4,994 research outputs found
Shuttle STS-2 mission communication systems RF coverage and performance predictions. Volume 1: Ascent
The RF communications capabilities and nominally expected performance for the ascent phase of the second orbital flight of the shuttle are provided. Predicted performance is given mainly in the form of plots of signal strength versus elapsed mission time for the STDN (downlink) and shuttle orbiter (uplink) receivers for the S-band PM and FM, and UHF systems. Performance of the NAV and landing RF systems is treated for RTLS abort, since in this case the spacecraft will loop around and return to the launch site. NAV and landing RF systems include TACAN, MSBLS, and C-band altimeter. Signal strength plots were produced by a computer program which combines the spacecraft trajectory, antenna patterns, transmit and receive performance characteristics, and system mathematical models. When available, measured spacecraft parameters were used in the predictions; otherwise, specified values were used. Specified ground station parameter values were also used. Thresholds and other criteria on the graphs are explained
Pairing Correlations in the Two-Dimensional Hubbard Model
We present the results of a quantum Monte Carlo study of the extended and
the pairing correlation functions for the two-dimensional Hubbard
model, computed with the constrained-path method. For small lattice sizes and
weak interactions, we find that the pairing correlations are
stronger than the extended pairing correlations and are positive when the
pair separation exceeds several lattice constants. As the system size or the
interaction strength increases, the magnitude of the long-range part of both
correlation functions vanishes.Comment: 4 pages, RevTex, 4 figures included; submitted to Phys. Rev. Let
A Constrained Path Quantum Monte Carlo Method for Fermion Ground States
We propose a new quantum Monte Carlo algorithm to compute fermion
ground-state properties. The ground state is projected from an initial
wavefunction by a branching random walk in an over-complete basis space of
Slater determinants. By constraining the determinants according to a trial
wavefunction , we remove the exponential decay of
signal-to-noise ratio characteristic of the sign problem. The method is
variational and is exact if is exact. We report results on the
two-dimensional Hubbard model up to size , for various electron
fillings and interaction strengths.Comment: uuencoded compressed postscript file. 5 pages with 1 figure. accepted
by PRL
Thermal transport in a granular metal array
We obtain the Kubo formula for the electronic thermal conductivity kappa(T)
of a granular metal array at low temperatures for the Ambegaokar-Eckern-Schoen
(AES) model and study the kinetic and potential contributions in the
diamagnetic (local) and paramagnetic (current-current) terms. For small values
of dimensionless intergrain tunneling conductance, g << 1, we show that
inelastic cotunneling processes contribute to thermal conductivity due to
non-cancellation of the diamagnetic and paramagnetic terms, unlike electrical
conductivity. We find that the electrical conductivity obeys the Arrhenius law,
sigma(T) ~ ge^{-E_c/T}, however kappa(T) decreases only algebraically, kappa(T)
\~ g^2 T^3/E_c^2. At large values of intergrain coupling, g >> 1, we find it
plausible that the Wiedemann-Franz law weakly deviates from the free-electron
theory due to Coulomb effects.Comment: 5 pages RevTeX, to appear in Physical Review Letter
Zone Determinant Expansions for Nuclear Lattice Simulations
We introduce a new approximation to nucleon matrix determinants that is
physically motivated by chiral effective theory. The method involves breaking
the lattice into spatial zones and expanding the determinant in powers of the
boundary hopping parameter.Comment: 20 pages, 6 figures, revtex4 (version to appear in PRC
Theory of monolayers with boundaries: Exact results and Perturbative analysis
Domains and bubbles in tilted phases of Langmuir monolayers contain a class
of textures knows as boojums. The boundaries of such domains and bubbles may
display either cusp-like features or indentations. We derive analytic
expressions for the textures within domains and surrounding bubbles, and for
the shapes of the boundaries of these regions. The derivation is perturbative
in the deviation of the bounding curve from a circle. This method is not
expected to be accurate when the boundary suffers large distortions, but it
does provide important clues with regard to the influence of various energetic
terms on the order-parameter texture and the shape of the domain or bubble
bounding curve. We also look into the effects of thermal fluctuations, which
include a sample-size-dependent effective line tension.Comment: replaced with published version, 21 pages, 16 figures include
The structure of EAS at E 0.1 EeV
The ratio of extensive air showers (EAS) total shower energy in the electromagnetic channel (E em) to the size of the shower at maximum development (N max) from a direct measurement of shower longitudinal development using the air fluorescence technique was calculated. The values are not inconsistent with values based upon track length integrals of the Gaisser-Hillas formula for shower development or the known relation between shower energy and size at maximum for pure electromagnetic cascades. Using Linsley's estimates for undetected shower energy based on an analysis of a wide variety of cosmic ray data, the following relation for total shower energy E vs N max is obtained. The Gaisser Hillas implied undetected shower energy fractions
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