Showing papers on "Energy (signal processing) published in 1997"

High energy neutrinos from cosmological gamma-ray burst fireballs

Abstract: Observations suggest that {gamma}-ray bursts (GRBs) are produced by the dissipation of the kinetic energy of a relativistic fireball. We show that a large fraction, {ge}10{percent}, of the fireball energy is expected to be converted by photomeson production to a burst of {approximately}10{sup 14}eV neutrinos. A km{sup 2} neutrino detector would observe at least several tens of events per year correlated with GRBs, and test for neutrino properties (e.g., flavor oscillations, for which upward moving {tau}{close_quote}s would be a unique signature, and coupling to gravity) with an accuracy many orders of magnitude better than is currently possible. {copyright} {ital 1997} {ital The American Physical Society}

Hyperdynamics: Accelerated Molecular Dynamics of Infrequent Events

Abstract: I derive a general method for accelerating the molecular-dynamics (MD) simulation of infrequent events in solids. A bias potential ( $\ensuremath{\Delta}{V}_{b}$) raises the energy in regions other than the transition states between potential basins. Transitions occur at an accelerated rate and the elapsed time becomes a statistical property of the system. $\ensuremath{\Delta}{V}_{b}$ can be constructed without knowing the location of the transition states and implementation requires only first derivatives. I examine the diffusion mechanisms of a 10-atom Ag cluster on the Ag(111) surface using a $220\ensuremath{\mu}\mathrm{s}$ hyper-MD simulation.

Quasiparticle Lifetime in a Finite System: A Nonperturbative Approach

Abstract: The problem of electron-electron lifetime in a quantum dot is studied beyond perturbation theory by mapping onto the problem of localization in the Fock space. Localized and delocalized regimes are identified, corresponding to quasiparticle spectral peaks of zero and finite width, respectively. In the localized regime, quasiparticle states are single-particle-like. In the delocalized regime, each eigenstate is a superposition of states with very different quasiparticle content. The transition energy is ${\ensuremath{\epsilon}}_{c}\ensuremath{\simeq}\ensuremath{\Delta}(g/\mathrm{ln}g{)}^{1/2}$, where $\ensuremath{\Delta}$ is mean level spacing, and $g$ is the dimensionless conductance. Near ${\ensuremath{\epsilon}}_{c}$ there is a broad critical region not described by the golden rule.

Experimental and theoretical approach to spin splitting in modulation-doped In x Ga 1 − x As/InP quantum wells for B→0

Abstract: Spin splitting of conduction-band energy levels in a modulation-doped InP/In{sub 0.77}Ga{sub 0.23}As/InP quantum well has been studied by Shubnikov{endash}de Haas oscillations. By analyzing the characteristic beating pattern of the oscillations the coupling constant {alpha} for spin-orbit interaction was determined. Biasing a gate on top of a Hall bar was used to modify the strength of the spin-orbit coupling. The measured spin-orbit coupling parameter {alpha} is quantitatively explained by utilizing a refined envelope-function-approximation theory for heterostructures. {copyright} {ital 1997} {ital The American Physical Society}

Finite-size scaling of the ground-state parameters of the two-dimensional Heisenberg model

Abstract: The ground-state parameters of the two-dimensional $S=1/2$ antiferromagnetic Heisenberg model are calculated using the stochastic series expansion quantum Monte Carlo method for $L\ifmmode\times\else\texttimes\fi{}L$ lattices with $L$ up to $16$. The finite-size results for the energy $E$, the sublattice magnetization $M$, the long-wavelength susceptibility ${\ensuremath{\chi}}_{\ensuremath{\perp}}(q=2\ensuremath{\pi}/L)$, and the spin stiffness ${\ensuremath{\rho}}_{s},$ are extrapolated to the thermodynamic limit using fits to polynomials in $1/L$, constrained by scaling forms previously obtained from renormalization-group calculations for the nonlinear $\ensuremath{\sigma}$ model and chiral perturbation theory. The results are fully consistent with the predicted leading finite-size corrections, and are of sufficient accuracy for extracting also subleading terms. The subleading energy correction $(\ensuremath{\sim}{1/L}^{4})$ agrees with chiral perturbation theory to within a statistical error of a few percent, thus providing numerical confirmation of the finite-size scaling forms to this order. The extrapolated ground- state energy per spin is $E=\ensuremath{-}0.669437(5)$. The result from previous Green's function Monte Carlo (GFMC) calculations is slightly higher than this value, most likely due to a small systematic error originating from ``population control'' bias in GFMC. The other extrapolated parameters are $M=0.3070(3)$, ${\ensuremath{\rho}}_{s}=0.175(2)$, ${\ensuremath{\chi}}_{\ensuremath{\perp}}=0.0625(9)$, and the spin-wave velocity $c=1.673(7)$. The statistical errors are comparable with those of previous estimates obtained by fitting loop algorithm quantum Monte Carlo data to finite-temperature scaling forms. Both $M$ and ${\ensuremath{\rho}}_{s}$ obtained from the finite-$T$ data are, however, a few error bars higher than the present estimates. It is argued that the $T=0$ extrapolations performed here are less sensitive to effects of neglected higher-order corrections, and therefore should be more reliable.

Pulsed second-harmonic generation in nonlinear, one-dimensional, periodic structures

Abstract: We present a numerical study of second-harmonic (SH) generation in a one-dimensional, generic, photonic band-gap material that is doped with a nonlinear ${\ensuremath{\chi}}^{(2)}$ medium. We show that a 20-period, 12-\ensuremath{\mu}m structure can generate short SH pulses (similar in duration to pump pulses) whose energy and power levels may be 2--3 orders of magnitude larger than the energy and power levels produced by an equivalent length of a phase-matched, bulk medium. This phenomenon comes about as a result of the combination of high electromagnetic mode density of states, low group velocity, and spatial phase locking of the fields near the photonic band edge. The structure is designed so that the pump pulse is tuned near the first-order photonic band edge, and the SH signal is generated near the band edge of the second-order gap. This maximizes the density of available field modes for both the pump and SH field. Our results show that the ${\ensuremath{\chi}}^{(2)}$ response is effectively enhanced by several orders of magnitude. Therefore, mm- or cm-long, quasi-phase-matched devices could be replaced by these simple layered structures of only a few micrometers in length. This has important applications to high-energy lasers, Raman-type sources, and frequency up- and down-conversion schemes.

Stabilization of Ternary Compounds via Ordered Arrays of Defect Pairs

Abstract: First-principles calculations show that the defect pair $({2V}_{\mathrm{Cu}}^{\ensuremath{-}}+{\mathrm{In}}_{\mathrm{Cu}}^{+})$ in CuInS${\mathrm{e}}_{2}$ has an unusually low formation energy, due both to the relative ease of forming Cu vacancies $({V}_{\mathrm{Cu}})$ and to the attractive interactions between ${V}_{\mathrm{Cu}}^{\ensuremath{-}}$ and ${\mathrm{In}}_{\mathrm{Cu}}^{2+}$. The defect pair is predicted to be electrically inactive. This explains the surprising electrical tolerance of CuInS${\mathrm{e}}_{2}$ to its huge $(\ensuremath{\sim}1%)$ concentration of native defects. An attractive interaction among the defect pairs is further predicted to lead to a crystallographic ordering of the pairs, explaining the observed, but hitherto surprising, structures CuI${\mathrm{n}}_{5}$S${\mathrm{e}}_{8}$, CuI${\mathrm{n}}_{3}$S${\mathrm{e}}_{5}$, C${\mathrm{u}}_{2}$I${\mathrm{n}}_{4}$S${\mathrm{e}}_{7}$, etc.

Nuclear incompressibility and density dependent NN interactions in the folding model for nucleus-nucleus potentials

Abstract: A generalized version of density dependence has been introduced into the M3Y effective nucleon-nucleon $(\mathrm{NN})$ interaction that was based on the $G$-matrix elements of the Paris $\mathrm{NN}$ potential. The density dependent parameters have been chosen to reproduce the saturation binding energy and density of normal nuclear matter within a Hartree-Fock scheme, but with various values for the corresponding nuclear incompressibility $K$ ranging from 176 to 270 MeV. We use these new density dependent interactions in the folding model to calculate the real parts of $\ensuremath{\alpha}$-nucleus and nucleus-nucleus optical potentials for those systems where strongly refractive scattering patterns have been observed. These provide some information on the potentials at short distances, where there is a strong overlap of the projectile and target density distributions, and hence where the density dependence of the interaction plays an important role. We try to infer, from careful optical model (OM) analyses, the sensitivity of the scattering data to different $K$ values. Results obtained for elastic $\ensuremath{\alpha}$ scattering on targets ranging from ${}^{12}$C to ${}^{208}$Pb allow us to determine unambiguously that the $K$ value favored in this approach is within the range of 240 to 270 MeV. Similar OM analyses have also been done on measurements of the elastic scattering of ${}^{12}$C+${}^{12}$C, ${}^{16}$O+${}^{12}$C, and ${}^{16}$O+${}^{16}$O at incident energies up to 94 MeV/nucleon. These data were found to be much less sensitive over such a narrow range of $K$ values. This lack of sensitivity is due mainly to the smaller maximum overlap density which occurs for these systems, compared to that which is formed in an $\ensuremath{\alpha}$-nucleus collision. This makes the effects of density dependence less substantial. Another reason is that a small difference between two folded heavy ion potentials can often be compensated for, in part, by a small overall renormalization of one of them. This renormalization is often allowed in optical model analyses, and interpreted, for example, as accounting for a contribution from a higher-order dynamic polarization potential. In an attempt to avoid this ambiguity, some OM analyses of the extensive and accurate data for ${}^{16}$O+${}^{16}$O scattering were done using the unrenormalized folded potentials, together with the explicit addition of a correction term, expressed in terms of cubic splines. This correction term can be interpreted as representing a contribution to the real potential from the dynamic polarization potential. The results of such a ``folding+spline'' analysis suggest a tendency to favor the same $K$ value range that was found in the OM analyses of $\ensuremath{\alpha}$-nucleus scattering.

Short Pulse X-Ray Laser at 32.6 nm Based on Transient Gain in Ne-like Titanium

Abstract: A novel two-step excitation scheme for an efficient table-top x-ray laser has been realized for the first time. A nanosecond pulse creates a plasma of neonlike ions of titanium, followed by a subpicosecond pulse which excites a nonstationary population inversion. With only a few joules of pump energy, a compact x-ray laser at 32.6 nm with a very high gain coefficient of $g\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}19{\mathrm{cm}}^{\ensuremath{-}1}$ and a gain-length product of $gL\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}9.5$ was achieved.

Band structure and transport properties of CrO 2

Abstract: The local-spin-density approximation is used to calculate the energy bands of both the ferromagnetic and paramagnetic phases of metallic ${\mathrm{CrO}}_{2}$. The Fermi level lies in a peak in the paramagnetic density of states, and the ferromagnetic phase is more stable. As predicted by Schwarz, the Fermi level lies in an insulating gap in the minority-spin bands between oxygen p and chromium d states (``half-metallic'' behavior). The resulting magnetic moment is 2${\mathrm{\ensuremath{\mu}}}_{\mathrm{B}}$ per Cr atom, in agreement with experiment. The ${\mathrm{A}}_{1\mathrm{g}}$ Raman frequency is predicted to be 587 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. Drude plasma frequencies are of order 2 eV, as seen experimentally by Chase. The measured resistivity is used to find the electron mean free path \ensuremath{\ell}, which is only a few angstroms at 600 K, but, nevertheless, resistivity continues to rise as temperature increases. This puts ${\mathrm{CrO}}_{2}$ into the category of ``bad metals'' in common with the high-${\mathrm{T}}_{\mathrm{c}}$ superconductors, the high-T metallic phase of ${\mathrm{VO}}_{2}$, and the ferromagnet ${\mathrm{SrRuO}}_{3}$. In common with both ${\mathrm{SrRuO}}_{3}$ and ${\mathrm{Sr}}_{2}$${\mathrm{RuO}}_{4}$, the measured specific-heat parameter \ensuremath{\gamma} is higher than band theory predicts by a renormalization factor close to 4.

Dust acoustic waves in strongly coupled dusty plasmas

Abstract: Dust grains, or solid particles of \ensuremath{\mu}m to sub-\ensuremath{\mu}m sizes, are observed in various low-temperature laboratory plasmas such as process plasmas and dust plasma crystals. The massive dust grains are generally highly charged, and it has been shown within the context of standard plasma theory that their presence can lead to new low-frequency modes such as dust acoustic waves. In certain laboratory plasmas, however, the dust may be strongly coupled, as characterized by the condition ${\ensuremath{\Gamma}}_{d}{=Q}_{d}^{2}\mathrm{exp}(\ensuremath{-}d/{\ensuremath{\lambda}}_{D}{)/dT}_{d}g~1,$ where ${Q}_{d}$ is the dust charge, $d$ is the intergrain spacing, ${T}_{d}$ is the dust thermal energy, and ${\ensuremath{\lambda}}_{D}$ is the plasma screening length. This paper investigates the dispersion relation for dust acoustic waves in a strongly coupled dusty plasma comprised of strongly coupled negatively charged dust grains, and weakly correlated classical ions and electrons. The dust grains are assumed to interact via a (screened Coulomb) Yukawa potential. The strongly coupled gas phase (liquid phase) is considered, and a quasilocalized charge approximation scheme is used, generalized to take into account electron and/or ion screening of the dust grains. The scheme relates the small-$k$ dispersion to the total correlation energy of the system, which is obtained from the results of published numerical simulations. Some effects of collisions of charged particles with neutrals are taken into account. Applications to laboratory dusty plasmas are discussed.

Electron Acceleration by a Laser Wakefield in a Relativistically Self-Guided Channel

Abstract: Acceleration of electrons to relativistic energies by a multidimensional self-modulated laser wakefield is discussed. Above a power threshold, a relativistically self-guided channel from an intense ultrashort laser pulse ( $I\ensuremath{\sim}4\ifmmode\times\else\texttimes\fi{}{10}^{18}\mathrm{W}/{\mathrm{cm}}^{2}$, $\ensuremath{\lambda}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1\ensuremath{\mu}\mathrm{m}$, $\ensuremath{\tau}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}400\mathrm{fs}$) was found to increase the laser propagation distance, decrease the electron beam divergence, and increase the energy of the electrons. These electron beam effects occurred even though the propagation distance became significantly longer than the conventional dephasing length.

Variation of electronic structure in La 1-x Sr x MnO 3 f(0<=x<=0.3) as investigated by optical conductivity spectra

Abstract: Optical conductivity spectra and their variation with temperature and doping level x have been investigated for single crystals of ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{MnO}}_{3}$n(0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.3). For the low-doped insulating crystal (x=0.1) which shows a ferromagnetic insulating state at low temperature, the spectral weight of the optical conductivity increases only in the inner-gap region around 0.5 eV, but no Drude part emerges due to carrier localization effect. For x\ensuremath{\geqslant}0.17, where the low-temperature ferromagnetic metallic state shows up, the optical conductivity spectrum above ${\mathrm{T}}_{\mathrm{c}}$ is characterized by interband transitions between the exchange-split conduction bands, and it gradually changes into that of intraband excitations below ${\mathrm{T}}_{\mathrm{c}}$. The energy scale (up to \ensuremath{\approx}2 eV) of the spectral weight transfer is determined by the effective Hund's-rule coupling energy. In the metallic phase, low-energy spectra arising from intraband excitations can be sorted into two parts: One is a nearly \ensuremath{\omega}-independent broad structure (incoherent part), and the other a sharp coherent Drude peak with anomalously low spectral weight. This can hardly be reconciled with the simple double-exchange theory, but indicates that another degree of freedom (e.g., the orbital ordering and/or electron-lattice interactions) should be taken into account.

Single top quark production via $W$ - gluon fusion at next-to-leading order

Abstract: Single-top-quark production via $W$-gluon fusion at hadron colliders provides an opportunity to directly probe the charged-current interaction of the top quark. We calculate the next-to-leading-order corrections to this process at the Fermilab Tevatron, the CERN Large Hadron Collider, and DESY HERA. Using a $b$-quark distribution function to sum collinear logarithms, we show that there are two independent corrections, of order $1/\mathrm{ln}{(m}_{t}^{2}{/m}_{b}^{2})$ and ${\ensuremath{\alpha}}_{s}.$ This observation is generic to processes involving a perturbatively derived heavy-quark distribution function at an energy scale large compared with the heavy-quark mass.

Distribution network reconfiguration for energy loss reduction

Abstract: A new method for energy loss reduction in distribution networks is presented It is based on known techniques and algorithms for radial network analysis-oriented element ordering, power summation method for power flow, statistical representation of load variations and a recently developed energy summation method for the computation of energy losses These methods, combined with the heuristic rules developed to lead the iterative process, make the energy loss minimization method effective, robust and fast It presents an alternative to the power minimization methods for operation and planning purposes

The maximum speed of dynamical evolution

Abstract: We discuss the problem of counting the maximum number of distinct states that an isolated physical system can pass through in a given period of time---its maximum speed of dynamical evolution. Previous analyses have given bounds in terms of the standard deviation of the energy of the system; here we give a strict bound that depends only on E-E0, the system's average energy minus its ground state energy. We also discuss bounds on information processing rates implied by our bound on the speed of dynamical evolution. For example, adding one Joule of energy to a given computer can never increase its processing rate by more than about 3x10^33 operations per second.

General relativistic energy conditions: The Hubble expansion in the epoch of galaxy formation

Abstract: The energy conditions of Einstein gravity (classical general relativity) are designed to extract as much information as possible from classical general relativity without enforcing a particular equation of state for the stress energy. This systematic avoidance of the need to specify a particular equation of state is particularly useful in a cosmological setting {emdash} since the equation of state for the cosmological fluid in a Friedmann-Robertson-Walker-type universe is extremely uncertain. I shall show that the energy conditions provide simple and robust bounds on the behavior of both the density and look-back time as a function of redshift. I shall show that current observations {ital suggest} that the so-called {ital strong energy condition} (SEC) is violated at some time between the epoch of galaxy formation and the present. This implies that no possible combination of {ital {open_quotes}normal{close_quotes}} matter is capable of fitting the observational data. {copyright} {ital 1997} {ital The American Physical Society}

Quantum mechanical time-delay matrix in chaotic scattering.

Abstract: We calculate the probability distribution of the matrix $Q\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{-}i\ensuremath{\Elzxh}{S}^{\ensuremath{-}1}\ensuremath{\partial}S/\ensuremath{\partial}E$ for a chaotic system with scattering matrix $S$ at energy $E$. The eigenvalues ${\ensuremath{\tau}}_{j}$ of $Q$ are the so-called proper delay times, introduced by Wigner and Smith to describe the time dependence of a scattering process. The distribution of the inverse delay times turns out to be given by the Laguerre ensemble from random-matrix theory.

High-resolution threshold photodetachment spectroscopy of OH–

Abstract: Relative photodetachment cross sections for rotational thresholds of the hydroxide anion were measured using a coaxial laser-ion beam spectrometer. The thresholds correspond to transitions from the J=1 and J=2 states of X${\mathrm{}}^{1}$${\mathrm{\ensuremath{\Sigma}}}^{+}$ ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$ to the J=3/2 state of X${\mathrm{}}^{2}$${\mathrm{\ensuremath{\Pi}}}_{3\mathrm{/}2}$ OH. Best fits of the data give cross sections that scale with energy above threshold as (E-${\mathrm{E}}_{\mathrm{th}}$${)}^{0.18\ifmmode\pm\else\textpm\fi{}0.06}$ and (E-${\mathrm{E}}_{\mathrm{th}}$${)}^{0.21\ifmmode\pm\else\textpm\fi{}0.05}$ for the two thresholds, respectively. This non-Wigner law energy dependence of the cross section results from the long-range ${\mathrm{r}}^{\mathrm{\ensuremath{-}}2}$ interaction between the dipole and the departing electron. Experimental results were compared with the predictions of a strong-coupling model of charge-dipole interaction devised by Engelking and good qualitative agreement was observed. When the resolution and range of the data for transitions that terminate in the lower \ensuremath{\Lambda}-doublet component is such that the \ensuremath{\Lambda}-doublet splitting is resolved, one observes the (E-${\mathrm{E}}_{\mathrm{th}}$${)}^{1\mathrm{/}2}$ behavior predicted by the Wigner law. This result is explained in terms of the effect of the \ensuremath{\Lambda} doubling upon the ${\mathrm{r}}^{\mathrm{\ensuremath{-}}2}$ charge-dipole interaction. From the threshold frequencies, an OH electron affinity of 14 741.02(3) ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ was determined.

Magnetic Excitations in the S = 1/2 Alternating Chain Compound \(VO\) 2 P 2 O 7

Abstract: Magnetic excitations in an array of $(\mathrm{VO}{)}_{2}{\mathrm{P}}_{2}{\mathrm{O}}_{7}$ single crystals have been measured using inelastic neutron scattering. Until now, $(\mathrm{VO}{)}_{2}{\mathrm{P}}_{2}{\mathrm{O}}_{7}$ has been thought of as a two-leg antiferromagnetic Heisenberg spin ladder with chains running in the $a$ direction. The present results show unequivocally that $(\mathrm{VO}{)}_{2}{\mathrm{P}}_{2}{\mathrm{O}}_{7}$ is best described as an alternating spin chain directed along the crystallographic $b$ direction. In addition to the expected magnon with magnetic zone-center energy gap $\ensuremath{\Delta}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3.1\mathrm{meV}$, a second excitation is observed at an energy just below $2\ensuremath{\Delta}$. The higher mode may be a triplet two-magnon bound state.

Speech transformation using log energy and orthogonal matrix

Abstract: Calculate the log frame energy value of each of a pre-determined number n of frames of an input speech signal and apply a matrix transform to the n log frame energy values to form a temporal matrix representing the input speech signal. The matrix transform may be a discrete cosine transform.

Generation mechanism of a dipole field by a magnetohydrodynamic dynamo

Abstract: Computer simulation of a magnetohydrodynamic dynamo in a rotating spherical shell is performed. A strong magnetic field is generated by thermal convection motion of an electrically conducting fluid. The generated magnetic field has a well-organized structure. The field is expanded by the spherical harmonics ${\mathrm{Y}}_{\mathrm{\ensuremath{\ell}}}^{\mathrm{m}}$. The leading moment is the dipole (\ensuremath{\ell}=1,m=0) which is 1.3 times larger than the second moment, octapole (\ensuremath{\ell}=3,m=0). These two moments occupy more than half of the total power of the magnetic field. Structure and the generation mechanism of the magnetic field are examined in detail. It is found that the well-known \ensuremath{\alpha}-\ensuremath{\omega} dynamo scenario gives a good explanation of the whole dynamo cycle.

Determination of electronic band structures of CaMnO 3 and LaMnO 3 using optical-conductivity analyses

Abstract: Reflectivity spectra of ${\mathrm{CaMnO}}_{3}$ and ${\mathrm{LaMnO}}_{3}$ were measured in a wide photon energy region between 5 meV and 30 eV at room temperature. Using the conductivity spectra obtained from the Kramers-Kronig analysis, electronic structures of the manganese oxides were investigated. In particular, the states near the Fermi energy ${\mathrm{E}}_{\mathrm{F}}$, O 2p and Mn 3d (${\mathrm{e}}_{\mathrm{g}}$,${\mathrm{t}}_{2\mathrm{g}}$), were studied in detail. It was found that the O 2p band is located closer to ${\mathrm{E}}_{\mathrm{F}}$ than the Mn ${\mathrm{t}}_{2\mathrm{g}}$ band. For ${\mathrm{LaMnO}}_{3}$, the filled Mn ${\mathrm{e}}_{\mathrm{g}\mathrm{\ensuremath{\uparrow}}}^{1}$ band is located closer to ${\mathrm{E}}_{\mathrm{F}}$ than any other valence band. The ${\mathrm{t}}_{2\mathrm{g}}$-${\mathrm{e}}_{\mathrm{g}}$ Hund exchange energy was found to be about 3.4 eV and the Jahn-Teller stabilization energy ${\mathrm{E}}_{0}$ was estimated to be less than 0.5 eV.

Resonant terahertz optical sideband generation from confined magnetoexcitons

Abstract: We have probed the internal structure and nonlinear response of magnetoexcitons in GaAs/AlGaAs quantum wells by resonantly driving one- and two-photon internal transitions with intense terahertz electric fields. Strong near-band-gap emission lines, or optical sidebands, appear at frequencies ${\ensuremath{\omega}}_{\mathrm{NIR}}\ifmmode\pm\else\textpm\fi{}2n{\ensuremath{\omega}}_{\mathrm{THz}}$, where ${\ensuremath{\omega}}_{\mathrm{NIR}}$ is the interband exciton-creation frequency, ${\ensuremath{\omega}}_{\mathrm{THz}}$ is the frequency of the driving field, and $n$ is an integer. The intensity of the sidebands exhibits pronounced enhancement when ${\ensuremath{\omega}}_{\mathrm{THz}}$ coincides with transitions between magnetically tuned energy levels in the excitons, providing new and accurate information on the internal dynamics of excitons.

A New Algorithm For Computing Liquid Crystal Stable Configurations: The Harmonic Mapping Case

Abstract: In this article, we propose a new algorithm for minimizing the energy of a nematic liquid crystal. Based on the equal elastic constants Oseen--Frank model, the problem reduces to finding harmonic minimizing maps that take values into the unit sphere of ${\Bbb R}^{3}$. The convergence of this algorithm is proved in a continuous setting. Then, numerous numerical results that show its efficiency are given.

System and method for energy measurement and verification with constant baseline reference

Abstract: Disclosed is an energy measurement and verification system and method which provides real time documentation of both baseline and post-retrofit energy consumption (58a, 58b). Actual energy savings are calculated with a unique combination of computer hardware, energy and data translation software, measurement protocols, and digital remote sensing equipment which monitor the energy consumption of the retrofit environment and a sample baseline environment (59).

Direct observation of the LO phonon bottleneck in wide GaAs/AlxGa1-xAs quantum wells

Abstract: We report the direct observation of a bottleneck in electron cooling in wide GaAs quantum wells. Intersubband lifetimes \ensuremath{\tau} and their dependence on intensity, lattice temperature ${\mathrm{T}}_{\mathrm{L}}$ , and well width have been measured using a ps excite-probe technique in wells with subband separation less than the longitudinal optical (LO) phonon energy. Above an electron temperature of about ${\mathrm{T}}_{\mathrm{e}}$ =35 K the lifetime depends on ${\mathrm{T}}_{\mathrm{e}}$ and is determined by LO-phonon emission. Below this bottleneck temperature acoustic phonons dominate the plasma cooling. An energy balance model of these interactions, with no adjustable parameters, gives good agreement with our results. At electron temperatures below 35 K we determine \ensuremath{\tau}=500 and 200 ps for samples of subband energy 19.5 and 26.6 meV, respectively.

Yang-Mills radiation in ultrarelativistic nuclear collisions

Abstract: The classical Yang-Mills radiation computed in the McLerran-Venugopalan model is shown to be equivalent to the gluon bremsstrahlung distribution to lowest (g{sup 6}) order in pQCD. The classical distribution is also shown to match smoothly onto the conventional pQCD minijet distribution at a scale k{sub {perpendicular}}{sup 2}{approximately}{chi}, characteristic of the initial parton transverse density of the system. The atomic number and energy dependence of {chi} is computed from available structure function information. The limits of applicability of the classical Yang-Mills description of nuclear collisions at RHIC and LHC energies are discussed. {copyright} {ital 1997} {ital The American Physical Society}

Free energy calculation methods: A theoretical and empirical comparison of numerical errors and a new method qualitative estimates of free energy changes

Abstract: We present a comparison of four free energy calculation methods: thermodynamic integration (TI); traditional free energy perturbation (FEP); Bennett's acceptance ratio method (IPS); and a method that is related to an implementation of the WHAM method (CRS). The theoretical bases of the methods are first described, then calculations of the solvation free energies of methane and ethane are performed to determine the magnitude of the errors for the different methods. We find that the methods give similar errors when many intermediate states (windows) are used, but the IPS and CRS methods give smaller errors than the TI and FEP methods when no intermediate states are used. We also present a new procedure (based on the CRS method) that uses coordinates from simulations of a set of solutes to calculate the salvation free energies of additional solutes for which no simulations were performed. Solvation free energies for nine solutes (methanol, dimethylether, methylamine, methylammonium, dimethylamine, fluoromethane, difluoromethane, trifluoromethane, and tetrafluoromethane) are estimated based only on simulations of set of small hydrophobic solutes (including methane, ethane, and propane). These estimates can be surprisingly accurate and appear to be useful for making rapid estimates of solvation free energies. © 1997 by John Wiley & Sons, Inc. J Comput Chem 18: 902–919, 1997