12,506 research outputs found
Aharonov-Bohm effect for an exciton in a finite width nano-ring
We study the Aharonov-Bohm effect for an exciton on a nano-ring using a 2D attractive fermionic Hubbard model. We extend previous results obtained for a 1D ring in which only azimuthal motion is considered, to a more general case of 2D annular lattices. In general, we show that the
existence of the localization effect, increased by the nonlinearity, makes the phenomenon in the 2D system similar to the 1D case. However, the introduction of radial motion introduces extra frequencies, different from the original isolated frequency corresponding to the excitonic Aharonov-
Bohm oscillations. If the circumference of the system becomes large enough, the Aharonov-Bohm effect is suppressed
Multifractal analysis with the probability density function at the three-dimensional Anderson transition
The probability density function (PDF) for critical wavefunction amplitudes
is studied in the three-dimensional Anderson model. We present a formal
expression between the PDF and the multifractal spectrum f(alpha) in which the
role of finite-size corrections is properly analyzed. We show the non-gaussian
nature and the existence of a symmetry relation in the PDF. From the PDF, we
extract information about f(alpha) at criticality such as the presence of
negative fractal dimensions and we comment on the possible existence of
termination points. A PDF-based multifractal analysis is hence shown to be a
valid alternative to the standard approach based on the scaling of general
inverse participation ratios.Comment: 4 pages, 7 figure
How structurally stable are global socioeconomic systems?
The stability analysis of socioeconomic systems has been centered on
answering whether small perturbations when a system is in a given quantitative
state will push the system permanently to a different quantitative state.
However, typically the quantitative state of socioeconomic systems is subject
to constant change. Therefore, a key stability question that has been
under-investigated is how strong the conditions of a system itself can change
before the system moves to a qualitatively different behavior, i.e., how
structurally stable the systems is. Here, we introduce a framework to
investigate the structural stability of socioeconomic systems formed by the
network of interactions among agents competing for resources. We measure the
structural stability of the system as the range of conditions in the
distribution and availability of resources compatible with the qualitative
behavior in which all the constituent agents can be self-sustained across time.
To illustrate our framework, we study an empirical representation of the global
socioeconomic system formed by countries sharing and competing for
multinational companies used as proxy for resources. We demonstrate that the
structural stability of the system is inversely associated with the level of
competition and the level of heterogeneity in the distribution of resources.
Importantly, we show that the qualitative behavior of the observed global
socioeconomic system is highly sensitive to changes in the distribution of
resources. We believe this work provides a methodological basis to develop
sustainable strategies for socioeconomic systems subject to constantly changing
conditions
Attraction between Neutral Dielectrics Mediated by Multivalent Ions in an Asymmetric Ionic Fluid
We study the interaction between two neutral plane-parallel dielectric bodies
in the presence of a highly asymmetric ionic fluid, containing multivalent as
well as monovalent (salt) ions. Image charge interactions, due to dielectric
discontinuities at the boundaries, as well as effects from ion confinement in
the slit region between the surfaces are taken fully into account, leading to
image-generated depletion attraction, ion correlation attraction and
steric-like repulsive interactions. We investigate these effects by employing a
combination of methods including explicit-ion and implicit-ion Monte-Carlo
simulations, as well as an effective interaction potential analytical theory.
The latter incorporates strong ion-image charge correlations, which develop in
the presence of high valency ions in the mixture. We show that the implicit-ion
simulations and the proposed analytical theory can describe the explicit
simulation results on a qualitative level, while excellent quantitative
agreement can be obtained for sufficiently large monovalent salt
concentrations. The resultant attractive interaction between the neutral
surfaces is shown to be significant, as compared with the usual van der Waals
interactions between semi-infinite dielectrics, and can thus play a significant
role at the nano scale.Comment: 9 pages, 4 figure
The influence of dispersal on a predator-prey system with two habitats
Dispersal between different habitats influences the dynamics and stability of
populations considerably. Furthermore, these effects depend on the local
interactions of a population with other species. Here, we perform a general and
comprehensive study of the simplest possible system that includes dispersal and
local interactions, namely a 2-patch 2-species system. We evaluate the impact
of dispersal on stability and on the occurrence of bifurcations, including
pattern forming bifurcations that lead to spatial heterogeneity, in 19
different classes of models with the help of the generalized modelling
approach. We find that dispersal often destabilizes equilibria, but it can
stabilize them if it increases population losses. If dispersal is nonrandom,
i.e. if emigration or immigration rates depend on population densities, the
correlation of stability with migration rates is positive in part of the
models. We also find that many systems show all four types of bifurcations and
that antisynchronous oscillations occur mostly with nonrandom dispersal
Quenched Charge Disorder and Coulomb Interactions
We develop a general formalism to investigate the effect of quenched fixed
charge disorder on effective electrostatic interactions between charged
surfaces in a one-component (counterion-only) Coulomb fluid. Analytical results
are explicitly derived for two asymptotic and complementary cases: i)
mean-field or Poisson-Boltzmann limit (including Gaussian-fluctuations
correction), which is valid for small electrostatic coupling, and ii)
strong-coupling limit, where electrostatic correlations mediated by counterions
become significantly large as, for instance, realized in systems with
high-valency counterions. In the particular case of two apposed and ideally
polarizable planar surfaces with equal mean surface charge, we find that the
effect of the disorder is nil on the mean-field level and thus the plates
repel. In the strong-coupling limit, however, the effect of charge disorder
turns out to be additive in the free energy and leads to an enhanced long-range
attraction between the two surfaces. We show that the equilibrium inter-plate
distance between the surfaces decreases for elevated disorder strength (i.e.
for increasing mean-square deviation around the mean surface charge), and
eventually tends to zero, suggesting a disorder-driven collapse transition.Comment: 13 pages, 2 figure
Presence and localization of a 30-kDa basic fibroblast growth factor-like protein in rodent testes
We have used a recently characterized rabbit antiserum against basic fibroblast growth factor (bFGF), which recognizes various forms of bFGF, to examine the presence and localization of bFGF in the testes of adult rats and mice and the 5-day-old rat. In Western blots of testicular homogenates of adult rats and mice and immature rats, immunoreactive single bands at approximately 30 kDa were detected. Immunocytochemistry revealed specific staining restricted to the tubular compartment. In 5-day-old rat testes, prespermatogonia were immunoreactive. The cytoplasm of pachytene spermatocytes was heavily stained in the adult testes of both species. Staining of these cells became evident around stage IV/V, was prominent in stage VII through IX and declined about stage XII/XIII (rat) or X-XI (mouse). Staining was seen in type A spermatogonia and in elongating spermatids in their cytoplasmatic lobes and along their flagellae. Sertoli cells were unstained. We propose that the pluripotential growth factor bFGF could be involved in the regulation of germ cell proliferation and differentiation in the adult and immature testis
Randomised controlled trial of primary school based intervention to reduce risk factors for obesity
OBJECTIVE: To assess if a school based intervention was
effective in reducing risk factors for obesity.
DESIGN: Group randomised controlled trial.
SETTING: 10 primary schools in Leeds.
PARTICIPANTS: 634 children aged 7-11 years.
INTERVENTION: Teacher training, modification of school
meals, and the development of school action plans
targeting the curriculum, physical education, tuck
shops, and playground activities.
MAIN OUTCOME MEASURES: Body mass index, diet,
physical activity, and psychological state.
RESULTS: Vegetable consumption by 24 hour recall was
higher in children in the intervention group than the
control group (weighted mean difference 0.3
portions/day, 95% confidence interval 0.2 to 0.4),
representing a difference equivalent to 50% of
baseline consumption. Fruit consumption was lower
in obese children in the intervention group ( - 1.0,
- 1.8 to - 0.2) than those in the control group. The
three day diary showed higher consumption of high
sugar foods (0.8, 0.1 to 1.6)) among overweight
children in the intervention group than the control
group. Sedentary behaviour was higher in overweight
children in the intervention group (0.3, 0.0 to 0.7).
Global self worth was higher in obese children in the
intervention group (0.3, 0.3 to 0.6). There was no
difference in body mass index, other psychological
measures, or dieting behaviour between the groups.
Focus groups indicated higher levels of self reported
behaviour change, understanding, and knowledge
among children who had received the intervention.
CONCLUSION: Although it was successful in producing
changes at school level, the programme had little
effect on children's behaviour other than a modest
increase in consumption of vegetables
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