9,462 research outputs found
A cluster model with random anisotropy for hysteresis jumps in CeNiCu alloys
Some Cerium compounds exhibit hysteresis cycles with sharp macroscopic jumps
in the magnetization at very low temperatures. This effect is attributed to the
formation of clusters in which the anisotropy competes with the applied
magnetic field. Here, we present a simple model where a lattice of
ferromagnetically coupled spins is separated in clusters of random sizes and
with random anisotropy. Within this model, we obtain hysteresis cycles
presenting jumps that behave in a similar way that the experimental ones, and
that disappear when increasing the temperature. The results are in good
agreement with the hysteresis cycles measured at very low temperatures in
CeNiCu and the comparison with these experimental results allows
to discriminate the relative importance of the mechanisms driving the thermal
evolution of the cycles.Comment: Accepted in PR
Integration of Dirac-Jacobi structures
We study precontact groupoids whose infinitesimal counterparts are
Dirac-Jacobi structures. These geometric objects generalize contact groupoids.
We also explain the relationship between precontact groupoids and homogeneous
presymplectic groupoids. Finally, we present some examples of precontact
groupoids.Comment: 10 pages. Brief changes in the introduction. References update
Physical activity trends in spanish adolescents
Este estudio se propone conocer la evolución de la práctica de actividad física moderada-vigorosa y examinar si la tendencia se mantiene en función de factores sociodemográficos, como el sexo y la edad, así como de factores socioeconómicos y su influencia según la comunidad autónoma. La muestra está compuesta de adolescentes españoles de 11 a 18 años de las ediciones 2002 y 2006 del estudio Health Behaviour in School-aged Children (HBSC). Los principales resultados muestran que, a pesar de un cierto aumento en el nivel de actividad física en los cuatro años que distan entre ambas ediciones
(especialmente en los adolescentes más pequeños), estos niveles se encuentran todavía lejos de las recomendaciones. Asimismo, los datos señalan como necesidades más urgentes promover un estilo de vida activo en los adolescentes de 13 años en adelante (especialmente en las chicas) y en los adolescentes de niveles socioeconómicos más bajos de determinadas comunidades autónomasThis study tries to find out the tendency of moderate-to-vigorous physical activity and to assess whether the trends are maintained with regard to socio-demographic factors such as sex and age, and also socio-economic factors and how they vary depending on the region. The sample comprises Spanish adolescents aged between 11 and 18 years, from the 2002 and 2006 surveys of the Health Behaviour in School-aged Children (HBSC) study. The main results reveal that, despite a certain increase in physical activity level in the four years span between both surveys (particularly in younger adolescents), these levels are still a long way from the recommendations. Likewise, data highlight, as immediate needs, the promotion of an active lifestyle among adolescents from 13 years onwards (particularly among the girls) and also among adolescents of lower socio-economic status in some regionsEsta investigación ha sido realizada gracias a los sucesivos convenios de colaboración firmados entre el Ministerio de Sanidad y Consumo y la Universidad de Sevilla para la ejecución del estudio HBSC (Health Behaviour in School-aged Children) en Españ
Magnetic Field scaling of Relaxation curves in Small Particle Systems
We study the effects of the magnetic field on the relaxation of the
magnetization of small monodomain non-interacting particles with random
orientations and distribution of anisotropy constants. Starting from a master
equation, we build up an expression for the time dependence of the
magnetization which takes into account thermal activation only over barriers
separating energy minima, which, in our model, can be computed exactly from
analytical expressions. Numerical calculations of the relaxation curves for
different distribution widths, and under different magnetic fields H and
temperatures T, have been performed. We show how a \svar scaling of the
curves, at different T and for a given H, can be carried out after proper
normalization of the data to the equilibrium magnetization. The resulting
master curves are shown to be closely related to what we call effective energy
barrier distributions, which, in our model, can be computed exactly from
analytical expressions. The concept of effective distribution serves us as a
basis for finding a scaling variable to scale relaxation curves at different H
and a given T, thus showing that the field dependence of energy barriers can be
also extracted from relaxation measurements.Comment: 12 pages, 9 figures, submitted to Phys. Rev.
Cost effectiveness analysis of larval therapy for leg ulcers
Objective: To assess the cost effectiveness of larval therapy compared with hydrogel in the management of leg ulcers. Design: Cost effectiveness and cost utility analyses carried out alongside a pragmatic multicentre, randomised, open trial with equal randomisation. Population: Intention to treat population comprising 267 patients with a venous or mixed venous and arterial ulcers with at least 25% coverage of slough or necrotic tissue. Interventions: Patients were randomly allocated to debridement with bagged larvae, loose larvae, or hydrogel. Main outcome measure: The time horizon was 12 months and costs were estimated from the UK National Health Service perspective. Cost effectiveness outcomes are expressed in terms of incremental costs per ulcer-free day (cost effectiveness analysis) and incremental costs per quality adjusted life years (cost utility analysis). Results: The larvae arms were pooled for the main analysis. Treatment with larval therapy cost, on average, 96.70 pound ((sic)109.61; $140.57) more per participant per year (95% confidence interval -491.9 pound to 685.8) pound than treatment with hydrogel. Participants treated with larval therapy healed, on average, 2.42 days before those in the hydrogel arm (95% confidence interval -0.95 to 31. 91 days) and had a slightly better health related quality of life, as the annual difference in QALYs was 0.011 (95% confidence interval -0.067 to 0.071). However, none of these differences was statistically significant. The incremental cost effectiveness ratio for the base case analysis was estimated at 8826 pound per QALY gained and 40 pound per ulcer-free day. Considerable uncertainty surrounds the outcome estimates. Conclusions: Debridement of sloughy or necrotic leg ulcers with larval therapy is likely to produce similar health benefits and have similar costs to treatment with hydrogel. Trial registration: Current Controlled Trials ISRCTN55114812 and National Research Register N0484123692
Effect of band filling in the Kondo lattice: A mean-field approach
The usual Kondo-lattice, including an antiferromagnetic exchange interaction
between nearest-neighboring localized spins, is treated here in a mean-field
scheme that introduces two mean-field parameters: one associated with the local
Kondo effect, and the other related to the magnetic correlations between
localized spins. Phases with short-range magnetic correlations or coexistence
between those and the Kondo effect are obtained. By varying the number of
electrons in the conduction band, we notice that the Kondo effect tends to be
suppressed away from half filling, while magnetic correlations can survive if
the Heisenberg coupling is strong enough. An enhanced linear coefficient of the
specific heat is obtained at low temperatures in the metallic state.Comment: 7 pages, ReVTeX two-column, 7 figure
Electromagnetic emission of white dwarf binary mergers
It has been recently proposed that the ejected matter from white dwarf (WD)
binary mergers can produce transient, optical and infrared emission similar to
the "kilonovae" of neutron star (NS) binary mergers. To confirm this we
calculate the electromagnetic emission from WD-WD mergers and compare with
kilonova observations. We simulate WD-WD mergers leading to a massive, fast
rotating, highly magnetized WD with an adapted version of the
smoothed-particle-hydrodynamics (SPH) code Phantom. We thus obtain initial
conditions for the ejecta such as escape velocity, mass and initial position
and distribution. The subsequent thermal and dynamical evolution of the ejecta
is obtained by integrating the energy-conservation equation accounting for
expansion cooling and a heating source given by the fallback accretion onto the
newly-formed WD and its magneto-dipole radiation. We show that magnetospheric
processes in the merger can lead to a prompt, short gamma-ray emission of up to
erg in a timescale of - s. The bulk of the ejecta
initially expands non-relativistically with velocity and then it
accelerates to due to the injection of fallback accretion energy. The
ejecta become transparent at optical wavelengths around days
post-merger with a luminosity - erg s. The X-ray
emission from the fallback accretion becomes visible around -
day post-merger with a luminosity of erg s. We also predict
the post-merger time at which the central WD should appear as a pulsar
depending on the value of the magnetic field and rotation period.Comment: 12 pages, Accepted for publication in JCA
Note About Hamiltonian Structure of Non-Linear Massive Gravity
We perform the Hamiltonian analysis of non-linear massive gravity action
studied recently in arXiv:1106.3344 [hep-th]. We show that the Hamiltonian
constraint is the second class constraint. As a result the theory possesses an
odd number of the second class constraints and hence all non physical degrees
of freedom cannot be eliminated.Comment: 15 page
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