2,728 research outputs found

### Wide-angle display developments by computer graphics

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

The Bogoliubov approximation is used to study the excited states of a dilute
gas of $N$ atomic bosons trapped in an isotropic harmonic potential
characterized by a frequency $\omega_0$ and an oscillator length $d_0 =
\sqrt{\hbar/m\omega_0}$. The self-consistent static Bose condensate has
macroscopic occupation number $N_0 \gg 1$, with nonuniform spherical condensate
density $n_0(r)$; by assumption, the depletion of the condensate is small ($N'
\equiv N - N_0\ll N_0$). The linearized density fluctuation operator $\hat
\rho'$ and velocity potential operator $\hat\Phi'$ satisfy coupled equations
that embody particle conservation and Bernoulli's theorem. For each angular
momentum $l$, introduction of quasiparticle operators yields coupled eigenvalue
equations for the excited states; they can be expressed either in terms of
Bogoliubov coherence amplitudes $u_l(r)$ and $v_l(r)$ that determine the
appropriate linear combinations of particle operators, or in terms of
hydrodynamic amplitudes $\rho_l'(r)$ and $\Phi_l'(r)$. The hydrodynamic picture
suggests a simple variational approximation for $l >0$ that provides an upper
bound for the lowest eigenvalue $\omega_l$ and an estimate for the
corresponding zero-temperature occupation number $N_l'$; 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

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 $A(p,\omega)$ 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 $m^*$, a mean-field potential $U$, and a gap
$\Delta$. 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

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

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

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

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

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