5,733 research outputs found
A host of traveling waves in a model of three-dimensional water-wave dynamics
We describe traveling waves in a basic model for three-dimensional water-wave
dynamics in the weakly nonlinear long-wave regime. Small solutions that are
periodic in the direction of translation (or orthogonal to it) form an
infinite-dimensional family. We characterize these solutions through spatial
dynamics, by reducing a linearly ill-posed mixed-type initial-value problem to
a center manifold of infinite dimension and codimension. A unique global
solution exists for arbitrary small initial data for the two-component bottom
velocity, specified along a single line in the direction of translation (or
orthogonal to it). A dispersive, nonlocal, nonlinear wave equation governs the
spatial evolution of bottom velocity.Comment: 22 pages with 1 figure, LaTeX2e with amsfonts, epsfig package
School Shootings: What’s the Plan?
The purpose of this research review was to gather the information that will help make changes with evidence-based approaches to help prevent and intervene with these horrendous crimes that have impacted the United States. Inspired by students\u27 and staff member\u27s outcries, these tragic events that attack the most innocent victims must be taken into complete consideration to guarantee their safety. This research review explores the concerns and issues as they relate to policies, code of ethics, strategies, safety planning, and training that have proven to work in preventing any future attacks. States and school officials have implemented programs, security measures, and mental health personnel to help prevent and intervene with these attacks. Another focus has been on gun violence, youths are no longer using guns for recreational purposes, youths are using guns to harm people. The American people request more gun control in our country, citizens are expressing their opinions, and thoughts on what can help with controlling gun violence in school shootings. Relying on evidence-based approaches and taking school security measures seriously has helped in developing policies and procedures that help keep schools safe and maintain a positive school climate as well as a culture that will improve school achievement
Preparation of an ultra-cold sample of ammonia molecules for precision measurements
We present experiments in which an ultra-cold sample of ammonia molecules is
released from an electrostatic trap and recaptured after a variable time. It is
shown that, by performing adiabatic cooling before releasing the molecules and
adiabatic re-compression after they are recaptured, we are able to observe
molecules even after more than 10 ms of free expansion. A coherent measurement
performed during this time will have a statistical uncertainty that decreases
approximately as the inverse of the square root of the expansion time. This
offers interesting prospects for high-resolution spectroscopy and precision
tests of fundamental physics theories
Coupled magnetic and elastic properties in LaPr(CaSr)MnO manganites
We investigate a series of manganese oxides, the
La0.225Pr0.4(Ca1-xSrx)0.375MnO3 system. The x = 0 sample is a prototype
compound for the study of phase separation in manganites, where ferromagnetic
and charge ordered antiferromagnetic phases coexist. Replacing Ca2+ by Sr2+
gradually turns the system into a homogeneous ferromagnet. Our results show
that the material structure plays a major role in the observed magnetic
properties. On cooling, at temperatures below 100 K, a strong contraction of
the lattice is followed by an increase in the magnetization. This is observed
both through thermal expansion and magnetostriction measurements, providing
distinct evidence of magneto-elastic coupling in these phase separated
compounds
Magnetocaloric effect in manganites: metamagnetic transitions for magnetic refrigeration
We present a study of the magnetocaloric effect in La5/8-yPryCa3/8MnO3
(y=0.3) and Pr0.5Ca0.09Sr0.41MnO3 manganites. The low temperature state of both
ystems is the result of a competition between the antiferromagnetic and
ferromagnetic phases. The samples display magnetocaloric effect evidenced in an
adiabatic temperature change during a metamagnetic transition from an
antiferromagnetic to a ferromagnetic phase . As additional features,
La5/8-yPryCa3/8MnO3 exhibits phase separation characterized by the coexistence
of antiferromagnetic and ferromagnetic phases and Pr0.5Ca0.09Sr0.41MnO3
displays inverse magnetocaloric effect in which temperature decreases while
applying an external magnetic field. In both cases, a significant part of the
magnetocaloric effect appears from non-reversible processes. As the traditional
thermodynamic description of the effect usually deals with reversible
transitions, we developed an alternative way to calculate the adiabatic
temperature change in terms of the change of the relative ferromagnetic
fraction induced by magnetic field. To evaluate our model, we performed direct
measurement of the sample's adiabatic temperature change by means of a
differential thermal analysis. An excellent agreement has been obtained between
experimental and calculated data. These results show that metamagnetic
transition in manganites play an important role in the study of magnetic
refrigeration.Comment: Acepted to be published in Applied Physics Letter
Estudio de los factores climaticos y geograficos que influyen en la presencia de casos de dengue y criaderos de Aedes aegypti en el municipio de Bello
El dengue es una enfermedad endémica en Colombia donde Antioquia es uno de los departamentos que mas casos aporta, y la gran mayorÃa provienen del municipio de Bello. En este trabajo se usaron datos satelitales de la región que daban cuenta de las condiciones climáticas y geográficas y datos georreferenciados de casos de la enfermedad y criaderos del vector del año 2008 para investigar las condiciones que influyen en la aparición de la enfermedad. Para su realización se analizó el problema desde un enfoque sistémico apoyándose en herramientas matemáticas y estadÃsticas como la regresión logÃstica y el agrupamiento difuso.
Con los resultados obtenidos se busca el desarrollo futuro de un modelo que permita predecir la presencia de los casos y los criaderos de la enfermedad y poder tomar medidas anticipadas para prevenirlas
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