481 research outputs found
Food Price Volatility over the Last Decade in Niger and Malawi: Extent, Sources and Impact on Child Malnutrition
Recently, considerable attention has rightly been paid to the nutritional impact of the sharp
hikes in international food prices which took place in 2007-8 and, again, in 2010-11. While sacrosanct,
this growing focus has somewhat obscured the effect of other factors which do affect malnutrition in
the Sub-Saharan Africa context, i.e. the long term impact of agricultural policies, huge and persistent
seasonal variation in domestic food prices, and the impact of famines which still regularly stalk the
continent. This paper focuses on the relative weight of these factors in explaining child malnutrition
(proxied by the number of child admissions to feeding centers) in Malawi and Niger, two prototypical
countries in the region. The analysis shows that the drivers of domestic food staple prices and of the
ensuing child malnutrition have to be found not only – or not primarily – in the changes of
international food prices but mainly in the impact of agricultural policies on food production, the
persistence of a strong food price seasonality, and recurrent and often poorly attended famines.
Indeed, even during years of declines in international food prices, these factors often exert a huge
upward pressures on domestic food prices and child malnutrition
Direct exoplanet detection and characterization using the ANDROMEDA method: Performance on VLT/NaCo data
Context. The direct detection of exoplanets with high-contrast imaging
requires advanced data processing methods to disentangle potential planetary
signals from bright quasi-static speckles. Among them, angular differential
imaging (ADI) permits potential planetary signals with a known rotation rate to
be separated from instrumental speckles that are either statics or slowly
variable. The method presented in this paper, called ANDROMEDA for ANgular
Differential OptiMal Exoplanet Detection Algorithm is based on a maximum
likelihood approach to ADI and is used to estimate the position and the flux of
any point source present in the field of view. Aims. In order to optimize and
experimentally validate this previously proposed method, we applied ANDROMEDA
to real VLT/NaCo data. In addition to its pure detection capability, we
investigated the possibility of defining simple and efficient criteria for
automatic point source extraction able to support the processing of large
surveys. Methods. To assess the performance of the method, we applied ANDROMEDA
on VLT/NaCo data of TYC-8979-1683-1 which is surrounded by numerous bright
stars and on which we added synthetic planets of known position and flux in the
field. In order to accommodate the real data properties, it was necessary to
develop additional pre-processing and post-processing steps to the initially
proposed algorithm. We then investigated its skill in the challenging case of a
well-known target, Pictoris, whose companion is close to the detection
limit and we compared our results to those obtained by another method based on
principal component analysis (PCA). Results. Application on VLT/NaCo data
demonstrates the ability of ANDROMEDA to automatically detect and characterize
point sources present in the image field. We end up with a robust method
bringing consistent results with a sensitivity similar to the recently
published algorithms, with only two parameters to be fine tuned. Moreover, the
companion flux estimates are not biased by the algorithm parameters and do not
require a posteriori corrections. Conclusions. ANDROMEDA is an attractive
alternative to current standard image processing methods that can be readily
applied to on-sky data
Field-induced level crossings in spin clusters: Thermodynamics and magneto-elastic instability
Quantum spin clusters with dominant antiferromagnetic Heisenberg exchange
interactions typically exhibit a sequence of field-induced level crossings in
the ground state as function of magnetic field. For fields near a level
crossing, the cluster can be approximated by a two-level Hamiltonian at low
temperatures. Perturbations, such as magnetic anisotropy or spin-phonon
coupling, sensitively affect the behavior at the level-crossing points. The
general two-level Hamiltonian of the spin system is derived in first-order
perturbation theory, and the thermodynamic functions magnetization, magnetic
torque, and magnetic specific heat are calculated. Then a magneto-elastic
coupling is introduced and the effective two-level Hamilitonian for the
spin-lattice system derived in the adiabatic approximation of the phonons. At
the level crossings the system becomes unconditionally unstable against lattice
distortions due to the effects of magnetic anisotropy. The resultant
magneto-elastic instabilities at the level crossings are discussed, as well as
the magnetic behavior.Comment: 13 pages, 8 figures, REVTEX
Spin dynamics in molecular ring nanomagnets: Significant effect of acoustic phonons and magnetic anisotropies
The nuclear spin-lattice relaxation rate 1/T_1_ is calculated for magnetic
ring clusters by fully diagonalizing their microscopic spin Hamiltonians.
Whether the nearest-neighbor exchange interaction J is ferromagnetic or
antiferromagnetic, 1/T_1_ versus temperature T in ring nanomagnets may be
peaked at around k_B_T=|J| provided the lifetime broadening of discrete energy
levels is in proportion to T^3^. Experimental findings for ferromagnetic and
antiferromagnetic Cu^II^ rings are reproduced with crucial contributions of
magnetic anisotropies as well as acoustic phonons.Comment: 5 pages with 5 figures embedded, to be published in J. Phys. Soc.
Jpn. 75, No. 10 (2006
Computer Vision in Human Analysis: From Face and Body to Clothes
For decades, researchers of different areas, ranging from artificial intelligence to computer vision, have intensively investigated human-centered data, i.e., data in which the human plays a significant role, acquired through a non-invasive approach, such as cameras. This interest has been largely supported by the highly informative nature of this kind of data, which provides a variety of information with which it is possible to understand many aspects including, for instance, the human body or the outward appearance. Some of the main tasks related to human analysis are focused on the body (e.g., human pose estimation and anthropocentric measurement estimation), the hands (e.g., gesture detection and recognition), the head (e.g., head pose estimation), or the face (e.g., emotion and expression recognition). Additional tasks are based on non-corporal elements, such as motion (e.g., action recognition and human behavior understanding) and clothes (e.g., garment-based virtual try-on and attribute recognition). Unfortunately, privacy issues severely limit the usage and the diffusion of this kind of data, making the exploitation of learning approaches challenging. In particular, privacy issues behind the acquisition and the use of human-centered data must be addressed by public and private institutions and companies.
Thirteen high-quality papers have been published in this Special Issue and are summarized in the following: four of them are focused on the human face (facial geometry, facial landmark detection, and emotion recognition), two on eye image analysis (eye status classification and 3D gaze estimation), five on the body (pose estimation, conversational gesture analysis, and action recognition), and two on the outward appearance (transferring clothing styles and fashion-oriented image captioning). These numbers confirm the high interest in human-centered data and, in particular, the variety of real-world applications that it is possible to develop
Tunneling Splittings in Mn12-Acetate Single Crystals
A Landau-Zener multi-crossing method has been used to investigate the tunnel
splittings in high quality Mn-acetate single crystals in the pure
quantum relaxation regime and for fields applied parallel to the magnetic easy
axis. With this method several individual tunneling resonances have been
studied over a broad range of time scales. The relaxation is found to be
non-exponential and a distribution of tunnel splittings is inferred from the
data. The distributions suggest that the inhomogeneity in the tunneling rates
is due to disorder that produces a non-zero mean value of the average
transverse anisotropy, such as in a solvent disorder model. Further, the effect
of intermolecular dipolar interaction on the magnetic relaxation has been
studied.Comment: Europhysics Letters (in press). 7 pages, including 3 figure
Electric Field Controlled Magnetic Anisotropy in a Single Molecule
We have measured quantum transport through an individual Fe
single-molecule magnet embedded in a three-terminal device geometry. The
characteristic zero-field splittings of adjacent charge states and their
magnetic field evolution are observed in inelastic tunneling spectroscopy. We
demonstrate that the molecule retains its magnetic properties, and moreover,
that the magnetic anisotropy is significantly enhanced by reversible electron
addition / subtraction controlled with the gate voltage. Single-molecule
magnetism can thus be electrically controlled
Nonadiabatic Landau Zener tunneling in Fe_8 molecular nanomagnets
The Landau Zener method allows to measure very small tunnel splittings \Delta
in molecular clusters Fe_8. The observed oscillations of \Delta as a function
of the magnetic field applied along the hard anisotropy axis are explained in
terms of topological quantum interference of two tunnel paths of opposite
windings. Studies of the temperature dependence of the Landau Zener transition
rate P gives access to the topological quantum interference between exited spin
levels. The influence of nuclear spins is demonstrated by comparing P of the
standard Fe_8 sample with two isotopically substituted samples. The need of a
generalized Landau Zener transition rate theory is shown.Comment: 5 pages, 6 figure
Observation of a Distribution of Internal Transverse Magnetic Fields in a Mn12-Based Single Molecule Magnet
A distribution of internal transverse magnetic fields has been observed in
single molecule magnet (SMM) Mn12-BrAc in the pure magnetic quantum tunneling
(MQT) regime. Magnetic relaxation experiments at 0.4 K are used to produce a
hole in the distribution of transverse fields whose angle and depth depend on
the orientation and amplitude of an applied transverse ``digging field.'' The
presence of such transverse magnetic fields can explain the main features of
resonant MQT in this material, including the tunneling rates, the form of the
relaxation and the absence of tunneling selection rules. We propose a model in
which the transverse fields originate from a distribution of tilts of the
molecular magnetic easy axes.Comment: 4 page
Q-dependence of the inelastic neutron scattering cross section for molecular spin clusters with high molecular symmetry
For powder samples of polynuclear metal complexes the dependence of the
inelastic neutron scattering intensity on the momentum transfer Q is known to
be described by a combination of so called interference terms. They reflect the
interplay between the geometrical structure of the compound and the spatial
properties of the wave functions involved in the transition. In this work, it
is shown that the Q-dependence is strongly interrelated with the molecular
symmetry of molecular nanomagnets, and, if the molecular symmetry is high
enough, is actually completely determined by it. A general formalism connecting
spatial symmetry and interference terms is developed. The arguments are
detailed for cyclic spin clusters, as experimentally realized by e.g. the
octanuclear molecular wheel Cr8, and the star like tetranuclear cluster Fe4.Comment: 8 pages, 1 figures, REVTEX
- …