathit{e}}^{3}$c/${\mathrm{\ensuremath{\Elzxh}}}^{3}$)\ensuremath{\le}4000. \textcopyright{} 1996 The American Physical Society.

Capillary gravity waves caused by a moving disturbance: Wave resistance.

Abstract: The dispersive property of capillary gravity waves is responsible for the complicated wave pattern generated at the free surface of a calm liquid by a disturbance moving with a velocity V greater than the minimum phase speed ${\mathit{c}}^{\mathrm{min}}$=(4g\ensuremath{\gamma}/\ensuremath{\rho}${)}^{1/4}$ (\ensuremath{\rho} is the liquid density, \ensuremath{\gamma} is the liquid-air surface tension, and g is the acceleration due to gravity). The disturbance may be produced by a small object immersed in the liquid or by the application of an external surface pressure distribution. The waves generated by the moving disturbance continually remove energy to infinity, and, consequently, the disturbance experiences a drag called the wave resistance. The wave resistance corresponding to a surface pressure distribution symmetrical about a point was analyzed by Havelock in the particular case of pure gravity waves (i.e., \ensuremath{\gamma}=0) for which the minimum phase speed reduces to zero. Here, we investigate the more general case of capillary gravity waves using a linearized theory. We also analyze the integral depression of the liquid, the momentum carried by the liquid, and the effective mass of the disturbance for velocities V smaller than ${\mathit{c}}^{\mathrm{min}}$. These results may possibly lead to a new method of probing soft surfaces. \textcopyright{} 1996 The American Physical Society.

Time-resolved fluorescence-line narrowing and energy-transfer studies in a Eu 3 + -doped fluorophosphate glass

Abstract: The optical properties of ${\mathrm{Eu}}^{3+}$-doped fluorophosphate glasses of composition (in mol %) 60 ${\mathrm{NaPO}}_{3}$-${15\mathrm{B}\mathrm{a}\mathrm{F}}_{2}$-(25-x)${\mathrm{YF}}_{3}$-x${\mathrm{EuF}}_{3}$ (x=0.5, 2, 5, 10, 15, 20, and 25) have been investigated in the 4.2--300 K temperature range by using optical absorption spectroscopy and time-resolved resonant laser-induced fluorescence line narrowing. From the room-temperature absorption spectra Judd-Ofelt parameters have been obtained and used to calculate the spontaneous emission probabilities from the $^{5}$${\mathit{D}}_{0}$ state. The spectral features of the time-resolved fluorescence line-narrowed $^{5}$${\mathit{D}}_{0}$${\ensuremath{\rightarrow}}^{7}$${\mathit{F}}_{0,1}$ emission spectra obtained under resonant excitation at different wavelengths along the $^{7}$${\mathit{F}}_{0}$${\ensuremath{\rightarrow}}^{5}$${\mathit{D}}_{0}$ transition as a function of concentration and temperature reveal the existence of energy migration between discrete regions of the inhomogeneous broadened spectral profile. From the concentration and time dependence of the average rate of excitation transfer, the electronic mechanism ruling the ion-ion interaction can be identified as a dipole-dipole energy transfer process. At low temperatures the average transfer rate parameter slightly depends on wavelength showing a temperature independent behavior. Above 77 K the weak dependence of the transfer rate on excitation wavelength (weak dependence on energy mismatch) together with its T${\mathrm{}}^{3}$ temperature dependence point to a transfer mechanism consistent with a two-site nonresonant two-phonon assisted process. The estimated average crystal field strength grows monotonically with the $^{7}$${\mathit{F}}_{0}$${\ensuremath{\rightarrow}}^{5}$${\mathit{D}}_{0}$ energy suggesting a large variation in the local environment of ${\mathrm{Eu}}^{3+}$ ions in these glasses. The slight increase with concentration of the $^{5}$${\mathit{D}}_{0}$ fluorescence decays together with their single exponential character suggest that the transfer process may be fast enough to drive the system of excited centers to thermal equilibrium. \textcopyright{} 1996 The American Physical Society.

Energy characterization based on clustering

Abstract: We illustrate a new method to characterize the energy dissipation of circuits by collapsing closely related input transition vectors and energy patterns into capacitive coefficients. Energy characterization needs to be done only once for each module (ALU, multiplier etc.) in order to build a library of these capacitive coefficients. A direct high-level energy simulator or profiler can then use the library of pre-characterized modules and a sequence of input vectors to compute the total energy dissipation. A heuristic algorithm which performs energy clustering under objective constraints has been devised. The worst case running time of this algorithm is O(m/sup 3/n), where m is the number of simulation points and n is the number of inputs of the circuit. The designer can experiment with the criterion function by setting the appropriate relative error norms to control the "goodness" of the clustering algorithm and the sampling error and confidence level to maintain the sufficiency of representation of each cluster. Experiments on circuits show a significant reduction of the energy table size under a specified criterion function, cluster sampling error and confidence level.

Experimental observation of high-brightness microbunching in a photocathode rf electron gun

Abstract: We report the measurement of very short, high-brightness bunches of electrons produced in a photocathode rf gun with no magnetic compression. The electron beam bunch length and the charge distribution along the bunch were measured by passing the energy chirped the electron beam through a momentum selection slit while varying the phase of the rf linac. The bunch compression as a function of rf gun phase and electric field at the cathode were investigated. The shortest measured bunch is 370{plus_minus}100 fs (at 95{percent} of the charge) with 2.5{times}10{sup 8} electrons (170 A peak current); the normalized rms emittance of this beam was measured to be 0.5{pi} mmmrad and the energy spread is 0.15{percent}. {copyright} {ital 1996 The American Physical Society.}

Optical spectroscopy of the charge-ordering transition in la1.67sr0.33nio4

Abstract: Optical spectra of a single crystal of ${\mathrm{La}}_{1.67}$${\mathrm{Sr}}_{0.33}$${\mathrm{NiO}}_{4}$, which undergoes a charge-ordering transition at ${\mathit{T}}_{\mathrm{CO}}$\ensuremath{\sim}240 K, have been investigated for wide ranges of photon energy (0.008--3 eV) and temperature (10--480 K). The opening of a charge gap as well as change of the gap magnitude (2\ensuremath{\Delta}) with temperature is clearly observed below ${\mathit{T}}_{\mathrm{CO}}$, with an anomalously large ratio of 2\ensuremath{\Delta}(T=0)/${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathrm{CO}}$ (\ensuremath{\sim}13). Conspicuous spectral change with temperature is also observed above ${\mathit{T}}_{\mathrm{CO}}$ over the energy region up to 2 eV, which suggests persistent fluctuation of the charge ordering or formation of small polarons above ${\mathit{T}}_{\mathrm{CO}}$. \textcopyright{} 1996 The American Physical Society.

Polarized and unpolarized neutron-scattering study of the dynamical spin susceptibility of YBa2Cu3O7.

Abstract: We report an extensive study of magnetic excitations in fully oxygenated Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$, using neutron scattering with and without spin polarization analysis. By calibrating the measured magnetic intensity against calculated structure factors of optical phonons and against antiferromagnetic spin waves measured in the same crystal after deoxygenation to Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.2}$, we establish an absolute intensity scale for the dynamical spin susceptibility, ${\ensuremath{\chi}}^{\ensuremath{'}\ensuremath{'}}(\mathrm{q}, \ensuremath{\omega})$. The integrated spectral weight of the sharp magnetic resonance at $\ensuremath{\hbar}\ensuremath{\omega}=40$ meV and ${\mathrm{q}}_{\ensuremath{\parallel}}=(\frac{\ensuremath{\pi}}{a}, \frac{\ensuremath{\pi}}{a})$ in the superconducting state is $\ensuremath{\int}d(\ensuremath{\hbar}\ensuremath{\omega}){\ensuremath{\chi}}_{\mathrm{res}}^{\ensuremath{'}\ensuremath{'}}(\mathrm{q}, \ensuremath{\omega})=(0.52\ifmmode\pm\else\textpm\fi{}0.1)$ at low temperatures. The energy and spectral weight of the resonance are measured up to $T=0.8{T}_{c}$. The resonance disappears in the normal state, and a conservative upper limit of 30 states/eV is established for the normal state dynamical susceptibility at ${\mathrm{q}}_{\ensuremath{\parallel}}=(\frac{\ensuremath{\pi}}{a}, \frac{\ensuremath{\pi}}{a})$ and $10 \mathrm{meV}l~\ensuremath{\hbar}\ensuremath{\omega}l~40 \mathrm{meV}$. Our results are compared to previous neutron-scattering data on Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$, theoretical interpretations of NMR data and current models of the 40 meV resonance.