47,418 research outputs found
LOX/hydrocarbon auxiliary propulsion system study
Liquid oxygen (LOX)/hydrocarbon propulsion concepts for a "second generation' orbiter auxiliary propulsion system was evaluated. The most attractive fuel and system design approach identified, and the technology advancements that are needed to provide high confidence for a subsequent system development were determined. The fuel candidates were ethanol, methane, propane, and ammonia. Even though ammonia is not a hydrocarbon, it was included for evaluation because it is clean burning and has a good technology base. The major system design options were pump versus pressure feed, cryogenic versus ambient temperature RCS propellant feed, and the degree of OMS-RCS integration. Ethanol was determined to be the best fuel candidate. It is an earth-storable fuel with a vapor pressure slightly higher than monomethyl hydrazine. A pump-fed OMS was recommended because of its high specific impulse, enabling greater velocity change and greater payload capability than a pressure fed system
Checkerboard order in the t--J model on the square lattice
We propose that the inhomogeneous patterns seen by STM in some underdoped
superconducting cuprates could be related to a bond-order-wave instability of
the staggered flux state, one of the most studied "normal" state proposed to
compete with the d-wave RVB superconductor. A checkerboard pattern is obtained
by a Gutzwiller renormalized mean-field theory of the t-J model for doping
around 1/8. It is found that the charge modulation is always an order of
magnitude smaller than the bond-order modulations. This is confirmed by an
exact optimization of the wavefunction by a variational Monte Carlo scheme. The
numerical estimates of the order parameters are however found to be strongly
reduced w.r.t their mean-field values
Multispectral scanner data applications evaluation. Volume 1: User applications study
A six-month systems study of earth resource surveys from satellites was conducted and is reported. SKYLAB S-192 multispectral scanner (MSS) data were used as a baseline to aid in evaluating the characteristics of future systems using satellite MSS sensors. The study took the viewpoint that overall system (sensor and processing) characteristics and parameter values should be determined largely by user requirements for automatic information extraction performance in quasi-operational earth resources surveys, the other major factor being hardware limitations imposed by state-of-the-art technology and cost. The objective was to use actual aircraft and spacecraft MSS data to outline parametrically the trade-offs between user performance requirements and hardware performance and limitations so as to allow subsequent evaluation of compromises which must be made in deciding what system(s) to build
Phonon lineshapes in the vortex state of the phonon-mediated superconductor YNiBC
We present an inelastic neutron scattering study of phonon lineshapes in the
vortex state of the type-II superconductor YNiBC. In a previous study
[Phys. Rev. Lett. \textbf{101}, 237002 (2008)] it was shown that certain
phonons exhibit a clear signature of the superconducting gap on
entering the superconducting state. Our interest was to find out whether or not
the lineshape of such phonons reflects the inhomogeneous nature of the vortex
state induced by a magnetic field smaller than the upper critical field
.We found that this is indeed the case because the observed phonon
lineshapes can be well described by a model considering the phonon as a local
probe of the spatial variation of the superconducting gap. We found that even
at , where the inter-vortex distance is less than \AA, the
phonon lineshape still shows evidence for a variation of the gap
Sensitivity of the Moment of Inertia of Neutron Stars to the Equation of State of Neutron-Rich Matter
The sensitivity of the stellar moment of inertia to the neutron-star matter
equation of state is examined using accurately-calibrated relativistic
mean-field models. We probe this sensitivity by tuning both the density
dependence of the symmetry energy and the high density component of the
equation of state, properties that are at present poorly constrained by
existing laboratory data. Particularly attractive is the study of the fraction
of the moment of inertia contained in the solid crust. Analytic treatments of
the crustal moment of inertia reveal a high sensitivity to the transition
pressure at the core-crust interface. This may suggest the existence of a
strong correlation between the density dependence of the symmetry energy and
the crustal moment of inertia. However, no correlation was found. We conclude
that constraining the density dependence of the symmetry energy - through, for
example, the measurement of the neutron skin thickness in 208Pb - will place no
significant bound on either the transition pressure or the crustal moment of
inertia.Comment: 25 pages, 8 figures, 5 table
Nonaxisymmetric Neutral Modes in Rotating Relativistic Stars
We study nonaxisymmetric perturbations of rotating relativistic stars.
modeled as perfect-fluid equilibria. Instability to a mode with angular
dependence sets in when the frequency of the mode vanishes. The
locations of these zero-frequency modes along sequences of rotating stars are
computed in the framework of general relativity. We consider models of
uniformly rotating stars with polytropic equations of state, finding that the
relativistic models are unstable to nonaxisymmetric modes at significantly
smaller values of rotation than in the Newtonian limit. Most strikingly, the
m=2 bar mode can become unstable even for soft polytropes of index , while in Newtonian theory it becomes unstable only for stiff polytropes
of index . If rapidly rotating neutron stars are formed by the
accretion-induced collapse of white dwarfs, instability associated with these
nonaxisymmetric, gravitational-wave driven modes may set an upper limit on
neutron-star rotation. Consideration is restricted to perturbations that
correspond to polar perturbations of a spherical star. A study of axial
perturbations is in progress.Comment: 57 pages, 9 figure
Evidence for crossed Andreev reflection in superconductor-ferromagnet hybrid structures
We have measured the non-local resistance of aluminum-iron spin-valve
structures fabricated by e-beam lithography and shadow evaporation. The sample
geometry consists of an aluminum bar with two or more ferromagnetic wires
forming point contacts to the aluminum at varying distances from each other. In
the normal state of aluminum, we observe a spin-valve signal which allows us to
control the relative orientation of the magnetizations of the ferromagnetic
contacts. In the superconducting state, at low temperatures and excitation
voltages well below the gap, we observe a spin-dependent non-local resistance
which decays on a smaller length scale than the normal-state spin-valve signal.
The sign, magnitude and decay length of this signal is consistent with
predictions made for crossed Andreev reflection (CAR).Comment: RevTeX, 4 page
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