21,683 research outputs found
Dynamics of atomic spin-orbit-state wave packets produced by short-pulse laser photodetachment
We analyse the experiment by Hultgren et al. [Phys. Rev. A {\bf 87}, 031404
(2013)] on orbital alignment and quantum beats in coherently excited atomic
fine-structure manifolds produced by short-pulse laser photodetachment of
C, Si and Ge negative ions, and derive a formula that describes the
beats. Analysis of the experimental data enables us to extract the non-coherent
background contribution for each species, and indicates the need for a full
density matrix treatment of the problem
Comment on "Direct photodetachment of F by mid-infrared few-cycle femtosecond laser pulses"
Multiphoton detachment of F by strong few-cycle laser pulses was studied
by Shearer and Monteith using a Keldysh-type approach [Phys. Rev. A 88, 033415
(2013)]. We believe that this work contained errors in the calculation of the
detachment amplitude and photoelectron spectra. We describe the necessary
corrections to the theory and show that the results, in particular, the
interference features of the photoelectron spectra, appear noticeably
different.Comment: 9 pages, 4 figure
Wind tunnel evaluation of a truncated NACA 64-621 airfoil for wind turbine applications
An experimental program to measure the aerodynamic performance of a NACA 64-621 airfoil with a truncated trailing edge for wind turbine applications has been conducted in the Ohio State University Aeronautical and Astronautical Research Laboratory 6 in. by 21 in. pressurized wind tunnel. The blunted or trailing edge truncated (TET) airfoil has an advantage over similar trailing edge airfoils because it is able to streamline a larger spar structure, while also providing aerodynamic properties that are quite good. Surface pressures were measured and integrated to determine the lift, pressure drag, and moment coefficients over angles of attack ranging from -14 to +90 deg at Mach 0.2 and Reynolds numbers of 1,000,000 and 600,000. Results are compared to the NACA 0025, 0030, and 0035 thick airfoils with sharp trailing edges. Comparison shows that the 30 percent thick NACA 64-621-TET airfoil has higher maximum lift, higher lift curve slope, lower drag at higher lift coefficients, and higher chordwise force coefficient than similar thick airfoils with sharp trailing edges
Liquid bridging of cylindrical colloids in near-critical solvents
Within mean field theory, we investigate the bridging transition between a
pair of parallel cylindrical colloids immersed in a binary liquid mixture as a
solvent which is close to its critical consolute point . We determine the
universal scaling functions of the effective potential and of the force between
the colloids. For a solvent which is at the critical concentration and close to
, we find that the critical Casimir force is the dominant interaction at
close separations. This agrees very well with the corresponding Derjaguin
approximation for the effective interaction between the two cylinders, while
capillary forces originating from the extension of the liquid bridge turn out
to be more important at large separations. In addition, we are able to infer
from the wetting characteristics of the individual colloids the first-order
transition of the liquid bridge connecting two colloidal particles to the
ruptured state. While specific to cylindrical colloids, the results presented
here provide also an outline for identifying critical Casimir forces acting on
bridged colloidal particles as such, and for analyzing the bridging transition
between them.Comment: 23 pages, 12 figure
Cheating and the evolutionary stability of mutualisms
Interspecific mutualisms have been playing a central role in the functioning of all ecosystems since the early history of life. Yet the theory of coevolution of mutualists is virtually nonexistent, by contrast with well-developed coevolutionary theories of competition, predator–prey and host–parasite interactions. This has prevented resolution of a basic puzzle posed by mutualisms: their persistence in spite of apparent evolutionary instability. The selective advantage of 'cheating', that is, reaping mutualistic benefits while providing fewer commodities to the partner species, is commonly believed to erode a mutualistic interaction, leading to its dissolution or reciprocal extinction. However, recent empirical findings indicate that stable associations of mutualists and cheaters have existed over long evolutionary periods. Here, we show that asymmetrical competition within species for the commodities offered by mutualistic partners provides a simple and testable ecological mechanism that can account for the long-term persistence of mutualisms. Cheating, in effect, establishes a background against which better mutualists can display any competitive superiority. This can lead to the coexistence and divergence of mutualist and cheater phenotypes, as well as to the coexistence of ecologically similar, but unrelated mutualists and cheaters
Off the shortest path: Betweenness on street network level to study pedestrian movement
Betweenness centrality is an important measure in network sciences that reflects the extent a node lies in between any pairs in a graph. The measure has been used by urban studies, to discuss the relationship between urban mobility and the spatial street network of a city, using Dijkstra shortest path betweenness centrality to describe human wayfinding procedures. As in reality, wayfinding is a more complex endeavor, results of studies using both random path or the most optimal shortest path approach might be misleading. In this paper we propose with the exploratory betweenness centrality (EBC) a more realistic set of measures that uses an exploratory path in calculating centrality rather than an optimal path in studying pedestrian movement. In particular we calculate EBC where the agent selects the longest street nearest to the destination (App-EBC) or any random street that is approaching the destination (Ran-EBC). In doing so, we compare how EBC and GBC correlate with aggregate pedestrian movement for two case studies in London. The result shows the EBC measures explains equal or greater variation of aggregate pedestrian movement than the GBC measure for both of the case studies, indicating the potential of using measures of EBC in modeling urban mobility
Laser-only adaptive optics achieves significant image quality gains compared to seeing-limited observations over the entire sky
Adaptive optics laser guide star systems perform atmospheric correction of
stellar wavefronts in two parts: stellar tip-tilt and high-spatial-order
laser-correction. The requirement of a sufficiently bright guide star in the
field-of-view to correct tip-tilt limits sky coverage. Here we show an
improvement to effective seeing without the need for nearby bright stars,
enabling full sky coverage by performing only laser-assisted wavefront
correction. We used Robo-AO, the first robotic AO system, to comprehensively
demonstrate this laser-only correction. We analyze observations from four years
of efficient robotic operation covering 15,000 targets and 42,000 observations,
each realizing different seeing conditions. Using an autoguider (or a
post-processing software equivalent) and the laser to improve effective seeing
independent of the brightness of a target, Robo-AO observations show a 39+/-19%
improvement to effective FWHM, without any tip-tilt correction. We also
demonstrate that 50% encircled-energy performance without tip-tilt correction
remains comparable to diffraction-limited, standard Robo-AO performance.
Faint-target science programs primarily limited by 50% encircled-energy (e.g.
those employing integral field spectrographs placed behind the AO system) may
see significant benefits to sky coverage from employing laser-only AO.Comment: Accepted for publication in The Astronomical Journal. 7 pages, 6
figure
Thermal expansion and magnetostriction of pure and doped RAgSb2 (R = Y, Sm, La) single crystals
Data on temperature-dependent, anisotropic thermal expansion in pure and
doped RAgSb2 (R = Y, Sm, La) single crystals are presented. Using the Ehrenfest
relation and heat capacity measurements, uniaxial pressure derivatives for long
range magnetic ordering and charge density wave transition temperatures are
evaluated and compared with the results of the direct measurements under
hydrostatic pressure. In-plane and c-axis pressure have opposite effect on the
phase transitions in these materials, with in-plane effects being significantly
weaker. Quantum oscillations in magnetostriction were observed for the three
pure compounds, with the possible detection of new frequencies in SmAgSb2 and
LaAgSb2. The uniaxial (along the c-axis) pressure derivatives of the dominant
extreme orbits (beta) were evaluated for YAgSb2 and LaAgSb2
Exotic fermion multiplets as a solution to baryon asymmetry, dark matter and neutrino masses
We propose an extension to the standard model where three exotic fermion
5-plets and one scalar 6-plet are added to the particle content. By demanding
that all interactions are renormalizable and standard model gauge invariant, we
show that the lightest exotic particle in this model can be a dark matter
candidate as long as the new 6-plet scalar does not develop a nonzero vacuum
expectation value. Furthermore, light neutrino masses are generated radiatively
at one-loop while the baryon asymmetry is produced by the CP-violating decays
of the second lightest exotic particle. We have demonstrated using concrete
examples that there is a parameter space where a consistent solution to the
problems of baryon asymmetry, dark matter and neutrino masses can be obtained.Comment: 17 pages, 2 figures (REVTeX4.1), v2: some refs added, v3: typos
corrected, Sec.VI.B, C modified, this version to appear in PR
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