115 research outputs found
Interlayer Exchange Coupling in (Ga,Mn)As-based Superlattices
The interlayer coupling between (Ga,Mn)As ferromagnetic layers in
all-semiconductor superlattices is studied theoretically within a tight-binding
model, which takes into account the crystal, band and magnetic structure of the
constituent superlattice components. It is shown that the mechanism originally
introduced to describe the spin correlations in antiferromagnetic EuTe/PbTe
superlattices, explains the experimental results observed in ferromagnetic
semiconductor structures, i.e., both the antiferromagnetic coupling between
ferromagnetic layers in IV-VI (EuS/PbS and EuS/YbSe) superlattices as well as
the ferromagnetic interlayer coupling in III-V ((Ga,Mn)As/GaAs) multilayer
structures. The model allows also to predict (Ga,Mn)As-based structures, in
which an antiferromagnetic interlayer coupling could be expected.Comment: 4 pages, 3 figure
Interlayer Exchange Coupling Mediated by Valence Band Electrons
The interlayer exchange coupling mediated by valence band electrons in
all-semiconductor IV-VI magnetic/nonmagnetic superlattices is studied
theoretically. A 3D tight-binding model, accounting for the band and magnetic
structure of the constituent superlattice components is used to calculate the
spin-dependent part of the total electronic energy. The antiferromagnetic
coupling between ferromagnetic layers in EuS/PbS superlattices is obtained, in
agreement with the experimental evidences. The results obtained for the
coupling between antiferromagnetic layers in EuTe/PbTe superlattices are also
presented.Comment: 8 pages, 6 figures, to be submitted to Phys.Rev.
Magnetic susceptibility of EuTe/PbTe Heisenberg superlattices: experimental and theoretical studies
We report results on the temperature dependence of the susceptibilities of a
set of MBE-grown short-period EuTe/PbTe antiferromagnetic superlattices having
different EuTe layer thicknesses. In-plane and orthogonal susceptibilities have
been measured and display a strong anisotropy at low temperature, confirming
the occurrence of a magnetic phase transition in the thicker samples, as seen
also in neutron diffraction studies. We suggest that dipolar interactions
stabilize antiferromagnetic long-range order in an otherwise isotropic system
and we present numerical and analytical results for the low-temperature
orthogonal susceptibility.Comment: 30 pages, 8 ps figures, RevTe
Spin Glass and Antiferromagnetic Behaviour in a Diluted fcc Antiferromagnet
We report on a Monte Carlo study of a diluted Ising antiferromagnet on a fcc
lattice. This is a typical model example of a highly frustrated
antiferromagnet, and we ask, whether sufficient random dilution of spins does
produce a spin glass phase. Our data strongly indicate the existence of a spin
glass transition for spin--concentration : We find a divergent spin
glass susceptibility and a divergent spin glass correlation length, whereas the
antiferromagnetic correlation length saturates in this regime. Furthermore, we
find a first order phase transition to an antiferromagnet for ,
which becomes continuous in the range . Finite size scaling is
employed to obtain critical exponents. We compare our results with experimental
systems as diluted frustrated antiferromagnets as .Comment: 29 pages (revtex) and 10 figures uuencoded and Z-compresse
Environmental factors associated with childhood norovirus diarrhoea in León, Nicaragua
Norovirus is detected in one in five diarrhoea episodes in children, yet little is known about environmental risk factors associated with this disease, especially in low-income settings. The objective of this study was to examine environmental risk factors, and spatial and seasonal patterns of norovirus diarrhoea episodes in children in León, Nicaragua. We followed a population-based cohort of children under age 5 years for norovirus diarrhoea over a 1-year period. At baseline, characteristics of each household were recorded. Households were geocoded and spatial locations of garbage dumps, rivers, and markets were collected. In bivariate analysis we observed younger children and those with animals in their households were more likely to have experienced norovirus episodes. In adjusted models, younger children remained at higher risk for norovirus episodes, but only modest associations were observed with family and environmental characteristics. We next identified symptomatic children living in the same household and within 500 m buffer zones around the household of another child infected with the same genotype. Norovirus diarrhoea episodes peaked early in the rainy season. These findings contribute to our understanding of environmental factors and norovirus infection
Magnetic interactions in EuTe epitaxial layers and EuTe/PbTe superlattices
The magnetic properties of antiferromagnetic (AFM) EuTe epitaxial layers and
short period EuTe/PbTe superlattices (SLs), grown by molecular beam epitaxy on
(111) BaF substrates, were studied by magnetization and neutron diffraction
measurements. Considerable changes of the N\'eel temperature as a function of
the EuTe layer thickness as well as of the strain state were found. A mean
field model, taking into account the variation of the exchange constants with
the strain-induced lattice distortions, and the nearest neighbor environment of
a Eu atoms, was developed to explain the observed changes in wide
range of samples. Pronounced interlayer magnetic correlations have been
revealed by neutron diffraction in EuTe/PbTe SLs with PbTe spacer thickness up
to 60 \AA. The observed diffraction spectra were analyzed, in a kinematical
approximation, assuming partial interlayer correlations characterized by an
appropriate correlation parameter. The formation of interlayer correlations
between the AFM EuTe layers across the nonmagnetic PbTe spacer was explained
within a framework of a tight-binding model. In this model, the interlayer
coupling stems from the dependence of the total electronic energy of the
EuTe/PbTe SL on the spin configurations in adjacent EuTe layers. The influence
of the EuTe and PbTe layer thickness fluctuations, inherent in the epitaxial
growth process, on magnetic properties and interlayer coupling is discussed.Comment: 17 pages, 19 figures, accepted to PR
Spin Glass Ordering in Diluted Magnetic Semiconductors: a Monte Carlo Study
We study the temperature-dilution phase diagram of a site-diluted Heisenberg
antiferromagnet on a fcc lattice, with and without the Dzyaloshinskii-Moriya
anisotropic term, fixed to realistic microscopic parameters for (IIB=Cd, Hg, Zn). We show that the dipolar Dzyaloshinskii-Moriya anisotropy
induces a finite-temperature phase transition to a spin glass phase, at
dilutions larger than 80%. The resulting probability distribution of the order
parameter P(q) is similar to the one found in the cubic lattice
Edwards-Anderson Ising model. The critical exponents undergo large finite size
corrections, but tend to values similar to the ones of the
Edwards-Anderson-Ising model.Comment: 4 pages plus 3 postscript figure
Social and spatial processes associated with childhood diarrheal disease in Matlab, Bangladesh
We develop novel methods for conceptualizing geographic space and social networks to evaluate their respective and combined contributions to childhood diarrheal incidence. After defining maternal networks according to direct familial linkages between females, and road networks using satellite imagery of the study area, we use a spatial econometrics model to evaluate the significance of correlation terms relating childhood diarrheal incidence to the incidence observed within respective networks. Disease was significantly clustered within road networks across time, but only inconsistently correlated within maternal networks. These methods could be widely applied to systems in which both social and spatial processes jointly influence health outcomes
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