1,610 research outputs found
First-principles study of the ferroelastic phase transition in CaCl_2
First-principles density-functional calculations within the local-density
approximation and the pseudopotential approach are used to study and
characterize the ferroelastic phase transition in calcium chloride (CaCl_2). In
accord with experiment, the energy map of CaCl_2 has the typical features of a
pseudoproper ferroelastic with an optical instability as ultimate origin of the
phase transition. This unstable optic mode is close to a pure rigid unit mode
of the framework of chlorine atoms and has a negative Gruneisen parameter. The
ab-initio ground state agrees fairly well with the experimental low temperature
structure extrapolated at 0K. The calculated energy map around the ground state
is interpreted as an extrapolated Landau free-energy and is successfully used
to explain some of the observed thermal properties. Higher-order anharmonic
couplings between the strain and the unstable optic mode, proposed in previous
literature as important terms to explain the soft-phonon temperature behavior,
are shown to be irrelevant for this purpose. The LAPW method is shown to
reproduce the plane-wave results in CaCl_2 within the precision of the
calculations, and is used to analyze the relative stability of different phases
in CaCl_2 and the chemically similar compound SrCl_2.Comment: 9 pages, 6 figures, uses RevTeX
Relaciones entre las dimensiones de las actitudes hacia las matemáticas en futuros maestros
En este trabajo se estudian las relaciones entre distintas dimensiones de las actitudes hacia las Matemáticas de los estudiantes del Grado en Educación Primaria de la Universidad de A Coruña del 1.er y 3.er curso recogidas durante tres años académicos consecutivos. Para ello se aplica el cuestionario de actitudes PAC de Naya-Riveiro, Soneira, Mato y Torre (2014) con una fiabilidad Alfa de Cronbach de 0.921 a una muestra de 308 estudiantes. El instrumento está formado por 19 ítems con cinco opciones de respuesta tipo Likert y tres dimensiones que miden el autoconcepto, la percepción que tiene el alumno de su profesor y el agrado hacia las Matemáticas. Los resultados muestran que existe una relación monótona creciente entre las distintas dimensiones de las actitudes en ambos cursos y que éstas se mantienen de un curso a otro
Influencia del software de geometría dinámica en la visualización matemática
Este trabajo tiene como objetivo exponer los aspectos teóricos relativos a la influencia del software de Geometría dinámica en el desarrollo de la visualización matemática con base a los resultados de las principales investigaciones realizadas en este ámbito. Se describe el marco teórico en el que encuadramos la presente investigación, y en particular se define el término visualización y se expone su importancia en el aprendizaje de la Geometría. Asimismo, se enuncian las dificultades que se encuentran en la práctica de la visualización y se explican las distintas pruebas existentes para evaluar la visualización. En último lugar, se detallan los beneficios del uso del software de Geometría dinámica para el desarrollo de la visualización
Bismuth radical catalysis in the activation and coupling of redox-active electrophiles
Radical cross-coupling reactions represent a revolutionary tool to forge C(sp3)–C and C(sp3)–heteroatom bonds, by means of transition metals, photoredox or electrochemical approaches. This study demonstrates how a low-valent bismuth complex is able to undergo one-electron oxidative addition with redox-active alkyl radical precursors in an autonomous manner, mimicking the behavior of first-row transition metals. This reactivity paradigm for bismuth gives rise to unique radical-equilibrium complexes, which could be fully characterized in solution and solid state. The resulting Bi(III)–C(sp3) intermediates display divergent reactivity patterns depending on the α-substituents of the alkyl fragment. Mechanistic investigations on this reactivity led to the development of a bismuth-catalyzed C(sp3)–N cross-coupling reaction that operates under mild conditions and accommodates synthetically relevant N-heterocycles as coupling partners
Phonons from neutron powder diffraction
The spherically averaged structure function \soq obtained from pulsed
neutron powder diffraction contains both elastic and inelastic scattering via
an integral over energy. The Fourier transformation of \soq to real space, as
is done in the pair density function (PDF) analysis, regularizes the data, i.e.
it accentuates the diffuse scattering. We present a technique which enables the
extraction of off-center phonon information from powder diffraction experiments
by comparing the experimental PDF with theoretical calculations based on
standard interatomic potentials and the crystal symmetry. This procedure
(dynamics from powder diffraction(DPD)) has been successfully implemented for
two systems, a simple metal, fcc Ni, and an ionic crystal, CaF. Although
computationally intensive, this data analysis allows for a phonon based
modeling of the PDF, and additionally provides off-center phonon information
from powder neutron diffraction
Spatial variability patterns of some Vertisol properties at a field scale using standardized data
Spatial variability of Vertisol properties is relevant for identifying those zones with physical degradation. In this sense, one has to face the problem of identifying the origin and distribution of spatial variability patterns. The objectives of the present work were (i) to quantify the spatial structure of different physical properties collected from a Vertisol, (ii) to search for potential correlations between different spatial patterns and (iii) to identify relevant components through multivariate spatial analysis. The study was conducted on a Vertisol (Typic Hapludert) dedicated to sugarcane (Saccharum officinarum L.) production during the last sixty years. We used six soil properties collected from a squared grid (225 points) (penetrometer resistance (PR), total porosity, fragmentation dimension (Df), vertical electrical conductivity (ECv), horizontal electrical conductivity (ECh) and soil water content (WC)). All the original data sets were z-transformed before geostatistical analysis. Three different types of semivariogram models were necessary for fitting individual experimental semivariograms. This suggests the different natures of spatial variability patterns. Soil water content rendered the largest nugget effect (C0 = 0.933) while soil total porosity showed the largest range of spatial correlation (A = 43.92 m). The bivariate geostatistical analysis also rendered significant cross-semivariance between different paired soil properties. However, four different semivariogram models were required in that case. This indicates an underlying co-regionalization between different soil properties, which is of interest for delineating management zones within sugarcane fields. Cross-semivariograms showed larger correlation ranges than individual, univariate, semivariograms (A ≥ 29 m). All the findings were supported by multivariate spatial analysis, which showed the influence of soil tillage operations, harvesting machinery and irrigation water distribution on the status of the investigated area
Drivers of Human‒wildlife interactions in a co-existence area: a case study of the Ngorongoro conservation area, Tanzania
Communities in Africa bordering national parks or protected areas commonly overlap with wildlife. However, it is unclear to what degree such overlaps result in interactions with wildlife. The Ngorongoro Conservation Area (NCA) was designated a multiple land-use conservation area in 1959. Maasai and Datoga pastoralists and Hadzabe hunter-gatherers reside with protected wildlife in NCA. The study was carried out in four Maasai villages within the NCA, including Kayapus, Endulen, Meshili, and Nainokanoka. A cross-sectional study was used to assess drivers of human‒wildlife interactions using questionnaire surveys, focus group discussions, and field visits. A total of 396 households participated in the survey. The collected data were analysed using qualitative data analysis techniques and descriptive statistics such as frequencies and means. The habitat, which comprises water, pasture, shelter, and space, accounted for 100% of interactions, indicating that it is the primary driver of human‒wildlife conflict. Other driving factors for human‒wildlife interactions are the increase in wildlife, collections of firewood, domestic animals kept, and influence of community sleeping arrangements, searching for traditional medicines, and killing of lions for ritual purposes or defense. Large household sizes (36 family members) coupled with climate change have also driven and fuelled human‒wildlife interactions. Challenges identified as threatening human‒wildlife co-existence are injuries, deaths, disease transmission, and destruction of property. To mitigate human‒wildlife conflicts, the following are recommended: the increase in boarding schools coupled with the increase in enrolment of students in boarding schools or providing reliable transport, distribution of tap water, increasing food assistance to the community living in poverty, controlling population increase through reallocation the population in other areas, introducing zero-grazing, using biogas, discouraging community sleeping arrangements, i.e., humans with calves in the same house, improving record-keeping of the wildlife attacks, provisional dissemination of research findings to the community
New Symmetries in Crystals and Handed Structures
For over a century, the structure of materials has been described by a
combination of rotations, rotation-inversions and translational symmetries. By
recognizing the reversal of static structural rotations between clockwise and
counterclockwise directions as a distinct symmetry operation, here we show that
there are many more structural symmetries than are currently recognized in
right- or left-handed handed helices, spirals, and in antidistorted structures
composed equally of rotations of both handedness. For example, though a helix
or spiral cannot possess conventional mirror or inversion symmetries, they can
possess them in combination with the rotation reversal symmetry. Similarly, we
show that many antidistorted perovskites possess twice the number of symmetry
elements as conventionally identified. These new symmetries predict new forms
for "roto" properties that relate to static rotations, such as rotoelectricity,
piezorotation, and rotomagnetism. They also enable symmetry-based search for
new phenomena, such as multiferroicity involving a coupling of spins, electric
polarization and static rotations. This work is relevant to structure-property
relationships in all material structures with static rotations such as
minerals, polymers, proteins, and engineered structures.Comment: 15 Pages, 4 figures, 3 Tables; Fig. 2b has error
Localization and Absorption of Light in 2D Composite Metal-Dielectric Films at the Percolation Threshold
We study in this paper the localization of light and the dielectric
properties of thin metal-dielectric composites at the percolation threshold and
around a resonant frequency where the conductivities of the two components are
of the same order. In particular, the effect of the loss in metallic components
are examined. To this end, such systems are modelized as random networks,
and the local field distribution as well as the effective conductivity are
determined by using two different methods for comparison: an exact resolution
of Kirchoff equations, and a real space renormalization group method. The
latter method is found to give the general behavior of the effective
conductivity but fails to determine the local field distribution. It is also
found that the localization still persists for vanishing losses. This result
seems to be in agreement with the anomalous absorption observed experimentally
for such systems.Comment: 14 page latex, 3 ps figures. submitte
The Relation of physical self-concept, anxiety, and BMI among mexicam university students
El propósito de la investigación es conocer y relacionar los resultados del
autoconcepto físico, con diferentes tipos de ansiedad y el IMC en los estudiantes
de enfermería y educación física dela península de Yucatán (México). La
muestra estuvo formada por 264 participantes, de los cuales 91 eran varones y
173 mujeres, con edades comprendidas entre 18 y 25 años. Nuestros datos
revelan que en el autoconcepto físico, los valores obtenidos por los estudiantes
de educación física son superiores al de los estudiantes de enfermería. Los
estudiantes de enfermería presentan mayores valores de ansiedad estado que
los de educación física. La percepción que tienen los estudiantes de ciclos
superiores en prácticamente todas las dimensiones del autoconcepto es superior
a la del alumnado de ciclos inferiores. Los niveles de ansiedad disminuyen
conforme pasan de ciclos inferiores a superioresThe purpose of this paper is to know and relate the results of the various
dimensions formed by the physical self-concept with respect to the different
types of anxiety and the BMI among nursing and physical education students in
the area of the Yucatán peninsula in Mexico. The study is based on a sample of
264 people, being 91 men and 173 women between the ages of 18 and 25. The
outcome shows that the values obtained by physical education students are
higher than those obtained by nursing students regarding the physical selfconcept.
Nursing students present a higher level of anxiety compared to
physical education students. The perception showed by junior and senior
students with regard to almost all dimensions of self-concept is higher than that
of freshmen and sophomore students. Anxiety level among students decreases
throughout the year
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