12,559 research outputs found
Remote sensing of atmospheric winds using a coherent, CW lidar and speckle-turbulence interaction
Speckle turbulence interaction has the potential for allowing single ended remote sensing of the path averaged vector crosswind in a plane perpendicular to the line of sight to a target. If a laser transmitter is used to illuminate a target, the resultant speckle field generated by the target is randomly perturbed by the atmospheric turbulence as it propagates back to the location of the transmitter-receiver. When a cross wind is present, this scintillation pattern will move with time across the receiver. A continuous wave (cw) laser transmitter of modest power level in conjunction with optical heterodyne detection was used to exploit the speckel turbulence interaction and measure the crosswind. The use of a cw transmitter at 10.6 microns and optical heterodyne detection has many advantages over direct detection and a double pulsed source in the visible or near infrared. These advantages include the availability of compact, reliable and inexpensive transmitters, better penetration of smoke, dust and fog; stable output power; low beam pointing jitter; and considerably reduced complexity in the receiver electronics
Revertant fibres and dystrophin traces in Duchenne muscular dystrophy: Implication for clinical trials
Duchenne muscular dystrophy (DMD) is characterised by the absence of dystrophin in muscle biopsies, although residual dystrophin can be present, either as dystrophin-positive (revertant) fibres or traces. As restoration of dystrophin expression is the end point of clinical trials, such residual dystrophin is a key factor in recruitment of patients and may also confound the analysis of dystrophin restoration in treated patients, if, as previously observed in the mdx mouse, revertant fibres increase with age. In 62% of the diagnostic biopsies reports of 65 DMD patients studied, traces or revertants were recorded with no correlation between traces or revertants, the patients' performance, or corticosteroids response. In nine of these patients, there was no increase in traces or revertants in biopsies taken a mean of 8.23 years (5.8-10.4 years) after the original diagnostic biopsy. This information should help in the design and execution of clinical trials focused on dystrophin restoration strategies. (C) 2010 Elsevier B.V. All rights reserved
Interplay of Rotational, Relaxational, and Shear Dynamics in Solid 4He
Using a high-sensitivity torsional oscillator technique, we mapped the
rotational and relaxational dynamics of solid helium-4 throughout the parameter
range of the proposed supersolidity. We found evidence that the same
microscopic excitations controlling the torsional oscillator motions are
generated independently by thermal and mechanical stimulation. Moreover, a
measure for the relaxation times of these excitations diverges smoothly without
any indication for a critical temperature or critical velocity of a supersolid
transition. Finally, we demonstrated that the combined temperature-velocity
dependence of the TO response is indistinguishable from the combined
temperature-strain dependence of the solid's shear modulus. This implies that
the rotational responses of solid helium-4 attributed to supersolidity are
associated with generation of the same microscopic excitations as those
produced by direct shear strain.Comment: 27 pages with 4 main figures and 7 supplemental figure
CydDC-mediated reductant export in Escherichia coli controls the transcriptional wiring of energy metabolism and combats nitrosative stress
The glutathione/cysteine exporter CydDC maintains redox balance in Escherichia coli. A cydD mutant strain was used to probe the influence of CydDC upon reduced thiol export, gene expression, metabolic perturbations, intracellular pH homeostasis, and tolerance to nitric oxide (NO). Loss of CydDC was found to decrease extracytoplasmic thiol levels, whereas overexpression diminished the cytoplasmic thiol content. Transcriptomic analysis revealed a dramatic up-regulation of protein chaperones, protein degradation (via phenylpropionate/phenylacetate catabolism), ?-oxidation of fatty acids, and genes involved in nitrate/nitrite reduction. 1H NMR metabolomics revealed elevated methionine and betaine and diminished acetate and NAD+ in cydD cells, which was consistent with the transcriptomics-based metabolic model. The growth rate and ?pH, however, were unaffected, although the cydD strain did exhibit sensitivity to the NO-releasing compound NOC-12. These observations are consistent with the hypothesis that the loss of CydDC-mediated reductant export promotes protein misfolding, adaptations to energy metabolism, and sensitivity to NO. The addition of both glutathione and cysteine to the medium was found to complement the loss of bd -type cytochrome synthesis in a cydD strain (a key component of the pleiotropic cydDC phenotype), providing the first direct evidence that CydDC substrates are able to restore the correct assembly of this respiratory oxidase. These data provide an insight into the metabolic flexibility of E. coli , highlight the importance of bacterial redox homeostasis during nitrosative stress, and report for the first time the ability of periplasmic low molecular weight thiols to restore haem incorporation into a cytochrome complex
On the structure of the energy distribution function in the hopping regime
The impact of the dispersion of the transport coefficients on the structure
of the energy distribution function for charge carriers far from equilibrium
has been investigated in effective-medium approximation for model densities of
states. The investigations show that two regimes can be observed in energy
relaxation processes. Below a characteristic temperature the structure of the
energy distribution function is determined by the dispersion of the transport
coefficients. Thermal energy diffusion is irrelevant in this regime. Above the
characteristic temperature the structure of the energy distribution function is
determined by energy diffusion. The characteristic temperature depends on the
degree of disorder and increases with increasing disorder. Explicit expressions
for the energy distribution function in both regimes are derived for a constant
and an exponential density of states.Comment: 16 page
Probability of local bifurcation type from a fixed point: A random matrix perspective
Results regarding probable bifurcations from fixed points are presented in
the context of general dynamical systems (real, random matrices), time-delay
dynamical systems (companion matrices), and a set of mappings known for their
properties as universal approximators (neural networks). The eigenvalue spectra
is considered both numerically and analytically using previous work of Edelman
et. al. Based upon the numerical evidence, various conjectures are presented.
The conclusion is that in many circumstances, most bifurcations from fixed
points of large dynamical systems will be due to complex eigenvalues.
Nevertheless, surprising situations are presented for which the aforementioned
conclusion is not general, e.g. real random matrices with Gaussian elements
with a large positive mean and finite variance.Comment: 21 pages, 19 figure
Evidence for a Superglass State in Solid 4He
Although solid helium-4 (4He) may be a supersolid it also exhibits many
phenomena unexpected in that context. We studied relaxation dynamics in the
resonance frequency f(T) and dissipation D(T) of a torsional oscillator
containing solid 4He. With the appearance of the "supersolid" state, the
relaxation times within f(T) and D(T) began to increase rapidly together. More
importantly, the relaxation processes in both D(T) and a component of f(T)
exhibited a complex synchronized ultraslow evolution towards equilibrium.
Analysis using a generalized rotational susceptibility revealed that, while
exhibiting these apparently glassy dynamics, the phenomena were quantitatively
inconsistent with a simple excitation freeze-out transition because the
variation in f was far too large. One possibility is that amorphous solid 4He
represents a new form of supersolid in which dynamical excitations within the
solid control the superfluid phase stiffness.Comment: 25 pages (12 main manuscript, 13 supporting material), 10 figures (4
main manuscript, 6 supporting material
Generalized Rotational Susceptibility Studies of Solid 4He
Using a novel SQUID-based torsional oscillator (TO) technique to achieve increased sensitivity and dynamic range, we studied TO’s containing solid [superscript 4]He. Below ∼250 mK, the TO resonance frequency f increases and its dissipation D passes through a maximum as first reported by Kim and Chan. To achieve unbiased analysis of such [superscript 4]He rotational dynamics, we implemented a new approach based upon the generalized rotational susceptibility χ[subscript 4He][superscript -1](ω,T). Upon cooling, we found that equilibration times within f(T) and D(T) exhibit a complex synchronized ultraslow evolution toward equilibrium indicative of glassy freezing of crystal disorder conformations which strongly influence the rotational dynamics. We explored a more specific χ[subscript 4He][superscript -1](ω,τ(T)) with τ(T) representing a relaxation rate for inertially active microscopic excitations. In such models, the characteristic temperature T* at which df/dT and D pass simultaneously through a maximum occurs when the TO angular frequency ω and the relaxation rate are matched: ωτ(T*)=1. Then, by introducing the free inertial decay (FID) technique to solid [superscript 4]He TO studies, we carried out a comprehensive map of f(T,V) and D(T,V) where V is the maximum TO rim velocity. These data indicated that the same microscopic excitations controlling the TO motions are generated independently by thermal and mechanical stimulation of the crystal. Moreover, a measure for their relaxation times τ(T,V) diverges smoothly everywhere without exhibiting a critical temperature or velocity, as expected in ωτ=1 models. Finally, following the observations of Day and Beamish, we showed that the combined temperature-velocity dependence of the TO response is indistinguishable from the combined temperature-strain dependence of the [superscript]4He shear modulus. Together, these observations imply that ultra-slow equilibration of crystal disorder conformations controls the rotational dynamics and, for any given disorder conformation, the anomalous rotational responses of solid [superscript 4]He are associated with generation of the same microscopic excitations as those produced by direct shear strain.National Science Foundation (U.S.) (Grants DMR-0806629 and NSF PHY05-51164)United States. Dept. of Energy (Grant DE-AC52-06NA25396
Dynamical Friction in a Gaseous Medium
Using time-dependent linear perturbation theory, we evaluate the dynamical
friction force on a massive perturber M_p traveling at velocity V through a
uniform gaseous medium of density rho_0 and sound speed c_s. This drag force
acts in the direction -\hat V, and arises from the gravitational attraction
between the perturber and its wake in the ambient medium. For supersonic motion
(M=V/c_s>1), the enhanced-density wake is confined to the Mach cone trailing
the perturber; for subsonic motion (M<1), the wake is confined to a sphere of
radius c_s t centered a distance V t behind the perturber. Inside the wake,
surfaces of constant density are hyperboloids or oblate spheroids for
supersonic or subsonic perturbers, respectively, with the density maximal
nearest the perturber. The dynamical drag force has the form F_df= - I 4\pi (G
M_p)^2\rho_0/V^2. We evaluate I analytically; its limits are I\to M^3/3 for
M>1. We compare our results to the
Chandrasekhar formula for dynamical friction in a collisionless medium, noting
that the gaseous drag is generally more efficient when M>1 but less efficient
when M<1. To allow simple estimates of orbit evolution in a gaseous protogalaxy
or proto-star cluster, we use our formulae to evaluate the decay times of a
(supersonic) perturber on a near-circular orbit in an isothermal \rho\propto
r^{-2} halo, and of a (subsonic) perturber on a near-circular orbit in a
constant-density core. We also mention the relevance of our calculations to
protoplanet migration in a circumstellar nebula.Comment: 17 pages, 5 postscript figures, to appear in ApJ 3/1/9
Job Growth in Early Transition: Comparing Two Paths
Small start-up firms are the engine of job creation in early transition and yet little is known about the characteristics of this new sector. We seek to identify patterns of job growth in this sector in terms of niches left from central planning and ask about differences in job creation across two different transition economies: Estonia, which experienced rapid destruction of the pre-existing firms, and the Czech Republic, which reduced the old sector gradually. We find job growth within industries to be quantitatively more important than job growth due to across-industry reallocation. Furthermore, the industrial composition of startups is strikingly similar in the two countries. We offer convergence to "western" industry firm-size distributions as an explanation. We also find regularities in wage evolution across new and old firms, including small differences in job quality across the two transition paths.http://deepblue.lib.umich.edu/bitstream/2027.42/39888/3/wp503.pd
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