7,311 research outputs found
Quasi-Local Energy Flux of Spacetime Perturbation
A general expression for quasi-local energy flux for spacetime perturbation
is derived from covariant Hamiltonian formulation using functional
differentiability and symplectic structure invariance, which is independent of
the choice of the canonical variables and the possible boundary terms one
initially puts into the Lagrangian in the diffeomorphism invariant theories.
The energy flux expression depends on a displacement vector field and the
2-surface under consideration. We apply and test the expression in Vaidya
spacetime. At null infinity the expression leads to the Bondi type energy flux
obtained by Lindquist, Schwartz and Misner. On dynamical horizons with a
particular choice of the displacement vector, it gives the area balance law
obtained by Ashtekar and Krishnan.Comment: 8 pages, added appendix, version to appear in Phys. Rev.
Extension of four-dimensional atmospheric models
The cloud data bank, the 4-D atmospheric model, and a set of computer programs designed to simulate meteorological conditions for any location above the earth are described in turns of space vehicle design and simulation of vehicle reentry trajectories. Topics discussed include: the relationship between satellite and surface observed cloud cover using LANDSAT 1 photographs and including the effects of cloud shadows; extension of the 4-D model to the altitude of 52 km; and addition of the u and v wind components to the 4-D model of means and variances at 1 km levels from the surface to 25 km. Results of the cloud cover analysis are presented along with the stratospheric model and the tropospheric wind profiles
Design of multivariable feedback control systems via spectral assignment
Applied research in the area of spectral assignment in multivariable systems is reported. A frequency domain technique for determining the set of all stabilizing controllers for a single feedback loop multivariable system is described. It is shown that decoupling and tracking are achievable using this procedure. The technique is illustrated with a simple example
Observation of the Presuperfluid Regime in a Two-Dimensional Bose Gas
In complementary images of coordinate-space and momentum-space density in a
trapped 2D Bose gas, we observe the emergence of pre-superfluid behavior. As
phase-space density increases toward degenerate values, we observe a
gradual divergence of the compressibility from the value predicted by
a bare-atom model, . grows to 1.7 before
reaches the value for which we observe the sudden emergence of a spike
at in momentum space. Momentum-space images are acquired by means of a 2D
focusing technique. Our data represent the first observation of non-meanfield
physics in the pre-superfluid but degenerate 2D Bose gas.Comment: Replace with the version appeared in PR
The structure of trailing vortices generated by model rotor blades
Hot-wire anemometry to analyze the structure and geometry of rotary wing trailing vortices is studied. Tests cover a range of aspect ratios and blade twist. For all configurations, measured vortex strength correlates well with maximum blade-bound circulation. Measurements of wake geometry are in agreement with classical data for high-aspect ratios. The detailed vortex structure is similar to that found for fixed wings and consists of four well defined regions--a viscous core, a turbulent mixing region, a merging region, and an inviscid outer region. A single set of empirical formulas for the entire set of test data is described
Head-on collisions of binary white dwarf--neutron stars: Simulations in full general relativity
We simulate head-on collisions from rest at large separation of binary white
dwarf -- neutron stars (WDNSs) in full general relativity. Our study serves as
a prelude to our analysis of the circular binary WDNS problem. We focus on
compact binaries whose total mass exceeds the maximum mass that a cold
degenerate star can support, and our goal is to determine the fate of such
systems. A fully general relativistic hydrodynamic computation of a realistic
WDNS head-on collision is prohibitive due to the large range of dynamical time
scales and length scales involved. For this reason, we construct an equation of
state (EOS) which captures the main physical features of NSs while, at the same
time, scales down the size of WDs. We call these scaled-down WD models
"pseudo-WDs (pWDs)". Using pWDs, we can study these systems via a sequence of
simulations where the size of the pWD gradually increases toward the realistic
case. We perform two sets of simulations; One set studies the effects of the NS
mass on the final outcome, when the pWD is kept fixed. The other set studies
the effect of the pWD compaction on the final outcome, when the pWD mass and
the NS are kept fixed. All simulations show that 14%-18% of the initial total
rest mass escapes to infinity. All remnant masses still exceed the maximum rest
mass that our cold EOS can support (1.92 solar masses), but no case leads to
prompt collapse to a black hole. This outcome arises because the final
configurations are hot. All cases settle into spherical, quasiequilibrium
configurations consisting of a cold NS core surrounded by a hot mantle,
resembling Thorne-Zytkow objects. Extrapolating our results to realistic WD
compactions, we predict that the likely outcome of a head-on collision of a
realistic, massive WDNS system will be the formation of a quasiequilibrium
Thorne-Zytkow-like object.Comment: 24 pages, 14 figures, matches PRD published version, tests of HRSC
schemes with piecewise polytropes adde
Efficient nonlinear room-temperature spin injection from ferromagnets into semiconductors through a modified Schottky barrier
We suggest a consistent microscopic theory of spin injection from a
ferromagnet (FM) into a semiconductor (S). It describes tunneling and emission
of electrons through modified FM-S Schottky barrier with an ultrathin heavily
doped interfacial S layer . We calculate nonlinear spin-selective properties of
such a reverse-biased FM-S junction, its nonlinear I-V characteristic, current
saturation, and spin accumulation in S. We show that the spin polarization of
current, spin density, and penetration length increase with the total current
until saturation. We find conditions for most efficient spin injection, which
are opposite to the results of previous works, since the present theory
suggests using a lightly doped resistive semiconductor. It is shown that the
maximal spin polarizations of current and electrons (spin accumulation) can
approach 100% at room temperatures and low current density in a nondegenerate
high-resistance semiconductor.Comment: 7 pages, 2 figures; provides detailed comparison with earlier works
on spin injectio
Neutrino Dimuon Production and the Strangeness Asymmetry of the Nucleon
We have performed the first global QCD analysis to include the CCFR and NuTeV
dimuon data, which provide direct constraints on the strange and anti-strange
parton distributions, and . To explore the strangeness
sector, we adopt a general parametrization of the non-perturbative functions satisfying basic QCD requirements. We find that the
strangeness asymmetry, as represented by the momentum integral , is sensitive to the dimuon data provided the
theoretical QCD constraints are enforced. We use the Lagrange Multiplier method
to probe the quality of the global fit as a function of and find
. Representative parton distribution sets spanning this
range are given. Comparisons with previous work are made.Comment: 23 pages, 4 figures; expanded version for publicatio
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