2,728 research outputs found

    Wide-angle display developments by computer graphics

    Get PDF
    Computer graphics can now expand its new subset, wide-angle projection, to be as significant a generic capability as computer graphics itself. Some prior work in computer graphics is presented which leads to an attractive further subset of wide-angle projection, called hemispheric projection, to be a major communication media. Hemispheric film systems have long been present and such computer graphics systems are in use in simulators. This is the leading edge of capabilities which should ultimately be as ubiquitous as CRTs (cathode-ray tubes). These assertions are not from degrees in science or only from a degree in graphic design, but in a history of computer graphics innovations, laying groundwork by demonstration. The author believes that it is timely to look at several development strategies, since hemispheric projection is now at a point comparable to the early stages of computer graphics, requiring similar patterns of development again

    Excited states of a static dilute spherical Bose condensate in a trap

    Full text link
    The Bogoliubov approximation is used to study the excited states of a dilute gas of NN atomic bosons trapped in an isotropic harmonic potential characterized by a frequency ω0\omega_0 and an oscillator length d0=/mω0d_0 = \sqrt{\hbar/m\omega_0}. The self-consistent static Bose condensate has macroscopic occupation number N01N_0 \gg 1, with nonuniform spherical condensate density n0(r)n_0(r); by assumption, the depletion of the condensate is small (NNN0N0N' \equiv N - N_0\ll N_0). The linearized density fluctuation operator ρ^\hat \rho' and velocity potential operator Φ^\hat\Phi' satisfy coupled equations that embody particle conservation and Bernoulli's theorem. For each angular momentum ll, introduction of quasiparticle operators yields coupled eigenvalue equations for the excited states; they can be expressed either in terms of Bogoliubov coherence amplitudes ul(r)u_l(r) and vl(r)v_l(r) that determine the appropriate linear combinations of particle operators, or in terms of hydrodynamic amplitudes ρl(r)\rho_l'(r) and Φl(r)\Phi_l'(r). The hydrodynamic picture suggests a simple variational approximation for l>0l >0 that provides an upper bound for the lowest eigenvalue ωl\omega_l and an estimate for the corresponding zero-temperature occupation number NlN_l'; both expressions closely resemble those for a uniform bulk Bose condensate.Comment: 5 pages, RevTeX, contributed paper accepted for Low Temperature Conference, LT21, August, 199

    Quantum Monte Carlo study of dilute neutron matter at finite temperatures

    Full text link
    We report results of fully non-perturbative, Path Integral Monte Carlo (PIMC) calculations for dilute neutron matter. The neutron-neutron interaction in the s channel is parameterized by the scattering length and the effective range. We calculate the energy and the chemical potential as a function of temperature at the density \dens=0.003\fm^{-3}. The critical temperature \Tc for the superfluid-normal phase transition is estimated from the finite size scaling of the condensate fraction. At low temperatures we extract the spectral weight function A(p,ω)A(p,\omega) from the imaginary time propagator using the methods of maximum entropy and singular value decomposition. We determine the quasiparticle spectrum, which can be accurately parameterized by three parameters: an effective mass mm^*, a mean-field potential UU, and a gap Δ\Delta. Large value of \Delta/\Tc indicates that the system is not a BCS-type superfluid at low temperatures.Comment: 4 pages, 3 figure

    Variation in Organic Standards Prior to the National Organic Program

    Get PDF
    Interest in establishing nationally uniform certification, labeling, and management standards for organic products grew out of concern that the existence of multiple standards led to consumer and supply chain confusion about, and lack of confidence in, these products. The National Organic Program Final Rule, issued in December 2000, is the result of this interest. We analyze the certification system that was in place prior to the new national rule to evaluate the extent of differences between certification standards and how the national rule is likely to impact the market for organic products. Our analysis suggests that most differences among US certification standards were minor. Also, the most important impacts of the national standard may be in facilitating trade in ingredients and products certified by different certifiers, increasing buyer confidence, and facilitating exports. However, the national rule may decrease the ability of organic certifiers and consumers to place differing emphasis on the multiple goals of organic production and may decrease the flexibility of organic standards to respond to changing market conditions, including new technologies.organic agriculture, organic certification standards, organic labeling, organic market, Agribusiness, Marketing,

    Thermodynamic properties of a dipolar Fermi gas

    Full text link
    Based on the semi-classical theory, we investigate the thermodynamic properties of a dipolar Fermi gas. Through a self-consistent procedure, we numerically obtain the phase space distribution function at finite temperature. We show that the deformations in both momentum and real space becomes smaller and smaller as one increases the temperature. For homogeneous case, we also calculate pressure, entropy, and heat capacity. In particular, at low temperature limit and in weak interaction regime, we obtain an analytic expression for the entropy, which agrees qualitatively with our numerical result. The stability of a trapped gas at finite temperature is also explored

    Asymmetric Two-component Fermion Systems in Strong Coupling

    Full text link
    We study the phase structure of a dilute two-component Fermi system with attractive interactions as a function of the coupling and the polarization or number difference between the two components. In weak coupling, a finite number asymmetry results in phase separation. A mixed phase containing symmetric superfluid matter and an asymmetric normal phase is favored. With increasing coupling strength, we show that the stress on the superfluid phase to accommodate a number asymmetry increases. Near the infinite-scattering length limit, we calculate the single-particle excitation spectrum and the ground-state energy at various polarizations. A picture of weakly-interacting quasi-particles emerges for modest polarizations. In this regime near infinite scattering length, and for modest polarizations, a homogeneous phase with a finite population of excited quasi-particle states characterized by a gapless spectrum should be favored over the phase separated state. These states may be realized in cold atom experiments.Comment: 4 pages, 3 figur

    Spin-dependent Hedin's equations

    Full text link
    Hedin's equations for the electron self-energy and the vertex were originally derived for a many-electron system with Coulomb interaction. In recent years it has been increasingly recognized that spin interactions can play a major role in determining physical properties of systems such as nanoscale magnets or of interfaces and surfaces. We derive a generalized set of Hedin's equations for quantum many-body systems containing spin interactions, e.g. spin-orbit and spin-spin interactions. The corresponding spin-dependent GW approximation is constructed.Comment: 5 pages, 1 figur
    corecore