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 $\rho$ increases toward degenerate values, we observe a gradual divergence of the compressibility $\kappa$ from the value predicted by a bare-atom model, $\kappa_{ba}$. $\kappa/\kappa_{ba}$ grows to 1.7 before $\rho$ reaches the value for which we observe the sudden emergence of a spike at $p=0$ 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, $s(x)$ and $\bar{s}(x)$. To explore the strangeness sector, we adopt a general parametrization of the non-perturbative $s(x), \bar{s}(x)$ functions satisfying basic QCD requirements. We find that the strangeness asymmetry, as represented by the momentum integral $[S^{-}]\equiv \int_0^1 x [s(x)-\bar{s}(x)] dx$, 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 $[S^-]$ and find $-0.001 < [S^-] < 0.004$. Representative parton distribution sets spanning this range are given. Comparisons with previous work are made.Comment: 23 pages, 4 figures; expanded version for publicatio