1,032 research outputs found
Magnetic excitations of perovskite rare-earth nickelates: RNiO
The perovskite nickelates RNiO (R: rare-earth) have been studied as
potential multiferroic compounds. A certain degree of charge disproportionation
in the Ni ions has been confirmed by high resolution synchrotron power
diffraction: instead of the nominal Ni valence, they can have the
mixed-valence state Ni and Ni, though agreement
has not been reached on the precise value of (e.g. for NdNiO,
and were reported). Also, the magnetic ground state
is not yet clear: collinear and non-collinear Ni-O magnetic structures have
been proposed to explain neutron diffraction and soft X-ray resonant
sccattering results in these compounds, and more recently a canted
antiferromagnetic spin arrangement was proposed on the basis of magnetic
susceptibility measurements. This scenario is reminiscent of the situation in
the half-doped manganites.
In order to gain insight into the ground state of these compounds, we studied
the magnetic excitations of some of the different phases proposed, using a
localized spin model for a simplified spin chain which could describe these
compounds. We first analize the stability of the collinear, orthogonal, and
intermediate phases in the classical case. We then explore the quantum ground
state indirectly, calculating the spin excitations obtained for each phase,
using the Holstein-Primakoff transformation and the linear spin-wave
approximation. For the collinear and orthogonal () phases, we
predict differences in the magnon spectrum which would allow to distinguish
between them in future inelastic neutron scattering experiments
Quantum magnons of the intermediate phase of half-doped manganite oxides
At half doping, the ground state of three-dimensional manganite perovskite
oxides like RCaMnO, where R is a trivalent ion such as La, Pr,
etc, is still unclear. Many experimental findings agree better with the
combined magnetic, charge, and orbital order characteristic of the
"intermediate phase", introduced by Efremov et al. in 2004 [Nature Mats. 3,
853]. This phase consists of spin dimers (thus incorporating aspects of the
Zener polaron phase (ZP) proposed in 2002 by Daoud-Aladine et al. [Phys. Rev.
Lett. 89, 097205]), though formed by a pair of parallel Mn spins of different
magnitude, in principle (thereby allowing for a degree of Mn charge
disproportionation: not necessarily as large as that of Mn-Mn in
Goodenough's original CE phase [Phys. Rev. 100, 564 (1955)]). In the
intermediate phase, consecutive spin dimers localed along the planar zig-zag
chains are oriented at a constant relative angle between them. Varying
Mn-charge disproportionation and , the intermediate phase should allow to
continuously interpolate between the two limiting cases of the CE phase and the
dimer phase denoted as "orthogonal intermediate phase". It is not easy
to find a microscopic model able to describe the phenomenological intermediate
phase adequately for the spin, charge, and orbital degrees of freedom
simultaneously. Here, we study the quantum spin excitations of a planar model
of interacting localized spins, which we found can stabilize the intermediate
phase classically. We compare the quantum magnons of the intermediate phase
with those of the CE and orthogonal phases, in the context of recent
experimental results.Comment: 5 pages, 8 figures Manuscript accepted 29 April 2013, by IEEE -
Transactions on Magnetic
The accretion disk in the post period-minimum cataclysmic variable SDSS J080434.20+510349.2
This study of SDSS0804 is primarily concerned with the double-hump shape in
the light curve and its connection with the accretion disk in this bounce-back
system. Time-resolved photometric and spectroscopic observations were obtained
to analyze the behavior of the system between superoutbursts. A geometric model
of a binary system containing a disk with two outer annuli spiral density waves
was applied to explain the light curve and the Doppler tomography. Observations
were carried out during 2008-2009, after the object's magnitude decreased to
V~17.7(0.1) from the March 2006 eruption. The light curve clearly shows a
sinusoid-like variability with a 0.07 mag amplitude and a 42.48 min
periodicity, which is half of the orbital period of the system. In Sept. 2010,
the system underwent yet another superoutburst and returned to its quiescent
level by the beginning of 2012. This light curve once again showed a
double-humps, but with a significantly smaller ~0.01mag amplitude. Other types
of variability like a "mini-outburst" or SDSS1238-like features were not
detected. Doppler tomograms, obtained from spectroscopic data during the same
period of time, show a large accretion disk with uneven brightness, implying
the presence of spiral waves. We constructed a geometric model of a bounce-back
system containing two spiral density waves in the outer annuli of the disk to
reproduce the observed light curves. The Doppler tomograms and the
double-hump-shape light curves in quiescence can be explained by a model system
containing a massive >0.7Msun white dwarf with a surface temperature of
~12000K, a late-type brown dwarf, and an accretion disk with two outer annuli
spirals. According to this model, the accretion disk should be large, extending
to the 2:1 resonance radius, and cool (~2500K). The inner parts of the disk
should be optically thin in the continuum or totally void.Comment: 12 pages, 15 figures, accepted for publication in A&
Cómo elegir una función de activación para el aprendizaje profundo
Activation functions are important in each layer of the neural network because they allow the network to learn complex relationships between the input data and the output data. They also introduce nonlinearity into the network, which is essential for learning patterns in data. Activation functions play a critical role in the training and optimization of deep learning models, and choosing the right activation function can significantly impact the model’s performance. This article presents a summary of the features of these functions.
Las funciones de activación son importantes en cada capa de la red neuronal porque permiten a la red aprender relaciones complejas entre los datos de entrada y los de salida. También introducen la no linealidad en la red, que es esencial para aprender patrones en los datos. Las funciones de activación desempeñan un papel fundamental en el entrenamiento y la optimización de los modelos de aprendizaje profundo, y la elección de la función de activación adecuada puede influir significativamente en el rendimiento del modelo. Este artÃculo presenta un resumen de las caracterÃsticas de estas funciones.
 
Evaluation of additional head of biceps brachii: a study with autopsy material
Additional head of the biceps brachii (AHBB) has been reported in different population groups with a frequency of 1–25%. The purpose of this study was to determine the incidence and morphologic expression of the AHBB as determined in a sample of the Colombian population. An exploration was conducted with 106 arms corresponding to unclaimed corpses autopsied at Institute of Legal and Forensic Medicine of Bucaramanga, Colombia. Using medial incision involvingskin, subcutaneous tissue, and brachial fascia, the heads of the biceps and their innervating branches were visualised. One AHBB was observed in 21 (19.8%) of the arms evaluated, with non-significant difference (p = 0.568) per side of presentation: 11 (52.4%) cases on the right side and 10 (47.6%) on the left side. All AHBBs were originated in the infero-medial segment of the humerus, with a mean thickness of 17.8 ± 6.8 mm. In 4 (19%) cases the fascicle was thin, less than 10 mm; in 7 (33.3%) cases it was of medium thickness, between 11 and 20 mm, whereas in 47.6% it was longer than 20 mm. The length of the AHBB was 118.3 ± 26.8 mm; its motor point supplied by the musculocutaneous nerve was located at 101.3 ± 20.9 mm of the bi-epicondylar line. The incidence of AHBB in this study is located at the upper segment of what has been reportedin the literature and could be a morphologic trait of the Colombian population; in agreement with prior studies, the origin was the infero-medial surface of the humerus
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