2,487 research outputs found
Distance From the Apiary as a Factor in Alfalfa Pollination
Experiments conducted in northern Utah indicated that the distribution of honey bees (Apis mellifera L.) on an alfalfa field is modified by various influences besides distance from the colonies. Because of these other factors, generalizations concerning the effect of distance from colonies on the distribution of foraging honey bees cannot be made on the basis of experiments herein reported. A slight negative relationship between bee populations and distance from colonies in alfalfa fields was found in some of the experiments reported. In two fields where the distance was less than 600 feet, horizontal stratification of the field population was found only in the one with an average population of over two bees per square yard. The other field had less than one bee per square yard and no significant changes in population were observed up to 550 feet from the apiary. In another location bee populations decreased beyond 3,000 feet, but other variable factors on the field made it impossible to attribute these decreases solely to increasing distance from the apiary
Helicity, polarization, and Riemann-Silberstein vortices
Riemann-Silberstein (RS) vortices have been defined as surfaces in spacetime
where the complex form of a free electromagnetic field given by F=E+iB is null
(F.F=0), and they can indeed be interpreted as the collective history swept out
by moving vortex lines of the field. Formally, the nullity condition is similar
to the definition of "C-lines" associated with a monochromatic electric or
magnetic field, which are curves in space where the polarization ellipses
degenerate to circles. However, it was noted that RS vortices of monochromatic
fields generally oscillate at optical frequencies and are therefore
unobservable while electric and magnetic C-lines are steady. Here I show that
under the additional assumption of having definite helicity, RS vortices are
not only steady but they coincide with both sets of C-lines, electric and
magnetic. The two concepts therefore become one for waves of definite frequency
and helicity. Since the definition of RS vortices is relativistically invariant
while that of C-lines is not, it may be useful to regard the vortices as a
wideband generalization of C-lines for waves of definite helicity.Comment: 5 pages, no figures. Submitted to J of Optics A, special issue on
Singular Optics; minor changes from v.
Limiting behaviour of Fréchet means in the space of phylogenetic trees
As demonstrated in our previous work on T4, the space of phylogenetic trees with four leaves, the topological structure of the space plays an important role in the non-classical limiting behaviour of the sample Fréchet means in T4. Nevertheless, the techniques used in that paper cannot be adapted to analyse Fréchet means in the space Tm of phylogenetic trees with m(⩾5)m(⩾5) leaves. To investigate the latter, this paper first studies the log map of Tm. Then, in terms of a modified version of this map, we characterise Fréchet means in Tm that lie in top-dimensional or co-dimension one strata. We derive the limiting distributions for the corresponding sample Fréchet means, generalising our previous results. In particular, the results show that, although they are related to the Gaussian distribution, the forms taken by the limiting distributions depend on the co-dimensions of the strata in which the Fréchet means lie
Coherent Quantum Optical Control with Subwavelength Resolution
We suggest a new method for quantum optical control with nanoscale
resolution. Our method allows for coherent far-field manipulation of individual
quantum systems with spatial selectivity that is not limited by the wavelength
of radiation and can, in principle, approach a few nanometers. The selectivity
is enabled by the nonlinear atomic response, under the conditions of
Electromagnetically Induced Transparency, to a control beam with intensity
vanishing at a certain location. Practical performance of this technique and
its potential applications to quantum information science with cold atoms,
ions, and solid-state qubits are discussed.Comment: 4 pages, 2 figures. V2: changes in presentation (text, figures,
tables) and new references - final version as published in Phys. Rev. Lett
Precision Measurement of the 29Si, 33S, and 36Cl Binding Energies
The binding energies of 29Si, 33S, and 36Cl have been measured with a
relative uncertainty using a flat-crystal spectrometer.
The unique features of these measurements are 1) nearly perfect crystals whose
lattice spacing is known in meters, 2) a highly precise angle scale that is
derived from first principles, and 3) a gamma-ray measurement facility that is
coupled to a high flux reactor with near-core source capability. The binding
energy is obtained by measuring all gamma-rays in a cascade scheme connecting
the capture and ground states. The measurements require the extension of
precision flat-crystal diffraction techniques to the 5 to 6 MeV energy region,
a significant precision measurement challenge. The binding energies determined
from these gamma-ray measurements are consistent with recent highly accurate
atomic mass measurements within a relative uncertainty of .
The gamma-ray measurement uncertainties are the dominant contributors to the
uncertainty of this consistency test. The measured gamma-ray energies are in
agreement with earlier precision gamma-ray measurements.Comment: 13 pages, 4 figure
Abrupt grain boundary melting in ice
The effect of impurities on the grain boundary melting of ice is investigated
through an extension of Derjaguin-Landau-Verwey-Overbeek theory, in which we
include retarded potential effects in a calculation of the full frequency
dependent van der Waals and Coulombic interactions within a grain boundary. At
high dopant concentrations the classical solutal effect dominates the melting
behavior. However, depending on the amount of impurity and the surface charge
density, as temperature decreases, the attractive tail of the dispersion force
interaction begins to compete effectively with the repulsive screened Coulomb
interaction. This leads to a film-thickness/temperature curve that changes
depending on the relative strengths of these interactions and exhibits a
decrease in the film thickness with increasing impurity level. More striking is
the fact that at very large film thicknesses, the repulsive Coulomb interaction
can be effectively screened leading to an abrupt reduction to zero film
thickness.Comment: 8 pages, 1 figur
Observations of the 8 December 1987 occultation of AG+40 deg 0783 by 324 Bamberga
The occultation of AG+40 deg 0783 by 324 Bamberga on 8 December 1987 was observed at 13 sites in the United States, Japan, and China. At four sites the event was observed photoelectrically; the other observations were visual. A least-squares fit of a circular limb profile to the data gives a diameter of 227.6 + or - 1.9 km. However, this solution is inconsistent with a negative visual observation near the northern edge of the ground track. The inconsistency cannot be removed by assuming an elliptical profile. The data suggest that Bamberga, despite its low-amplitude lightcurve, may depart significantly from a spherical or ellipsoidal shape. The asteroid also appears to be at least 10 percent smaller than indicated by infrared radiometry
Elastic constants of beta-eucryptite: A density functional theory study
The five independent elastic constants of hexagonal -eucryptite have
been determined using density functional theory (DFT) total energy
calculations. The calculated values agree well, to within 15%, with the
experimental data. Using the calculated elastic constants, the linear
compressibility of -eucryptite parallel to the c-axis, , and
perpendicular to it, , have been evaluated. These values are in close
agreement to those obtained from experimentally known elastic constants, but
are in contradiction to the direct measurements based on a three-terminal
technique. The calculated compressibility parallel to the c-axis was found to
positive as opposed to the negative value obtained by direct measurements. We
have demonstrated that must be positive and discussed the implications
of a positive in the context of explaining the negative bulk thermal
expansion of -eucryptite.Comment: 3 eps figures, submitted for publicatio
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