6,263 research outputs found
Cooling method prolongs life of hot-wire transducer
To cool a hot-wire transducer, the two ends of the wire are supported on thermally and electrically conductive rods, surrounded by a fluid cooling medium. By keeping the supporting rods at a substantially constant temperature, the probe is prevented from overheating
A site-specific standard for comparing dynamic solar ultraviolet protection characteristics of established tree canopies
A standardised procedure for making fair and comparable assessments of the ultraviolet protection of an established tree canopy that takes into account canopy movement and the changing position of the sun is presented for use by government, planning, and environmental health authorities. The technique utilises video image capture and replaces the need for measurement by ultraviolet radiometers for surveying shade quality characteristics of trees growing in public parks, playgrounds and urban settings. The technique improves upon tree shade assessments that may be based upon single measurements of the ultraviolet irradiance observed from a fixed point of view. The presented technique demonstrates how intelligent shade audits can be conducted without the need for specialist equipment, enabling the calculation of the Shade Protection Index (SPI) and Ultraviolet Protection Factor (UPF) for any discreet time interval and over a full calendar year
Preliminary survey of propulsion using chemical energy stored in the upper atmosphere
Ram-jet cycles that use the chemical energy of dissociated oxygen for propulsion in the ionosphere are presented. After a review of the properties and compositions of the upper atmosphere, the external drag, recombination kinetics, and aerodynamic-heating problems of an orbiting ram jet are analyzed. The study indicates that the recombination ram jet might be useful for sustaining a satellite at an altitude of about 60 miles. Atmospheric composition and recombination-rate coefficients were too uncertain for more definite conclusions. The ram jet is a marginal device even in the optimistic view
Decoherence of charge qubit coupled to interacting background charges
The major contribution to decoherence of a double quantum dot or a Josephson
junction charge qubit comes from the electrostatic coupling to fluctuating
background charges hybridized with the conduction electrons in the reservoir.
However, estimations according to previously developed theories show that
finding a sufficient number of effective fluctuators in a realistic
experimental layout is quite improbable. We show that this paradox is resolved
by allowing for a short-range Coulomb interaction of the fluctuators with the
electrons in the reservoir. This dramatically enhances both the number of
effective fluctuators and their contribution to decoherence, resulting in the
most dangerous decoherence mechanism for charge qubits.Comment: 4 pages, 1 figur
Spin-Mediated Mott Excitons
Motivated by recent experiments on Mott insulators, in both iridates and
ultracold atoms, we theoretically study the effects of magnetic order on the
Mott-Hubbard excitons. In particular, we focus on spin-mediated doublon-holon
pairing in Hubbard materials. We use several complementary theoretical
techniques: mean-field theory to describe the spin degrees of freedom, the
self-consistent Born approximation to characterize individual charge
excitations across the Hubbard gap, and the Bethe-Salpeter equation to identify
bound states of doublons and holons. The binding energy of the Hubbard exciton
is found to increase with increasing the N{\'e}el order parameter, while the
exciton mass decreases. We observe that these trends rely significantly on the
retardation of the effective interaction, and require consideration of multiple
effects from changing the magnetic order. Our results are consistent with the
key qualitative trends observed in recent experiments on iridates. Moreover,
the findings could have direct implications on ultracold atom Mott insulators,
where the Hubbard model is the exact description of the system and the
microscopic degrees of freedom can be directly accessed.Comment: 11 pages, 11 figure
Spin Susceptibility of an Ultra-Low Density Two Dimensional Electron System
We determine the spin susceptibility in a two dimensional electron system in
GaAs/AlGaAs over a wide range of low densities from 2cm to
4cm. Our data can be fitted to an equation that describes
the density dependence as well as the polarization dependence of the spin
susceptibility. It can account for the anomalous g-factors reported recently in
GaAs electron and hole systems. The paramagnetic spin susceptibility increases
with decreasing density as expected from theoretical calculations.Comment: 5 pages, 2 eps figures, to appear in PR
Multilocus sequence types of invasive Corynebacterium diphtheriae isolated in the Rio de Janeiro urban area, Brazil
Invasive infections caused by Corynebacterium diphtheriae in vaccinated and non-vaccinated individuals have been reported increasingly. In this study we used multilocus sequence typing (MLST) to study genetic relationships between six invasive strains of this bacterium isolated solely in the urban area of Rio de Janeiro, Brazil, during a 10-year period. Of note, all the strains rendered negative results in PCR reactions for the tox gene, and four strains presented an atypical sucrose-fermenting ability. Five strains represented new sequence types. MLST results did not support the hypothesis that invasive (sucrose-positive) strains of C. diphtheriae are part of a single clonal complex. Instead, one of the main findings of the study was that such strains can be normally found in clonal complexes with strains related to non-invasive disease. Comparative analyses with C. diphtheriae isolated in different countries provided further information on the geographical circulation of some sequence types
Fermi edge singularity in neutral electron-hole system
In neutral dense electron-hole (e-h) systems at low temperatures, theory
predicts Cooper-pair-like excitons at the Fermi energy and a BCS-like exciton
condensation. Optical excitation allows creating e-h systems with the densities
controlled by the excitation power. However, the intense optical excitations
required to achieve high densities cause substantial heating of the e-h system
that prevents the realization of dense and cold e-h systems in conventional
semiconductors. In this work, we study e-h systems created by optical
excitation in separated electron and hole layers. The layer separation
increases the e-h recombination time and, in turn, the density for a given
optical excitation by orders of magnitude and, as a result, enables the
realization of the dense and cold e-h system. We found a strong enhancement of
photoluminescence intensity at the Fermi energy of the neutral dense ultracold
e-h system that evidences the emergence of excitonic Fermi edge singularity due
to the Cooper-pair-like excitons at the Fermi energy
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Thermal Behavior in the Lens Process
Direct laser metal deposition processing is a promising manufacturing technology which
could significantly impact the length oftime between initial concept and finished part. For
adoption ofthis technology in the manufacturing environment, further understanding is required
to ensure robust components with appropriate properties are routinelyfabricated. This requires a
complete understanding ofthe thermal history.during part fabrication and control ofthis behavior.
This paper will describe our research to understand the thermal behavior for the Laser Engineered
Net Shaping (LENS) process!, where a component is fabricated by focusing a laser beam onto a
substrate to create a molten pool in which powder particles are simultaneously injected to build
each layer. The substrate is moved beneath the l~ser beam to deposit a thin cross section, thereby
creating the desired geometry for each layer. After deposition of each layer, the powder delivery
nozzle and focusing lens assembly is incremented in the positive Z-direction, thereby building a
three dimensional component layer additively.
It is important to control the thermal behavior to reproducibly fabricate parts. The
ultimate intent is to monitor the thermal signatures and to incorporate sensors and feedback
algorithms to control part fabrication. With appropriate control, the geometric properties
(accuracy, surface finish, low warpage) as well as the materials' properties (e.g. strength,
ductility) of a component can be dialed into the part through the fabrication parameters. Thermal
monitoring techniques will be described, and their particular benefits highlighted. Preliminary
details in correlating thermal behavior with processing results will be discussed.Mechanical Engineerin
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