1,130 research outputs found
Diversity, genetic structure and evidence of outcrossing in British populations of the rock fern Adiantum capillus-veneris using microsatellites
Microsatellites were isolated and a marker system was developed in the fern Adiantum capillus-veneris. Polymorphic markers were then used to study the genetic diversity and structure of populations within the UK and Ireland where this species grows at the northern edge of its range, requiring a specific rock habitat and limited to a few scattered populations. Three dinucleotide loci detected a high level of diversity (23 alleles and 28 multilocus genotypes) across the UK and Ireland, with nearly all variation partitioned among rather than within populations. Of 17 populations represented by multiple samples, all except four were monomorphic. Heterozygosity was detected in three populations, all within Glamorgan, Wales (UK), showing evidence of outcrossing. We make inferences on the factors determining the observed levels and patterns of genetic variation and the possible evolutionary history of the populations
Comparing various multi-component global heliosphere models
Modeling of the global heliosphere seeks to investigate the interaction of
the solar wind with the partially ionized local interstellar medium. Models
that treat neutral hydrogen self-consistently and in great detail, together
with the plasma, but that neglect magnetic fields, constitute a sub-category
within global heliospheric models. There are several different modeling
strategies used for this sub-category in the literature. Differences and
commonalities in the modeling results from different strategies are pointed
out. Plasma-only models and fully self-consistent models from four research
groups, for which the neutral species is modeled with either one, three, or
four fluids, or else kinetically, are run with the same boundary parameters and
equations. They are compared to each other with respect to the locations of key
heliospheric boundary locations and with respect to the neutral hydrogen
content throughout the heliosphere. In many respects, the models' predictions
are similar. In particular, the locations of the termination shock agree to
within 7% in the nose direction and to within 14% in the downwind direction.
The nose locations of the heliopause agree to within 5%. The filtration of
neutral hydrogen from the interstellar medium into the inner heliosphere,
however, is model dependent, as are other neutral results including the
hydrogen wall. These differences are closely linked to the strength of the
interstellar bow shock. The comparison also underlines that it is critical to
include neutral hydrogen into global heliospheric models.Comment: 10 pages, 4 figures, submitted to a special section at A&A of an ISSI
team "Determination of the physical Hydrogen parameters of the LIC from
within the Heliosphere
Wavefunction and level statistics of random two dimensional gauge fields
Level and wavefunction statistics have been studied for two dimensional
clusters of the square lattice in the presence of random magnetic fluxes.
Fluxes traversing lattice plaquettes are distributed uniformly between - (1/2)
Phi_0 and (1/2) Phi_0 with Phi_0 the flux quantum. All considered statistics
start close to the corresponding Wigner-Dyson distribution for small system
sizes and monotonically move towards Poisson statistics as the cluster size
increases. Scaling is quite rapid for states close to the band edges but really
difficult to observe for states well within the band. Localization properties
are discussed considering two different scenarios. Experimental measurement of
one of the considered statistics --wavefunction statistics seems the most
promising one-- could discern between both possibilities. A real version of the
previous model, i.e., a system that is invariant under time reversal, has been
studied concurrently to get coincidences and differences with the Hermitian
model.Comment: 12 twocolumnn pages in revtex style, 17 postscript figures, to be
published in PRB, send comments to [email protected]
Magnetotransport in inhomogeneous magnetic fields
Quantum transport in inhomogeneous magnetic fields is investigated
numerically in two-dimensional systems using the equation of motion method. In
particular, the diffusion of electrons in random magnetic fields in the
presence of additional weak uniform magnetic fields is examined. It is found
that the conductivity is strongly suppressed by the additional uniform magnetic
field and saturates when the uniform magnetic field becomes on the order of the
fluctuation of the random magnetic field. The value of the conductivity at this
saturation is found to be insensitive to the magnitude of the fluctuation of
the random field. The effect of random potential on the magnetoconductance is
also discussed.Comment: 5 pages, 5 figure
Multi-Exciton Spectroscopy of a Single Self Assembled Quantum Dot
We apply low temperature confocal optical microscopy to spatially resolve,
and spectroscopically study a single self assembled quantum dot. By comparing
the emission spectra obtained at various excitation levels to a theoretical
many body model, we show that: Single exciton radiative recombination is very
weak. Sharp spectral lines are due to optical transitions between confined
multiexcitonic states among which excitons thermalize within their lifetime.
Once these few states are fully occupied, broad bands appear due to transitions
between states which contain continuum electrons.Comment: 12 pages, 4 figures, submitted for publication on Jan,28 199
Localization length in a random magnetic field
Kubo formula is used to get the d.c conductance of a statistical ensemble of
two dimensional clusters of the square lattice in the presence of random
magnetic fluxes. Fluxes traversing lattice plaquettes are distributed uniformly
between minus one half and plus one half of the flux quantum. The localization
length is obtained from the exponential decay of the averaged conductance as a
function of the cluster side. Standard results are recovered when this
numerical approach is applied to Anderson model of diagonal disorder. The
localization length of the complex non-diagonal model of disorder remains well
below 10 000 (in units of the lattice constant) in the main part of the band in
spite of its exponential increase near the band edges.Comment: 12 two-column pages including 10 figures (epsfig), revtex, to appear
in PR
Magnetic field dependence of the exciton energy in a quantum disk
The groundstate energy and binding energy of an exciton, confined in a^M
quantum disk, are calculated as a function of an external magnetic field. The
confinement potential is a hard wall of finite height. The diamagnetic shift is
investigated for magnetic fields up to 40. Our results are applied to
self-assembled quantum dots and very good
agreement with experiments is obtained. Furthermore, we investigated the
influence of the dot size on the diamagnetic shift by changing the disk radius.
The exciton excited states are found as a function of the magnetic field. The
relative angular momentum is not a quantum number and changes with the magnetic
field strength.Comment: 10 pages, 17 figure
The velocity peaks in the cold dark matter spectrum on Earth
The cold dark matter spectrum on earth is expected to have peaks in velocity
space. We obtain estimates for the sizes and locations of these peaks. To this
end we have generalized the secondary infall model of galactic halo formation
to include angular momentum of the dark matter particles. This new model is
still spherically symmetric and it has self-similar solutions. Our results are
relevant to direct dark matter search experiments.Comment: 12 pages including 1 table and 4 figures, LaTeX, REVTEX 3.0 versio
Hidden degree of freedom and critical states in a two-dimensional electron gas in the presence of a random magnetic field
We establish the existence of a hidden degree of freedom and the critical
states of a spinless electron system in a spatially-correlated random magnetic
field with vanishing mean. Whereas the critical states are carried by the
zero-field contours of the field landscape, the hidden degree of freedom is
recognized as being associated with the formation of vortices in these special
contours. It is argued that, as opposed to the coherent backscattering
mechanism of weak localization, a new type of scattering processes in the
contours controls the underlying physics of localization in the random magnetic
field system. In addition, we investigate the role of vortices in governing the
metal-insulator transition and propose a renormalization-group diagram for the
system under study.Comment: 17 pages, 16 figures; Figs. 1, 7, 9, and 10 have been reduced in
quality for e-submissio
Neutral H density at the termination shock: a consolidation of recent results
We discuss a consolidation of determinations of the density of neutral
interstellar H at the nose of the termination shock carried out with the use of
various data sets, techniques, and modeling approaches. In particular, we focus
on the determination of this density based on observations of H pickup ions on
Ulysses during its aphelion passage through the ecliptic plane. We discuss in
greater detail a novel method of determination of the density from these
measurements and review the results from its application to actual data. The H
density at TS derived from this analysis is equal to 0.087 \pm 0.022 cm-3, and
when all relevant determinations are taken into account, the consolidated
density is obtained at 0.09 \pm 0.022 cm-3. The density of H in CHISM based on
literature values of filtration factor is then calculated at 0.16 \pm 0.04
cm-3.Comment: Submitted to Space Science Review
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