15,683 research outputs found
Electron-phonon coupling in the C60 fullerene within the many-body GW approach
We study the electron-phonon coupling in the C60 fullerene within the
first-principles GW approach, focusing on the lowest unoccupied t1u three-fold
electronic state which is relevant for the superconducting transition in
electron doped fullerides. It is shown that the strength of the coupling is
significantly enhanced as compared to standard density functional theory
calculations with (semi)local functionals, with a 48% increase of the
electron-phonon potential Vep. The calculated GW value for the contribution
from the Hg modes of 93 meV comes within 4% of the most recent experimental
values. The present results call for a reinvestigation of previous density
functional based calculations of electron-phonon coupling in covalent systems
in general.Comment: 4 pages, 0 figur
Monolithic microwave integrated circuit water vapor radiometer
A proof of concept Monolithic Microwave Integrated Circuit (MMIC) Water Vapor Radiometer (WVR) is under development at the Jet Propulsion Laboratory (JPL). WVR's are used to remotely sense water vapor and cloud liquid water in the atmosphere and are valuable for meteorological applications as well as for determination of signal path delays due to water vapor in the atmosphere. The high cost and large size of existing WVR instruments motivate the development of miniature MMIC WVR's, which have great potential for low cost mass production. The miniaturization of WVR components allows large scale deployment of WVR's for Earth environment and meteorological applications. Small WVR's can also result in improved thermal stability, resulting in improved calibration stability. Described here is the design and fabrication of a 31.4 GHz MMIC radiometer as one channel of a thermally stable WVR as a means of assessing MMIC technology feasibility
Renormalized field theory and particle density profile in driven diffusive systems with open boundaries
We investigate the density profile in a driven diffusive system caused by a
plane particle source perpendicular to the driving force. Focussing on the case
of critical bulk density we use a field theoretic renormalization
group approach to calculate the density as a function of the distance
from the particle source at first order in (: spatial
dimension). For we find reasonable agreement with the exact solution
recently obtained for the asymmetric exclusion model. Logarithmic corrections
to the mean field profile are computed for with the result for .Comment: 32 pages, RevTex, 4 Postscript figures, to appear in Phys. Rev.
The optimal schedule for pulsar timing array observations
In order to maximize the sensitivity of pulsar timing arrays to a stochastic
gravitational wave background, we present computational techniques to optimize
observing schedules. The techniques are applicable to both single and
multi-telescope experiments. The observing schedule is optimized for each
telescope by adjusting the observing time allocated to each pulsar while
keeping the total amount of observing time constant. The optimized schedule
depends on the timing noise characteristics of each individual pulsar as well
as the performance of instrumentation. Several examples are given to illustrate
the effects of different types of noise. A method to select the most suitable
pulsars to be included in a pulsar timing array project is also presented.Comment: 16 pages, 6 figures, accepted by MNRA
X-ray Observations of XSS J12270-4859 in a New Low State: A Transformation to a Disk-Free Rotation-Powered Pulsar Binary
We present XMM-Newton and Chandra observations of the low-mass X-ray binary
XSS J12270--4859, which experienced a dramatic decline in optical/X-ray
brightness at the end of 2012, indicative of the disappearance of its accretion
disk. In this new state, the system exhibits previously absent
orbital-phase-dependent, large-amplitude X-ray modulations with a decline in
flux at superior conjunction. The X-ray emission remains predominantly
non-thermal but with an order of magnitude lower mean luminosity and
significantly harder spectrum relative to the previous high flux state. This
phenomenology is identical to the behavior of the radio millisecond pulsar
binary PSR J1023+0038 in the absence of an accretion disk, where the X-ray
emission is produced in an intra-binary shock driven by the pulsar wind. This
further demonstrates that XSS J12270-4859 no longer has an accretion disk and
has transformed to a full-fledged eclipsing "redback" system that hosts an
active rotation-powered millisecond pulsar. There is no evidence for diffuse
X-ray emission associated with the binary that may arise due to outflows or a
wind nebula. An extended source situated 1.5' from XSS J12270--4859 is unlikely
to be associated, and is probably a previously uncatalogued galaxy cluster.Comment: 8 pages, 6 figures; accepted for publication in the Astrophysical
Journa
Pattern measurements of a low-sidelobe horn antenna
The techniques and results of power pattern measurements of a corrugated horn antenna designed for low sidelobes are reported. The power pattern was measured to levels 90 dB below the main beam maximum in both the E- and H-planes. The measured patterns were found to be in good agreement with predictions from existing theory for the performance of corrugated scalar feeds
The beamformer and correlator for the Large European Array for Pulsars
The Large European Array for Pulsars combines Europe's largest radio
telescopes to form a tied-array telescope that provides high signal-to-noise
observations of millisecond pulsars (MSPs) with the objective to increase the
sensitivity of detecting low-frequency gravitational waves. As part of this
endeavor we have developed a software correlator and beamformer which enables
the formation of a tied-array beam from the raw voltages from each of
telescopes. We explain the concepts and techniques involved in the process of
adding the raw voltages coherently. We further present the software processing
pipeline that is specifically designed to deal with data from widely spaced,
inhomogeneous radio telescopes and describe the steps involved in preparing,
correlating and creating the tied-array beam. This includes polarization
calibration, bandpass correction, frequency dependent phase correction,
interference mitigation and pulsar gating. A link is provided where the
software can be obtained.Comment: 10 pages, 6 figures, accepted for publication in Astronomy and
Computin
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