Showing papers on "Energy (signal processing) published in 1995"

Accurate nucleon-nucleon potential with charge-independence breaking

Abstract: The authors present a new high-quality nucleon-nucleon potential with explicit charge dependence and charge asymmetry, which they designate Argonne {upsilon}{sub 18}. The model has a charge-independent part with fourteen operator components that is an updated version of the Argonne {upsilon}{sub 14} potential. Three additional charge-dependent and one charge-asymmetric operators are added, along with a complete electromagnetic interaction. The potential has been fit directly to the Nijmegen pp and np scattering data base, low-energy nn scattering parameters, and deuteron binding energy. With 40 adjustable parameters it gives a {chi}{sup 2} per datum of 1.09 for 4,301 pp and np data in the range 0--350 MeV.

Periodic boundary conditions in ab initio calculations.

Abstract: The convergence of the electrostatic energy in calculations using periodic boundary conditions is considered in the context of periodic solids and localized aperiodic systems in the gas and condensed phases. Conditions for the absolute convergence of the total energy in periodic boundary conditions are obtained, and their implications for calculations of the properties of polarized solids under the zero-field assumption are discussed. For aperiodic systems the exact electrostatic energy functional in periodic boundary conditions is obtained. The convergence in such systems is considered in the limit of large supercells, where, in the gas phase, the computational effort is proportional to the volume. It is shown that for neutral localized aperiodic systems in either the gas or condensed phases, the energy can always be made to converge as O(${\mathit{L}}^{\mathrm{\ensuremath{-}}5}$) where L is the linear dimension of the supercell. For charged systems, convergence at this rate can be achieved after adding correction terms to the energy to account for spurious interactions induced by the periodic boundary conditions. These terms are derived exactly for the gas phase and heuristically for the condensed phase.

Probing the nuclear liquid-gas phase transition.

Abstract: Fragment distributions resulting from $\mathrm{Au}+\mathrm{Au}$ collisions at an incident energy of $E/A\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}600\phantom{\rule{0ex}{0ex}}\mathrm{MeV}$ are studied. From the measured fragment and neutron distributions the mass and the excitation energy of the decaying prefragments were determined. A temperature scale was derived from observed yield ratios of He and Li isotopes. The relation between this isotope temperature and the excitation energy of the system exhibits a behavior which is expected for a phase transition. The nuclear vapor regime takes over at an excitation energy of 10 MeV per nucleon, a temperature of 5 MeV, and may be characterized by a density of 0.15--0.3 normal nuclear density.

Waste energy control and management in power amplifiers

Abstract: An amplifying apparatus for linearly amplifying a desired signal using a pair of coupled non-linear amplifiers is disclosed. The amplifying apparatus comprises a limiter for separating amplitude variations from the desired signal and producing a constant amplitude signal bearing the phase of the desired signal and an amplitude related signal. In addition, a drive signal generater produces two drive signals each dependent on the constant amplitude signal and the amplitude related signal such that each drive signal depends on the phase of the desired signal and such that the sum of the squares of the amplitudes of the drive signals is constant. Finally, a coupler couples the two drive signals to produce two constant amplitude signals for driving the pair of non-linear power amplifiers and for coupling the outputs of the power amplifiers to produce two amplified output signals, one of which is the linearly amplified desired signal and the other of which is a waste energy signal.

ac Josephson Effect in a Single Quantum Channel.

Abstract: We have calculated all the components of the current in a short one-dimensional channel between two superconductors for arbitrary voltages and transparencies $D$ of the channel. We demonstrate that in the ballistic limit ( $D\ensuremath{\simeq}1$) the crossover between the quasistationary evolution of the Josephson phase difference $\ensuremath{\phi}$ at small voltages and transport by multiple Andreev reflections at larger voltages can be described as the Landau-Zener transition induced by finite reflection in the channel. For perfect transmission and vanishing energy relaxation rates the stationary current-phase relation is never recovered, and $I(\ensuremath{\phi}){\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}I}_{c}\ensuremath{\mid}\mathrm{sin}\ensuremath{\phi}/2\ensuremath{\mid}\mathrm{sgn}V$ for arbitrary small voltages.

General features of latent track formation in magnetic insulators irradiated with swift heavy ions

Abstract: A thermal spike model is proposed for the analysis of latent track formation in insulators. The model predicts that above a threshold electronic stopping power ${\mathit{S}}_{\mathit{e}\mathit{t}}$ ${\mathit{R}}_{\mathit{e}}^{2}$\ensuremath{\sim}ln${\mathit{S}}_{\mathit{e}}$ for 1\ensuremath{\le}${\mathit{S}}_{\mathit{e}}$/${\mathit{S}}_{\mathit{e}\mathit{t}}$\ensuremath{\le}2.7 followed by ${\mathit{R}}_{\mathit{e}}^{2}$\ensuremath{\sim}${\mathit{S}}_{\mathit{e}}$ for 2.7\ensuremath{\le}${\mathit{S}}_{\mathit{e}}$/${\mathit{S}}_{\mathit{e}\mathit{t}}$ (${\mathit{R}}_{\mathit{e}}$ being the effective track radius). A simple expression is derived for the evaluation of ${\mathit{S}}_{\mathit{e}\mathit{t}}$ from material parameters. A good agreement is found with literature data on latent tracks in magnetic insulators. The analysis showed that the width of the temperature distribution is a(0)=(4.5\ifmmode\pm\else\textpm\fi{}0.25) nm and the energy deposited in the spike is 0.17${\mathit{S}}_{\mathit{e}}$.

Escaping the symmetry dilemma through a pair-density interpretation of spin-density functional theory

Abstract: In the standard interpretation of spin-density functional theory, a self-consistent Kohn-Sham calculation within the local spin density (LSD) or generalized gradient approximation (GGA) leads to a prediction of the total energy E, total electron density n(r)=${\mathit{n}}_{\mathrm{\ensuremath{\uparrow}}}$(r)+${\mathit{n}}_{\mathrm{\ensuremath{\downarrow}}}$(r), and spin magnetization density m(r)=${\mathit{n}}_{\mathrm{\ensuremath{\uparrow}}}$(r)-${\mathit{n}}_{\mathrm{\ensuremath{\downarrow}}}$(r). This interpretation encounters a serious ``symmetry dilemma'' for ${\mathrm{H}}_{2}$, ${\mathrm{Cr}}_{2}$, and many other molecules. Without changing LSD or GGA calculational methods and results, we escape this dilemma through an alternative interpretation in which the third physical prediction is not m(r) but the on-top electron pair density P(r,r), a quantity more directly related to the total energy in the absence of an external magnetic field. This alternative interpretation is also relevant to antiferromagnetic solids. We argue that the nonlocal exchange-correlation energy functional, which must be approximated, is most nearly local in the alternative spin-density functional theory presented here, less so in the standard theory, and far less so in total-density functional theory. Thus, in LSD or GGA, predictions of spin magnetization densities and moments are not so robust as predictions of total density and energy. The alternative theory helps to explain the surprising accuracy of LSD and GGA energies, and suggests that the correct solution of the Kohn-Sham equations in LSD or GGA is the fully self-consistent broken-symmetry single determinant of lowest total energy.

Theoretical study of electronic, magnetic, and structural properties of alpha -fe2o3 (hematite)

Abstract: Antiferromagnetic rhombohedral \ensuremath{\alpha}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ has been studied by calculations of the ground-state spin-polarized wave function and total energy, using the ab initio periodic unrestricted Hartree-Fock approach. All-electron basis sets of contracted Gaussian-type functions are employed to represent the O and Fe atoms (18 and 27 orbitals, respectively); Fe is alternatively described by a large-core pseudopotential plus 18 valence-shell orbitals. Computations have been performed for both the antiferromagnetic (AF) and ferromagnetic (FM) structures; the correct relative stability is reproduced, with \ensuremath{\Delta}E(AF-FM)=-0.0027 hartree per formula unit (including a correction for correlation energy). The dependence of \ensuremath{\Delta}E(AF-FM) on variations of the Fe-O bond lengths and Fe-O-Fe' angles involved in superexchange is analyzed, finding that for some configurations the FM structure becomes more stable than the AF one. The athermal equation of state, equilibrium crystal structure, elastic bulk modulus, and binding energy have been computed and compared to experimental quantities. An analysis of the density of electronic states show that the band gap is of p-d rather than d-d type, confirming the charge-transfer-insulator nature of hematite as inferred from photoelectron spectra. The overall shape of the valence band is also fully consistent with spectroscopic results. Mulliken electron population data indicate a charge back transfer of 0.29\ensuremath{\Vert}e\ensuremath{\Vert} from ${\mathrm{O}}^{2\mathrm{\ensuremath{-}}}$ to the d shell of ${\mathrm{Fe}}^{3+}$, causing a partial spin pairing with a magnetic moment of 4.7${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$.

Self-consistent approximation to the Kohn-Sham exchange potential.

Abstract: A scheme of approximation of the Kohn-Sham exchange potential ${\mathit{v}}_{\mathit{x}}$ is proposed, making use of a partitioning of ${\mathit{v}}_{\mathit{x}}$ into the long-range Slater ${\mathit{v}}_{\mathit{S}}$ and the short-range response ${\mathit{v}}_{\mathrm{resp}}$ components. A model potential ${\mathit{v}}_{\mathrm{resp}}^{\mathrm{mod}}$ has been derived from dimensional arguments. It possesses the proper short-range behavior and the atomic-shell stepped structure characteristic for ${\mathit{v}}_{\mathrm{resp}}$. When combined with the accurate ${\mathit{v}}_{\mathit{S}}$, ${\mathit{v}}_{\mathrm{resp}}^{\mathrm{mod}}$ provides an excellent approximation to the exchange potential of the optimized potential model ${\mathit{v}}_{\mathit{x}}^{\mathrm{OPM}}$. With the generalized-gradient approximation to ${\mathit{v}}_{\mathit{S}}$ ${\mathit{v}}_{\mathrm{resp}}^{\mathrm{mod}}$ provides an efficient density-functional-theory approach that fits closely the form of the accurate exchange potential and yields reasonably accurate exchange and total energies as well as the energy of the highest occupied orbital.

Measurement of the electric polarizability of sodium with an atom interferometer

Abstract: We have constructed an atom interferometer with interfering beams that are physically isolated by a metal foil. By applying an interaction to one of the two interfering beams, we can measure ground-state energy shifts with a spectroscopic precision of 6.6\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}14}$ eV/\ensuremath{\surd}min , or 16 Hz/\ensuremath{\surd}min . Applying an electric field to one beam of the interferometer, we have measured the phase shift induced from the quadratic Stark effect. By analyzing these phase shifts, we have determined the ground-state polarizability of sodium, with much improved accuracy, to be 24.11(6${)}_{\mathrm{statistical}}$(6${)}_{\mathrm{systematic}}$\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}24}$ ${\mathrm{cm}}^{3}$.

Collective dynamics in water by high energy resolution inelastic X-ray scattering.

Abstract: A propagating excitation with a velocity of sound of $3200\ifmmode\pm\else\textpm\fi{}100$ $\mathrm{m}/\mathrm{s}$ is measured in ${\mathrm{H}}_{2}\mathrm{O}$ at 294 $\mathrm{K}$ between 4 and 14 ${\mathrm{nm}}^{\ensuremath{-}1}$, using inelastic x-ray scattering with 5 $\mathrm{meV}$ energy resolution. The existence of fast sound is therefore demonstrated in an energy-momentum region much wider than that of previous neutron measurements on ${\mathrm{D}}_{2}\mathrm{O}$. The equivalence of the fast sound velocity in ${\mathrm{H}}_{2}\mathrm{O}$ and in ${\mathrm{D}}_{2}\mathrm{O}$ rules out models where this mode propagates only on the hydrogen network. These results show the ability of inelastic x-ray scattering to study the collective dynamics of liquids.

Quantum Monte Carlo determination of electronic and structural properties of Sin clusters (n <= 20).

Abstract: Variational and fixed-node diffusion Monte Carlo methods are applied to study the structural and valence electronic properties of ${\mathrm{Si}}_{n}$ clusters. Binding energies for $n\ensuremath{\le}7$ agree within $\ensuremath{\approx}4%$ with experiments and within $\ensuremath{\approx}2%$ when the fixed-node error is decreased by using natural orbitals. For $n\ensuremath{\ge}9$ we show that the local density approximation overbinds by $\ensuremath{\approx}25%$. We determined unambiguously (i) the role of correlation in the energy ordering for different structures, including our new lowest energy structure of ${\mathrm{Si}}_{20}$, and (ii) a different ground state for ${\mathrm{Si}}_{13}{}^{\ensuremath{-}}$ than the one predicted by the local density approximation.

Dominant density parameters and local pseudopotentials for simple metals

Abstract: The properties of the simple metals are controlled largely by three density parameters: the equilibrium average valence electron density 3/4\ensuremath{\pi}${\mathit{r}}_{\mathit{s}}^{3}$, the valence z, and the density on the surface of the Wigner-Seitz cell, represented here by the equilibrium number ${\mathit{N}}_{\mathrm{int}}$ of valence electrons in the interstitial region. To demonstrate this fact, and as a refinement of the ``stabilized jellium'' or ``structureless pseudopotential'' model, we propose a structured local electron-ion pseudopotential w(r) which depends upon either ${\mathit{r}}_{\mathit{s}}$ and z (``universal'' choice for ${\mathit{N}}_{\mathrm{i}\mathrm{n}\mathrm{t})}$, or ${\mathit{r}}_{\mathit{s}}$, z, and ${\mathit{N}}_{\mathrm{int}}$ for each metal (``individual'' potential). Calculated binding energies, bulk moduli, and pressure derivatives of bulk moduli, evaluated in second-order perturbation theory, are in good agreement with experiment for 16 simple metals, and the bulk moduli are somewhat better than those calculated from first-principles nonlocal norm-conserving pseudopotentials. Structural energy differences agree with those from a nonlocal pseudopotential calculation for Na, Mg, and Al, but not for Ca and Sr. Our local pseudopotential w(r) is analytic for all r, and displays an exponential decay of the core repulsion as r\ensuremath{\rightarrow}\ensuremath{\infty}. The decay length agrees with that of the highest atomic core orbital of s or p symmetry, corroborating the physical picture behind this ``evanescent core'' form. The Fourier transform or form factor w(Q) is also analytic, and decays rapidly as Q\ensuremath{\rightarrow}\ensuremath{\infty}; its first and only zero ${\mathit{Q}}_{0}$ is close to conventional or empirical values. In comparison with nonlocal pseudopotentials, local ones have the advantages of computational simplicity, physical transparency, and suitability for tests of density functional approximations against more-exact many-body methods.

Thermoelectric power: A simple, instructive probe of high-Tc superconductors.

Abstract: The thermoelectric power $S(T)$ is studied for $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ and Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$ as a function of Zn substitution and hole concentration $p$. A strong enhancement in $S(T)$ is correlated with the smooth opening of an energy gap in the normal-state spectrum, as observed from NMR and heat capacity data, which appears to be responsible for the $p$-dependent variation in ${T}_{c}$. The data are consistent with the local suppression of this energy gap in the immediate neighborhood of a Zn atom. Studies of $S(T)$ provide a simple means to explore the energy scales associated with the superconducting phase diagram and give some key insights into the origins of cuprate superconductivity.

Adaptive system for optimizing disk drive power consumption

Abstract: A disk drive power manager that compares energy usage with a predetermined profile to select one of several reduced-power operating modes. An energy usage register is updated at regular intervals according to the energy used during each time interval. An attenuated average energy usage value is obtained by attenuating the energy usage measures over time and integrating the results. The average energy level is compared with several predetermined energy threshold profiles to select a full- or reduced-power operating mode. Operating mode selection automatically adapts to the usage pattern to optimally balance power consumption with operational accessibility.

Multiphonon-assisted tunneling through deep levels: A rapid energy-relaxation mechanism in nonideal quantum-dot heterostructures.

Abstract: The presence of a deep-level trap coupled to a quantum-dot heterostructure is shown to provide a rapid energy-relaxation pathway through which electrons may thermalize. A capture process is considered whereby a free conduction-band electron is captured into the ground conduction-band state of a quantum dot by multiphonon-assisted tunneling through the trap. As an example calculation, transition rates for a 5 nm radius ${\mathrm{In}}_{0.5}$${\mathrm{Ga}}_{0.5}$As/GaAs quantum dot coupled to the defect M1 are calculated as a function of separation between the quantum dot and the deep level. For separations less than \ensuremath{\approxeq}10 nm these rates are found to be in excess of ${10}^{10}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ at 4.2 K. The result suggests that the presence of point defects may serve to enhance the luminescence efficiency of quantum-dot material. The physical situation described in this paper could only arise if the spatial distribution of defects were strongly correlated with that of the quantum-dot structures, e.g., through formation of interface states or point defects as a consequence of the growth process. With this caveat, the proposed mechanism may possibly explain the failure to observe a significant phonon bottleneck effect in recent work on ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As quantum-dot structures [e.g., Appl. Phys. Lett. 64, 2815 (1994)].

Method and apparatus for providing an improved feature set in speech recognition by performing noise cancellation and background masking

Abstract: The invention relates to a method and apparatus for generating noise-attenuated feature vectors for use in recognizing speech, more particularly to a system and method providing a feature set for speech recognition that is robust to adverse noise conditions. This is done by receiving, through an input, a set of signal frames, at least some containing speech sounds, and then classifying the frames in the set of signal frames into classification groups on the basis of their energy levels. Each classification group is characterized by a mean energy value. In a specific example of implementation, the invention makes use of channel energy values to condition the frames in the set of signal frames. The frames in the set of signal frames are attenuated or noise reduced by altering the energy of the frames on the basis of the frames containing non-speech sounds. In a specific example of implementation, the invention compresses the energy of the frames in the set of signal frames such that the energy lies within a range. The invention also allows separate energy ranges to be defined for each channel.

(3+1)-dimensional optical soliton dragging logic.

Abstract: A review of the varieties of optical solitons and their possible interactions, combined with the requirements for a robust digital logic gate motivate the use of (3+1)-dimensional optical solitons (light bullets) as information carriers and soliton dragging gates as switches. Soliton dragging is the asymmetric interaction between two initially overlapping, orthogonally polarized solitons propagating at different angles so that a weak signal soliton can drag a strong pump out of a spatial aperture, thereby implementing a phase-insensitive, high-contrast, logical switch with gain. Light bullets may be an ideal choice for use in these soliton dragging gates but are unstable in Kerr media, but stable (for sufficient pulse energy) in materials with physically reasonable saturating or negative ${\mathit{n}}_{4}$${\mathit{I}}^{2}$ nonlinearities. An efficient technique for the propagation of spherically symmetric (3+1)-dimensional field envelopes is developed and used to verify the theoretical stability predictions. A split-step numerical algorithm that models the propagation and phase-independent interaction of arbitrary (3+1)-dimensional, vector e.m. fields in anisotropic media with up to sixth-order tensor nonlinearities is developed and used to demonstrate the features of the gates. NOT and single-stage, two- and four-input NOR light-bullet dragging logic gates are simulated and their performance over a range of operating parameters is presented. It is shown that, with material parameters in the range of those currently available from highly nonlinear organic crystals, high-contrast, all-optical, soliton logic gates with a clock rate greater than 1 THz, latency of a few picoseconds, and switching energy of 25 pJ may be possible.

Skyrme crystal in a two-dimensional electron gas.

Abstract: The ground state of a two-dimensional electron gas at Landau level filling factors near $\ensuremath{ u}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$ is a Skyrme crystal with long-range order in the positions and orientations of the topologically and electrically charged elementary excitations of the $\ensuremath{ u}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$ ferromagnetic ground state. The lowest energy Skyrme crystal is a square lattice with opposing postures for topological excitations on opposite sublattices. The filling factor dependence of the electron spin polarization, calculated for the square lattice Skyrme crystal, is in excellent agreement with recent experiments.

Signal acquisition in a multi-user communication system using multiple walsh channels

Abstract: A technique for using energy received by subscriber units over multiple orthogonal channels within a spread spectrum communication system to acquire signal timing by controlling signal amplitude integration intervals used in detecting such timing. Received signals are despread and respective amplitudes integrated over periods that are divisible by factors of 2 into the length of Walsh functions used to generate orthogonal signal channels. Non-coherent combinations of the results of this integration are subsequently formed over periods that commence and terminate on Walsh function boundaries, and used to determine when a correct time offset has been selected for despreading signals. Additional advantages are realized by assigning signals that consistently provide a higher energy content such as paging, synchronization, and most frequently assigned traffic channels to specific orthogonal channels within the communication system. In exemplary embodiments, Walsh functions of length 128 are used as channelizing codes and a pilot signal is assigned to channel 0. This results in traffic channels or paging and synchronization functions being assigned to channel 64 when the integration periods are 64 chips long, and to channels 32, 64, and 96 when the periods are 32 chips long. In this manner, additional energy is available during the integration process for use in determining when correct signal acquisition timing offsets have been selected, without the use of additional hardware.

Higher order differential energy operators

Abstract: Instantaneous signal operators /spl Upsi//sub k/(x)=x/spl dot/x/sup (k-1)/-xx/sup (k)/ of integer orders k are proposed to measure the cross energy between a signal x and its derivatives. These higher order differential energy operators contain as a special case, for k=2, the Teager-Kaiser (1990) operator. When applied to (possibly modulated) sinusoids, they yield several new energy measurements useful for parameter estimation or AM-FM demodulation. Applying them to sampled signals involves replacing derivatives with differences that lead to several useful discrete energy operators defined on an extremely short window of samples. >

Influence of electron-electron scattering on shot noise in diffusive contacts.

Abstract: Shot noise in contacts with a small elastic mean free path in the presence of electron-electron scattering is calculated using the Boltzmann-Langevin approach and the approximation of effective electron tempreature. In the case of strong electron-electron scattering, the ratio between the shot noise ${\mathit{S}}_{\mathit{I}}$ and its classical value 2e\ensuremath{\Vert}I\ensuremath{\Vert} increases from 1/3 to \ensuremath{\surd}3 /4 owing to an increasing band of partially occupied states. The influence of electron-phonon scattering on the shot noise at finite temperatures is also considered. As this scattering decreases the energy of electron gas, it suppresses the shot noise at high voltages and increases the low-voltage boundary of the shot-noise range at finite temperatures. However, the noise decrease with increasing contact length more slowly than its reciprocal.

Prediction of the lowest energy structure of clusters using a genetic algorithm

Abstract: An optimization approach using a genetic algorithm in the search for a global minimum is described. The method is based on the use of control variables to form the genotypes in each generation. This procedure allows an accurate representation of the control variables, and consequently, a high resolution determination of the optimum solution. A set of genetic operators, appropriate for the operation on genes represented by real numbers, is introduced. The method is used to predict the lowest energy structures of ${\mathrm{Ar}}_{\mathit{n}}$${\mathrm{H}}_{2}$ microclusters with n=4, 5, 6, 7, and 12. Comparison between the performance of this optimization approach and the well established simulated annealing method clearly demonstrates the superiority of the genetic algorithm based search.

Quantum-confined Stark effects in semiconductor quantum dots

Abstract: Quantum-confined Stark effects (QCSE) on excitons, i.e., the influence of a uniform electric field on the confined excitons in semiconductor quantum dots (QD's), have been studied by using a numerical matrix-diagonalization scheme. The energy levels and the wave functions of the ground and several excited states of excitons in CdS and ${\mathrm{CdS}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Se}}_{\mathit{x}}$ quantum dots as functions of the size of the quantum dot and the applied electric field have been obtained. The electron and hole distributions and wave function overlap inside the QD's have also been calculated for different QD sizes and electric fields. It is found that the electron and hole wave function overlap decreases under an electric field, which implies an increased exciton recombination lifetime due to QCSE. The energy level redshift and the enhancement of the exciton recombination lifetime are due to the polarization of the electron-hole pair under the applied electric field.

Instability and blow-up of solutions to a generalized Boussinesq equation

Abstract: In this paper we investigate conditions for the finite-time blow-up of solutions of the generalized Boussinesq equation (BQ) \[ u_{tt} - u_{xx} + (f(u) + u_{xx} )_{xx} = 0,\quad x \in {\bf R},t > 0. \] The conditions are expressed in terms of the energy of the ground state. In particular, there exist initial data arbitrarily close to the stationary state of lowest energy whose solutions blow up in finite time.

Approach to ergodicity in quantum wave functions

Abstract: According to theorems of Shnirelman and followers, in the semiclassical limit the quantum wave functions of classically ergodic systems tend to the microcanonical density on the energy shell. Here we develop a semiclassical theory that relates the rate of approach to the decay of certain classical fluctuations. For uniformly hyperbolic systems, we find that the variance of the quantum matrix elements is proportional to the variance of the integral of the associated classical operator over trajectory segments of length ${\mathit{T}}_{\mathit{H}}$ and inversely proportional to ${\mathit{T}}_{\mathit{H}}^{2}$, where ${\mathit{T}}_{\mathit{H}}$=h\ensuremath{\rho}\ifmmode\bar\else\textasciimacron\fi{} is the Heisenberg time, \ensuremath{\rho}\ifmmode\bar\else\textasciimacron\fi{} being the mean density of states. Since for these systems the classical variance increases linearly with ${\mathit{T}}_{\mathit{H}}$, the variance of the matrix elements decays like 1/${\mathit{T}}_{\mathit{H}}$. For nonhyperbolic systems, such as Hamiltonians with a mixed phase space and the stadium billiard, our results predict a slower decay due to sticking in marginally unstable regions. Numerical computations supporting these conclusions are presented for the bakers map and the hydrogen atom in a magnetic field. (c) 1995 The American Physical Society

Size dependence of the optical response of spherical sodium clusters.

Abstract: The optical response of spherical ${{\mathrm{Na}}_{j}}^{+}$ clusters ($9\ensuremath{\le}j\ensuremath{\le}93$) shows one large maximum at about 2.7 eV. As a function of the inverse cluster radius its peak position and root mean square energy extrapolate linearly, but with different slopes, to the bulk Mie plasmon. The slopes are compared to two theoretical predictions: the static spill-out equation of cluster science and the dynamic screening theory of surface science as generalized to clusters. Good agreement is obtained. The similarity of collective excitations of surfaces and clusters is emphasized.

Hearing the zero locus of a magnetic field

Abstract: We investigate the ground state of a two-dimensional quantum particle in a magnetic field where the field vanishes nondegenerately along a closed curve. We show that the ground state concentrates on this curve ase/h tends to infinity, wheree is the charge, and that the ground state energy grows like (e/h)2/3. These statements are true for any energy level, the level being fixed as the charge tends to infinity. If the magnitude of the gradient of the magnetic field is a constantb 0 along its zero locus, then we get the precise asymptotics(e/h) 2/3 (b 0) 2/3 E * +O(1) for every energy level. The constantE * ≃ .5698 is the infimum of the ground state energiesE(β) of the anharmonic oscillator family $$ - \frac{{d^2 }}{{dy^2 }} + \left( {\frac{1}{2}y^2 - \beta } \right)^2$$ .

Electronic structure of PrNiO3 studied by photoemission and x-ray-absorption spectroscopy: Band gap and orbital ordering

Abstract: The electronic structure of ${\mathrm{PrNiO}}_{3}$ has been studied by photoemission and x-ray-absorption spectroscopy. By analyzing the spectra using configuration-interaction calculations on a ${\mathrm{NiO}}_{6}$ cluster model, it has been found that the charge-transfer energy \ensuremath{\Delta} is \ensuremath{\sim}1 eV and the Ni 3d and O 2p orbitals are strongly hybridized in the ground state. From the cluster-model calculation, the magnetic moment of Ni 3d is estimated to be \ensuremath{\sim}0.9${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$, which is close to the ionic value of ${\mathrm{Ni}}^{3+}$ and in good agreement with that obtained from neutron-diffraction experiments. Using the electronic-structure parameters deduced from the cluster-model analysis, we have performed unrestricted Hartree-Fock calculations on a Ni 3d--O 2p perovskite-type lattice model in order to study the effect of ${\mathrm{GdFeO}}_{3}$-type distortion on the orbital polarization and band gap.

Explicitly correlated Gaussian functions in variational calculations: The ground state of the beryllium atom

Abstract: Explicitly correlated Gaussian functions are applied to extensive variational calculations of the $^{1}$S ground state of the beryllium atom. The convergence of the energy with respect to the basis-set expansion length is investigated. The nonrelativistic clamped-nuclei energy computed from a 1200-term wave function equals -14.667 355 hartree and is in error by about 1 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. This is the lowest variational upper bound to the beryllium ground-state energy reported to date and it shows that recent empirical estimates of the nonrelativistic energy of the Be atom lie slightly too high. Several expectation values, including powers of interparticle distances and the Dirac \ensuremath{\delta} function, are computed. The nuclear magnetic shielding constant, the magnetic susceptibility, the specific mass shifts, the transition isotope shift, and the electron density at the nucleus position are evaluated.