19,752 research outputs found
A high-throughput ab initio review of platinum-group alloy systems
We report a comprehensive study of the binary systems of the platinum group
metals with the transition metals, using high-throughput first-principles
calculations. These computations predict stability of new compounds in 38
binary systems where no compounds have been reported in the literature
experimentally, and a few dozen of as yet unreported compounds in additional
systems. Our calculations also identify stable structures at compound
compositions that have been previously reported without detailed structural
data and indicate that some experimentally reported compounds may actually be
unstable at low temperatures. With these results we construct enhanced
structure maps for the binary alloys of platinum group metals. These are much
more complete, systematic and predictive than those based on empirical results
alone.Comment: 24 pages, 12 figure
Data processing method for a weak, moving telemetry signal
Method of processing data from a spacecraft, where the carrier has a low signal-to-noise ratio and wide unpredictable frequency shifts, consists of analogue recording of the noisy signal along with a high-frequency tone that is used as a clock to trigger a digitizer
Overlapping Protein-Encoding Genes in Pseudomonas fluorescens Pf0-1
The annotated genome sequences of prokaryotes seldom include overlapping genes encoded opposite each other by the same stretch of DNA. However, antisense transcription is becoming recognized as a widespread phenomenon in eukaryotes, and examples have been linked to important biological processes. Pseudomonas fluorescens inhabits aquatic and terrestrial environments, and can be regarded as an environmental generalist. The genetic basis for this ecological success is not well understood. In a previous search for soil-induced genes in P. fluorescens Pf0-1, ten antisense genes were discovered. These were termed ‘cryptic’ genes, as they had escaped detection by gene-hunting algorithms, and lacked easily recognizable promoters. In this communication, we designate such genes as ‘non-predicted’ or ‘hidden’. Using reverse transcription PCR, we show that at each of six non-predicted gene loci chosen for study, transcription occurs from both ‘sense’ and ‘antisense’ DNA strands. Further, at least one of these hidden antisense genes, iiv14, encodes a protein, as does the sense transcript, both identified by poly-histidine tags on the C-terminus of the proteins. Mutational and complementation studies showed that this novel antisense gene was important for efficient colonization of soil, and multiple copies in the wildtype host improved the speed of soil colonization. Introduction of a stop codon early in the gene eliminated complementation, further implicating the protein in colonization of soil. We therefore designate iiv14 “cosA”. These data suggest that, as is the case with eukaryotes, some bacterial genomes are more densely coded than currently recognized
Optimization of the leak conductance in the squid giant axon
We report on a theoretical study showing that the leak conductance density,
\GL, in the squid giant axon appears to be optimal for the action potential
firing frequency. More precisely, the standard assumption that the leak current
is composed of chloride ions leads to the result that the experimental value
for \GL is very close to the optimal value in the Hodgkin-Huxley model which
minimizes the absolute refractory period of the action potential, thereby
maximizing the maximum firing frequency under stimulation by sharp, brief input
current spikes to one end of the axon. The measured value of \GL also appears
to be close to optimal for the frequency of repetitive firing caused by a
constant current input to one end of the axon, especially when temperature
variations are taken into account. If, by contrast, the leak current is assumed
to be composed of separate voltage-independent sodium and potassium currents,
then these optimizations are not observed.Comment: 9 pages; 9 figures; accepted for publication in Physical Review
The phase-dependent linear conductance of a superconducting quantum point contact
The exact expression for the phase-dependent linear conductance of a weakly
damped superconducting quantum point contact is obtained. The calculation is
performed by summing up the complete perturbative series in the coupling
between the electrodes. The failure of any finite order perturbative expansion
in the limit of small voltage and small quasi-particle damping is analyzed in
detail. In the low transmission regime this nonperturbative calculation yields
a result which is at variance with standard tunnel theory. Our result predicts
the correct sign of the quasi-particle pair interference term and exhibits an
unusual phase-dependence at low temperatures in qualitative agreement with the
available experimental data.Comment: 12 pages (revtex) + 1 postscript figure. Submitted to Phys. Rev. Let
An experimental and computational investigation of the flow field about a transonic airfoil in supercritical flow with turbulent boundary-layer separation
A combined experimental and computational research program is described for testing and guiding turbulence modeling within regions of separation induced by shock waves incident in turbulent boundary layers. Specifically, studies are made of the separated flow the rear portion of an 18%-thick circular-arc airfoil at zero angle of attack in high Reynolds number supercritical flow. The measurements include distributions of surface static pressure and local skin friction. The instruments employed include highfrequency response pressure cells and a large array of surface hot-wire skin-friction gages. Computations at the experimental flow conditions are made using time-dependent solutions of ensemble-averaged Navier-Stokes equations, plus additional equations for the turbulence modeling
Ka-band (32-GHz) performance of 70-meter antennas in the Deep Space Network
Two models are provided of the Deep Space Network (DSN) 70 m antenna performance at Ka-band (32 GHz) and, for comparison purposes, one at X-band (8.4 GHz). The baseline 70 m model represents expected X-band and Ka-band performance at the end of the currently ongoing 64 m to 70 m mechanical upgrade. The improved 70 m model represents two sets of Ka-band performance estimates (the X-band performance will not change) based on two separately developed improvement schemes: the first scheme, a mechanical approach, reduces tolerances of the panels and their settings, the reflector structure and subreflector, and the pointing and tracking system. The second, an electronic/mechanical approach, uses an array feed scheme to compensate fo lack of antenna stiffness, and improves panel settings using microwave holographic measuring techniques. Results are preliminary, due to remaining technical and cost uncertainties. However, there do not appear to be any serious difficulties in upgrading the baseline DSN 70 m antenna network to operate efficiently in an improved configuration at 32 GHz (Ka-band). This upgrade can be achieved by a conventional mechanical upgrade or by a mechanical/electronic combination. An electronically compensated array feed system is technically feasible, although it needs to be modeled and demonstrated. Similarly, the mechanical upgrade requires the development and demonstration of panel actuators, sensors, and an optical surveying system
Plasma Leptin Levels and Incidence of Heart Failure, Cardiovascular Disease, and Total Mortality in Elderly Individuals
OBJECTIVE: Obesity predisposes individuals to congestive heart failure (CHF) and cardiovascular disease (CVD). Leptin regulates energy homeostasis, is elevated in obesity, and influences ventricular and vascular remodeling. We tested the hypothesis that leptin levels are associated with greater risk of CHF, CVD, and mortality in elderly individuals. RESEARCH DESIGN AND METHODS: We evaluated 818 elderly (mean age 79 years, 62% women) Framingham Study participants attending a routine examination at which plasma leptin was assayed. RESULTS: Leptin levels were higher in women and strongly correlated with BMI (P < 0.0001). On follow-up (mean 8.0 years), 129 (of 775 free of CHF) participants developed CHF, 187 (of 532 free of CVD) experienced a first CVD event, and 391 individuals died. In multivariable Cox regression models adjusting for established risk factors, log-leptin was positively associated with incidence of CHF and CVD (hazard ratio [HR] per SD increment 1.26 [95% CI 1.03–1.55] and 1.28 [1.09–1.50], respectively). Additional adjustment for BMI nullified the association with CHF (0.97 [0.75–1.24]) but only modestly attenuated the relation to CVD incidence (1.23 [1.00–1.51], P = 0.052). We observed a nonlinear, U-shaped relation between log-leptin and mortality (P = 0.005 for quadratic term) with greater risk of death evident at both low and high leptin levels. CONCLUSIONS: In our moderate-sized community-based elderly sample, higher circulating leptin levels were associated with a greater risk of CHF and CVD, but leptin did not provide incremental prognostic information beyond BMI. Additional investigations are warranted to elucidate the U-shaped relation of leptin to mortality.National Institutes of Health's National Heart, Lung, and Blood Institute (N01-HC25195, N01-HV28178, K24-HL04334, R01-DK080739
Semiclassical theory of spin-polarized shot noise in mesoscopic diffusive conductors
We study fluctuations of spin-polarized currents in a three-terminal
spin-valve system consisting of a diffusive normal metal wire connected by
tunnel junctions to three ferromagnetic terminals. Based on a spin-dependent
Boltzmann-Langevin equation, we develop a semiclassical theory of charge and
spin currents and the correlations of the currents fluctuations. In the three
terminal system, we show that current fluctuations are strongly affected by the
spin-flip scattering in the normal metal and the spin polarizations of the
terminals, which may point in different directions. We analyze the dependence
of the shot noise and the cross-correlations on the spin-flip scattering rate
in the full range of the spin polarizations and for different magnetic
configurations. Our result demonstrate that noise measurements in
multi-terminal devices allow to determine the spin-flip scattering rate by
changing the polarizations of ferromagnetic terminals.Comment: 12 pages, 5 figure
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