21,276 research outputs found
Stability of ferromagnetism in the half-metallic pnictides and similar compounds: A first-principles study
Based on first-principles electron structure calculations and employing the
frozen-magnon approximation we study the exchange interactions in a series of
transition-metal binary alloys crystallizing in the zinc-blende structure and
calculate the Curie temperature within both the mean-field approximation (MFA)
and random-phase approximation (RPA). We study two Cr compounds, CrAs and CrSe,
and four Mn compounds: MnSi, MnGe, MnAs and MnC. MnC, MnSi and MnGe are
isovalent to CrAs and MnAs is isoelectronic with CrSe. Ferromagnetism is
particular stable for CrAs, MnSi and MnGe: All three compounds show Curie
temperatures around 1000 K. On the other hand, CrSe and MnAs show a tendency to
antiferromagnetism when compressing the lattice. In MnC the half-metallic gap
is located in the majority-spin channel contrary to the other five compounds.
The large half-metallic gaps, very high Curie temperatures, the stability of
the ferromagnetism with respect to the variation of the lattice parameter and a
coherent growth on semiconductors make MnSi and CrAs most promising candidates
for the use in spintronics devises.Comment: 17 pages, 6 figure
Nonlinear normal modes of a two degree of freedom oscillator with a bilateral elastic stop
A study of the non linear modes of a two degree of freedom mechanical system
with bilateral elastic stop is considered. The issue related to the
non-smoothness of the impact force is handled through a regularization
technique. In order to obtain the Nonlinear Normal Mode (NNM), the harmonic
balance method with a large number of harmonics, combined with the asymptotic
numerical method, is used to solve the regularized problem. These methods are
present in the software "package" MANLAB. The results are validated from
periodic orbits obtained analytically in the time domain by direct integration
of the non regular problem. The two NNMs starting respectively from the two
linear normal modes of the associated underlying linear system are discussed.
The energy-frequency plot is used to present a global vision of the behavior of
the modes. The dynamics of the modes are also analyzed comparing each periodic
orbits and modal lines. The first NNM shows an elaborate dynamics with the
occurrence of multiple impacts per period. On the other hand, the second NNM
presents a more simple dynamics with a localization of the displacement on the
first mass
Tomographic approach to the violation of Bell's inequalities for quantum states of two qutrits
The tomographic method is employed to investigate the presence of quantum
correlations in two classes of parameter-dependent states of two qutrits. The
violation of some Bell's inequalities in a wide domain of the parameter space
is shown. A comparison between the tomographic approach and a recent method
elaborated by Wu, Poulsen and Molmer shows the better adequacy of the former
method with respect to the latter one.Comment: 9 pages, 4 figure
Ensinar Ciência no pré-escolar. Contributos para aprendizagens de outras áreas/domÃnios curriculares. Relato de experiências realizadas em jardins de infância
O ensino das ciências deve começar no jardim de infância, através de uma abordagem transversal em que se evidencia o papel das ciências e em particular das actividades experimentais, como contexto privilegiado para o desenvolvimento das outras áreas do currÃculo. Na comunicação oral serão apresentados diversos exemplos de actividades cientÃficas realizadas em salas de jardim de infância, em que se privilegia uma abordagem transversal dos conteúdos, mobilizadora de conhecimentos, capacidades e atitudes de diferentes áreas e domÃnios (matemática, comunicação, expressões plástica e dramática, formação pessoal e social)
Improvements on analytic modelling of stellar spots
In this work we present the solution of the stellar spot problem using the
Kelvin-Stokes theorem. Our result is applicable for any given location and
dimension of the spots on the stellar surface. We present explicitely the
result up to the second degree in the limb darkening law. This technique can be
used to calculate very efficiently mutual photometric effects produced by
eclipsing bodies occulting stellar spots and to construct complex spot shapes.Comment: Resubmitted to MNRAS after accounting for minor comments of second
review, 9 pages, 5 figures, software available at
http://eduscisoft.com/KSINT
A Hidden Twelve-Dimensional SuperPoincare Symmetry In Eleven Dimensions
First, we review a result in our previous paper, of how a ten-dimensional
superparticle, taken off-shell, has a hidden eleven-dimensional superPoincare
symmetry. Then, we show that the physical sector is defined by three
first-class constraints which preserve the full eleven-dimensional symmetry.
Applying the same concepts to the eleven dimensional superparticle, taken
off-shell, we discover a hidden twelve dimensional superPoincare symmetry that
governs the theory.Comment: 13 page
Orbital moment of a single Co atom on a Pt(111) surface - a view from correlated band theory
The orbital magnetic moment of a Co adatom on a Pt(111) surface is calculated
in good agreement with experimental data making use of the LSDA+U method. It is
shown that both electron correlation induced orbital polarization and
structural relaxation play essential roles in orbital moment formation. The
microscopic origins of the orbital moment enhancement are discussed
A study of atom localization in an optical lattice by analysis of the scattered light
We present an experimental study of a four beam optical lattice using the
light scattered by the atoms in the lattice. We use both intensity correlations
and observations of the transient behavior of the scattering when the lattice
is suddenly switched on. We compare results for 3 different configurations of
the optical lattice. We create situations in which the Lamb-Dicke effect is
negligible and show that, in contrast to what has been stated in some of the
literature, the damping rate of the 'coherent' atomic oscillations can be much
smaller than the inelastic photon scattering rate.Comment: An old pape
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