1,770 research outputs found
Small, low power analog-to-digital converter
A small, low-power, high-speed, 8-bit analog-to-digital converter using silicon chip integrated circuits is suitable for use in airborne test data systems. The successive approximation method of analog-to-digital conversion is used to generate the digital output
Time-Resolved Magnetic Relaxation of a Nanomagnet on Subnanosecond Time Scales
We present a two-current-pulse temporal correlation experiment to study the
intrinsic subnanosecond nonequilibrium magnetic dynamics of a nanomagnet during
and following a pulse excitation. This method is applied to a model
spin-transfer system, a spin valve nanopillar with perpendicular magnetic
anisotropy. Two-pulses separated by a short delay (< 500 ps) are shown to lead
to the same switching probability as a single pulse with a duration that
depends on the delay. This demonstrates a remarkable symmetry between magnetic
excitation and relaxation and provides a direct measurement of the magnetic
relaxation time. The results are consistent with a simple finite temperature
Fokker-Planck macrospin model of the dynamics, suggesting more coherent
magnetization dynamics in this short time nonequilibrium limit than near
equilibrium
Ute Self-Determination in Setting Tribal Resource Development Policy
The Ute Indian Tribe of northeastern Utah has large land holdings rich in water and energy resources. Revenues from these sources have created capital that can potentially be used by the Tribe to further develop its resources and expand its agricultural and business enterprises. Realities, however, complicate this simple scenario in that the Tribal goals are broader than pure economic gain and that increased incomes may mean sacrifices in terms of these other goals. The Tribe faces the problem of how best to develop its water resources in a way that best meets its multiplicity of needs.
The research team worked with the Utes to articulate their resources development goals, to operationalize those goals through the identification of measureable indicators, to identify promising development policies, to predict what those policies would achieve through changing the indicators, and to set before the Utes the trade offs among the goals. The procedures used in working with the Indians are described. The results were used to construct and use a linear programming model to identify Ute-specific development policies with their impacts and the trade offs among them. The model provides a framework that the Tribe can use to assess the alternatives for developing its water resources
X-ray emission from the giant magnetosphere of the magnetic O-type star NGC 1624-2
We observed NGC 1624-2, the O-type star with the largest known magnetic field
Bp~20 kG), in X-rays with the ACIS-S camera onboard the Chandra X-ray
Observatory. Our two observations were obtained at the minimum and maximum of
the periodic Halpha emission cycle, corresponding to the rotational phases
where the magnetic field is the closest to equator-on and pole-on,
respectively. With these observations, we aim to characterise the star's
magnetosphere via the X-ray emission produced by magnetically confined wind
shocks. Our main findings are:
(i) The observed spectrum of NGC 1624-2 is hard, similar to the magnetic
O-type star Theta 1 Ori C, with only a few photons detected below 0.8 keV. The
emergent X-ray flux is 30% lower at the Halpha minimum phase.
(ii) Our modelling indicated that this seemingly hard spectrum is in fact a
consequence of relatively soft intrinsic emission, similar to other magnetic
Of?p stars, combined with a large amount of local absorption (~1-3 x 10^22
cm^-2). This combination is necessary to reproduce both the prominent Mg and Si
spectral features, and the lack of flux at low energies. NGC 1624-2 is
intrinsically luminous in X-rays (log LX emission ~ 33.4) but 70-95% of the
X-ray emission produced by magnetically confined wind shocks is absorbed before
it escapes the magnetosphere (log LX ISM corrected ~ 32.5).
(iii) The high X-ray luminosity, its variation with stellar rotation, and its
large attenuation are all consistent with a large dynamical magnetosphere with
magnetically confined wind shocks.Comment: Accepted in MNRAS 13 pages, 10 figures, 4 table
The EXPLORE Project I: A Deep Search for Transiting Extrasolar Planets
(Abridged) We discuss the design considerations of the EXPLORE (EXtra-solar
PLanet Occultation REsearch) project, a series of transiting planet searches
using 4-m-class telescopes to continuously monitor a single field of stars in
the Galactic Plane in each ~2 week observing campaign. We discuss the general
factors which determine the efficiency and the number of planets found by a
transit search, including time sampling strategy and field selection. The
primary goal is to select the most promising planet candidates for radial
velocity follow-up observations. We show that with very high photometric
precision light curves that have frequent time sampling and at least two
detected transits, it is possible to uniquely solve for the main parameters of
the eclipsing system (including planet radius) based on several important
assumptions about the central star. Together with a measured spectral type for
the star, this unique solution for orbital parameters provides a powerful
method for ruling out most contaminants to transiting planet candidates. For
the EXPLORE project, radial velocity follow-up observations for companion mass
determination of the best candidates are done on 8-m-class telescopes within
two or three months of the photometric campaigns. This same-season follow-up is
made possible by the use of efficient pipelines to produce high quality light
curves within weeks of the observations. We conclude by presenting early
results from our first search, EXPLORE I, in which we reached <1% rms
photometric precision (measured over a full night) on ~37,000 stars to I <=
18.2.Comment: accepted by ApJ. Main points unchanged but more thorough discussion
of some issues. 36 pages, including 14 figure
Reorientation of Spin Density Waves in Cr(001) Films induced by Fe(001) Cap Layers
Proximity effects of 20 \AA thin Fe layers on the spin density waves (SDWs)
in epitaxial Cr(001) films are revealed by neutron scattering. Unlike in bulk
Cr we observe a SDW with its wave vector Q pointing along only one {100}
direction which depends dramatically on the film thickness t_{Cr}. For t_{Cr} <
250 \AA the SDW propagates out-of-plane with the spins in the film plane. For
t_{Cr} > 1000 \AA the SDW propagates in the film plane with the spins
out-of-plane perpendicular to the in-plane Fe moments. This reorientation
transition is explained by frustration effects in the antiferromagnetic
interaction between Fe and Cr across the Fe/Cr interface due to steps at the
interface.Comment: 4 pages (RevTeX), 3 figures (EPS
The EXPLORE Project: A Deep Search for Transiting Extra-Solar Planets
Searching for transits provides a very promising technique for finding
close-in extra-solar planets. Transiting planets present the advantage of
allowing one to determine physical properties such as mass and radius
unambiguously. The EXPLORE (EXtra-solar PLanet Occultation REsearch) project is
a transit search project carried out using wide-field CCD imaging cameras on
4-m class telescopes, and 8-10m class telescopes for radial velocity
verification of the photometric candidates. We describe some of the
considerations that go into the design of the EXPLORE transit search to
maximize the discovery rate and minimize contaminating objects that mimic
transiting planets. We show that high precision photometry (2 to 10 millimag)
and high time sampling (few minutes) are crucial for sifting out contaminating
signatures, such as grazing binaries. We have completed two searches using the
8k MOSAIC camera at the CTIO4m and the CFH12k camera at CFHT, with runs
covering 11 and 16 nights, respectively. We obtained preliminary light curves
for approximately 47,000 stars with better than ~1% photometric precision. A
number of light curves with flat-bottomed eclipses consistent with being
produced by transiting planets has been discovered. Preliminary results from
follow-up spectroscopic observations using the VLT UVES spectrograph and the
Keck HIRES spectrograph obtained for a number of the candidates are presented.
Data from four of these can be interpreted consistently as possible planet
candidates, although further data are still required for definitive
confirmations.Comment: 11 pages. To appear in the Proceedings of the SPIE conference:
Astronomical Telescopes and Instrumentatio
Asymmetric switching behavior in perpendicularly magnetized spin-valve nanopillars due to the polarizer dipole field
We report the free layer switching field distributions of spin-valve
nanopillars with perpendicular magnetization. While the distributions are
consistent with a thermal activation model, they show a strong asymmetry
between the parallel to antiparallel and the reverse transition, with energy
barriers more than 50% higher for the parallel to antiparallel transitions. The
inhomogeneous dipolar field from the polarizer is demonstrated to be at the
origin of this symmetry breaking. Interestingly, the symmetry is restored for
devices with a lithographically defined notch pair removed from the midpoint of
the pillar cross-section along the ellipse long axis. These results have
important implications for the thermal stability of perpendicular magnetized
MRAM bit cells.Comment: Submitted to Applied Physics Letters on November 4, 2011. Consists of
4 pages, 3 figure
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