192,543 research outputs found
Are topological defects responsible for the 300 EeV cosmic rays?
We use of a hybrid matrix--Monte Carlo method to simulate the cascade through
the cosmic background radiation initiated by UHE particles and radiation
emitted by topological defects. We follow the cascade over cosmological
distances and calculate the intensities of hadrons, gamma-rays and neutrinos
produced. We compare our results with the observed cosmic ray intensity at 300
EeV and lower energies, and conclude that topological defects are most unlikely
to be the origin of the most energetic cosmic ray events.Comment: 3 pages, compressed and uuencoded PostScript (111kb); Nucl. Phys. B.,
Proc. Suppl., vol 48, in press (TAUP95 Workshop
A solvable model for excitonic complexes in one dimension
It is known experimentally that stable few-body clusters containing
negatively-charged electrons (e) and positively-charged holes (h) can exist in
low-dimensional semiconductor nanostructures. In addition to the familiar
exciton (e+h), three-body 'charged excitons' (2e+h and 2h+e) have also been
observed. Much less is known about the properties of such charged excitons
since three-body problems are generally very difficult to solve, even
numerically. Here we introduce a simple model, which can be considered as an
extended Calogero model, to calculate analytically the energy spectra for both
a charged exciton and a neutral exciton in a one-dimensional nanostructure,
such as a finite-length quantum wire. Apart from its physical motivation, the
model is of mathematical interest in that it can be related to the Heun (or
Heine) equation and, as shown explicitly, highly accurate, closed form
solutions can be obtained.Comment: 14 pages, 3 figures, To appear in J. Math. Phy
Three dimensional turbulent boundary layers: Data sets for two-space coordinate flows
Sets of data (flows) from eight original sources on three-dimensional turbulent boundary layers were reevaluated and tabulated in a common format. The flows studied were all of the type describable in only two space coordinates, e.g., flow over a swept wing of infinite span. The principal data in each set are profiles of the main and crossflow components of mean velocity. Turbulent shear stress vector profiles were available for two flows, Bradshaw and Terrell (1969) and Johnson (1970). Free stream pressure gradient, wall shear stress coefficient and angle, integral thickness and left and right hand sides of the momentum integral equations were evaluated in a consistent manner for each flow
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Ancient Cosmic Dust from Triassic Halite
We describe the discovery of fossil micrometeorites in ancient Triassic rock salt; the first to be found in salt and the oldest complete micrometeorites found to date. We present an estimated flux rate of micrometeorites to Earth at this time
Atomic resolution STM imaging of a twisted single-wall carbon nanotube
We present atomically-resolved STM images of single-wall carbon nanotubes
(SWNTs) embedded in a crystalline nanotube rope. Although they may be
interpreted as of a chiral nanotube, the images are more consistently explained
a an achiral armchair tube with a quenched twist distortion. The existence of
quenched twists in SWNTs in ropes might explain the fact that both as-grown
bulk nanotube material and individual ropes have insulator-like conductivity at
low temperature.Comment: preprint, 4 pages, and 4 gif figure
Set Systems Containing Many Maximal Chains
The purpose of this short problem paper is to raise an extremal question on
set systems which seems to be natural and appealing. Our question is: which set
systems of a given size maximise the number of -element chains in the
power set ? We will show that for each fixed
there is a family of sets containing
such chains, and that this is asymptotically best possible. For smaller set
systems we are unable to answer the question. We conjecture that a `tower of
cubes' construction is extremal. We finish by mentioning briefly a connection
to an extremal problem on posets and a variant of our question for the grid
graph.Comment: 5 page
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