23,908 research outputs found
Radio wave propagation experiments to probe the ionosphere
Ionospheric bias corrections associated with radio tracking of spacecraft depend on the following measuring techniques for integrated electron content: (1) Faraday rotation measurements from an earth synchronous satellite; (2) ranging measurements at two frequencies; and (3) group and phase velocity measurements obtained from tracking data. The extraction of the integrated electron content directly from tracking data is achieved by comparison of range-rate measurements based on Doppler shift with differentiated range measurements based on tone delay. This method is most desirable because the measured corrections pertain directly to the spacecraft whose orbit is being determined and can be used in near earth as well as deep space tracking data
Lunar far-side communication satellites
Data relay and tracking capability of lunar communication satellite
Mirages, anti-mirages, and further surprises in quantum corrals with non-magnetic impurities
We investigate the local density of states (LDOS) for non-interacting
electrons in a hard wall ellipse in the presence of a single non-magnetic
scattering center. Using a T-matrix analysis we calculate the local Green's
function and observe a variety of quantum mirage effects for different impurity
positions. Locating the impurity near positions with LDOS maxima for the
impurity free corral can either lead to a reduction or an enhancement of the
LDOS at the mirror image point, i.e. a mirage or anti-mirage effect, or even
suppress LDOS maxima in the entire area of the corral.Comment: 6 pages, 7 figure
Controlling diffusive transport in confined geometries
We analyze the diffusive transport of Brownian particles in narrow channels
with periodically varying cross-section. The geometrical confinements lead to
entropic barriers, the particle has to overcome in order to proceed in
transport direction. The transport characteristics exhibit peculiar behaviors
which are in contrast to what is observed for the transport in potentials with
purely energetic barriers. By adjusting the geometric parameters of the channel
one can effectively tune the transport and diffusion properties. A prominent
example is the maximized enhancement of diffusion for particular channel
parameters. The understanding of the role of channel-shape provides the
possibility for a design of stylized channels wherein the quality of the
transport can be efficiently optimized.Comment: accepted for publication in Acta Physica Polonica
Silicon ingot casting: Heat exchanger method. Multi-wire slicing: Fixed abrasive slicing technique, phase 3
In the area of ingot casting the proof of concept of heat exchanger method (HEM) was established. It was also established that HEM cast silicon yielded solar cell performance comparable to Czochralski grown material. Solar cells with conversion efficiencies of up to 15% were fabricated. It was shown that square cross-section ingots can be cast. In the area of crystal slicing, it was established that silicon can be sliced efficiently with the fixed abrasive slicing technique approach. This concept was carried forward to 10 cm diameter workpiece
Silicon ingot casting: Heat Exchange Method (HEM). Multi-wire slicing: Fixed Abrasive Slicing Technique (FAST). Phase 3 and phase 4: Silicon sheet growth development for the large area sheet task of the low-cost solar array project
Several areas of silicon sheet growth development are addressed including: silicon ingot casting, heat exchanger method, multiwire slicing, and fixed abrasive slicing technique
Disorder-Induced Static Antiferromagnetism in Cuprate Superconductors
Using model calculations of a disordered d-wave superconductor with on-site
Hubbard repulsion, we show how dopant disorder can stabilize novel states with
antiferromagnetic order. We find that the critical strength of correlations or
impurity potential necessary to create an ordered magnetic state in the
presence of finite disorder is reduced compared to that required to create a
single isolated magnetic droplet. This may explain why in cuprates like LSCO
low-energy probes have identified a static magnetic component which persists
well into the superconducting state, whereas in cleaner systems like YBCO it is
absent or minimal. Finally we address the case of nominally clean LSCO samples
at optimal doping, where such ordered magnetic moments are absent, but where
they can be induced by small concentrations of strong scatterers.Comment: 4 pages, 5 figure
A variational framework for flow optimization using semi-norm constraints
When considering a general system of equations describing the space-time
evolution (flow) of one or several variables, the problem of the optimization
over a finite period of time of a measure of the state variable at the final
time is a problem of great interest in many fields. Methods already exist in
order to solve this kind of optimization problem, but sometimes fail when the
constraint bounding the state vector at the initial time is not a norm, meaning
that some part of the state vector remains unbounded and might cause the
optimization procedure to diverge. In order to regularize this problem, we
propose a general method which extends the existing optimization framework in a
self-consistent manner. We first derive this framework extension, and then
apply it to a problem of interest. Our demonstration problem considers the
transient stability properties of a one-dimensional (in space) averaged
turbulent model with a space- and time-dependent model "turbulent viscosity".
We believe this work has a lot of potential applications in the fluid
dynamics domain for problems in which we want to control the influence of
separate components of the state vector in the optimization process.Comment: 30 page
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