990 research outputs found
Theory of Ferromagnetism in Doped Excitonic Condensates
Nesting in a semimetal can lead to an excitonic insulator state with
spontaneous coherence between conduction and valence bands and a gap for
charged excitations. In this paper we present a theory of the ferromagnetic
state that occurs when the density of electrons in the conduction band and
holes in the valence band differ. We find an unexpectedly rich doping-field
phase diagram and an unusual collective excitation spectrum that includes two
gapless collective modes. We predict regions of doping and external field in
which phase-separated condensates of electrons and holes with parallel spins
and opposing spins coexist.Comment: 5 pages, 3 postscript file
CaB_6: a new semiconducting material for spin electronics
Ferromagnetism was recently observed at unexpectedly high temperatures in
La-doped CaB_6. The starting point of all theoretical proposals to explain this
observation is a semimetallic electronic structure calculated for CaB_6 within
the local density approximation. Here we report the results of parameter-free
quasiparticle calculations of the single-particle excitation spectrum which
show that CaB_6 is not a semimetal but a semiconductor with a band gap of 0.8
eV. Magnetism in La_xCa_{1-x}B_6 occurs just on the metallic side of a Mott
transition in the La-induced impurity band.Comment: 4 pages, 1 postscript figur
A risk profile for identifying community-dwelling elderly with a highrisk of recurrent falling: results of a 3-year prospective study
Introduction: The aim of the prospective study reported here was to develop a risk profile that can be used to identify community-dwelling elderly at a high risk of recurrent falling. Materials and methods: The study was designed as a 3-year prospective cohort study. A total of 1365 community-dwelling persons, aged 65 years and older, of the population-based Longitudinal Aging Study Amsterdam participated in the study. During an interview in 1995/1996, physical, cognitive, emotional and social aspects of functioning were assessed. A follow-up on the number of falls and fractures was conducted during a 3-year period using fall calendars that participants filled out weekly. Recurrent fallers were identified as those who fell at least twice within a 6-month period during the 3-year follow-up. Results: The incidence of recurrent falls at the 3-year follow-up point was 24.9% in women and 24.4% in men. Of the respondents, 5.5% reported a total of 87 fractures that resulted from a fall, including 20 hip fractures, 21 wrist fractures and seven humerus fractures. Recurrent fallers were more prone to have a fall-related fracture than those who were not defined as recurrent fallers (11.9% vs. 3.4%; OR: 3.8; 95% CI: 2.3-6.1). Backward logistic regression analysis identified the following predictors in the risk profile for recurrent falling: two or more previous falls, dizziness, functional limitations, weak grip strength, low body weight, fear of falling, the presence of dogs/cats in the household, a high educational level, drinking 18 or more alcoholic consumptions per week and two interaction terms (high educationx18 or more alcohol consumptions per week and two or more previous falls x fear of falling) (AUC=0.71). Discussion: At a cut-off point of 5 on the total risk score (range 0-30), the model predicted recurrent falling with a sensitivity of 59% and a specificity of 71%. At a cut-off point of 10, the sensitivity and specificity were 31% and 92%, respectively. A risk profile including nine predictors that can easily be assessed seems to be a useful tool for the identification of community-dwelling elderly with a high risk of recurrent falling. © International Osteoporosis Foundation and National Osteoporosis Foundation 2006
Stress-driven instability in growing multilayer films
We investigate the stress-driven morphological instability of epitaxially
growing multilayer films, which are coherent and dislocation-free. We construct
a direct elastic analysis, from which we determine the elastic state of the
system recursively in terms of that of the old states of the buried layers. In
turn, we use the result for the elastic state to derive the morphological
evolution equation of surface profile to first order of perturbations, with the
solution explicitly expressed by the growth conditions and material parameters
of all the deposited layers. We apply these results to two kinds of multilayer
structures. One is the alternating tensile/compressive multilayer structure,
for which we determine the effective stability properties, including the effect
of varying surface mobility in different layers, its interplay with the global
misfit of the multilayer film, and the influence of asymmetric structure of
compressive and tensile layers on the system stability. The nature of the
asymmetry properties found in stability diagrams is in agreement with
experimental observations. The other multilayer structure that we study is one
composed of stacked strained/spacer layers. We also calculate the kinetic
critical thickness for the onset of morphological instability and obtain its
reduction and saturation as number of deposited layers increases, which is
consistent with recent experimental results. Compared to the single-layer film
growth, the behavior of kinetic critical thickness shows deviations for upper
strained layers.Comment: 27 pages, 11 figures; Phys. Rev. B, in pres
Exogenous Ligand-Free Nickel-Catalyzed Carboxylate O-Arylation:Insight into Ni<sup>I</sup>/Ni<sup>III</sup> Cycles**
Nickel-catalyzed cross-coupling reactions have become a powerful methodology to construct C-heteroatom bonds. However, many protocols suffer from competitive off-cycle reaction pathways and require non-equimolar amounts of coupling partners to suppress them. Here, we report on mechanistic examination of carboxylate O-arylation under thermal conditions, in both the presence and absence of an exogeneous bipyridine-ligand. Furthermore, spectroscopic studies of the novel ligand-free carboxylate O-arylation reaction unveiled the resting state of the nickel catalyst, the crucial role of the alkylamine base and the formation of an off-cycle NiI−NiII dimer upon reduction. This study provides insights into the competition between productive catalysis and deleterious pathways (comproportionation and protodehalogenation) in the commonly proposed self-sustained NiI/NiIII catalytic cycle. Thereby we show that for productive nickel-catalyzed carboxylate O-arylation a choice must be made between either mild conditions or equimolar ratios of substrates
Prepyramid-to-pyramid transition of SiGe islands on Si(001)
The morphology of the first three-dimensional islands appearing during
strained growth of SiGe alloys on Si(001) was investigated by scanning
tunneling microscopy. High resolution images of individual islands and a
statistical analysis of island shapes were used to reconstruct the evolution of
the island shape as a function of size. As they grow, islands undergo a
transition from completely unfacetted rough mounds (prepyramids) to partially
{105} facetted islands and then they gradually evolve to {105} facetted
pyramids. The results are in good agreement with the predictions of a recently
proposed theoretical model
Weak Ferromagnetism and Excitonic Condensates
We investigate a model of excitonic ordering (i.e electron-hole pair
condensation) appropriate for the divalent hexaborides. We show that the
inclusion of imperfectly nested electron hole Fermi surfaces can lead to the
formation of an undoped excitonic metal phase. In addition, we find that weak
ferromagnetism with compensated moments arises as a result of gapless
excitations. We study the effect of the low lying excitations on the density of
states, Fermi surface topology and optical conductivity and compare to
available experimental data.Comment: 10 Pages, 8 Figures, RevTe
Point defects, ferromagnetism and transport in calcium hexaboride
The formation energy and local magnetic moment of a series of point defects
in CaB are computed using a supercell approach within the generalized
gradient approximation to density functional theory. Based on these results,
speculations are made as to the influence of these defects on electrical
transport. It is found that the substitution of Ca by La does not lead to the
formation of a local moment, while a neutral B vacancy carries a moment of
2.4 Bohr magnetons, mostly distributed over the six nearest-neighbour B atoms.
A plausible mechanism for the ferromagnetic ordering of these moments is
suggested. Since the same broken B-B bonds appear on the preferred (100)
cleavage planes of the CaB structure, it is argued that internal surfaces
in polycrystals as well as external surfaces in general will make a large
contribution to the observed magnetization.Comment: Calculated defect formation energies had to be corrected, due to the
use of a wrong reference energy for the perfect crystal in the original pape
POSIWID and determinism in design for behaviour change
Copyright @ 2012 Social Services Research GroupWhen designing to influence behaviour for social or environmental benefit, does designers' intent matter? Or are the effects on behaviour more important, regardless of the intent involved? This brief paper explores -- in the context of design for behaviour change -- some treatments of design, intentionality, purpose and responsibility from a variety of fields, including Stafford Beer's "The purpose of a system is what it does" and Maurice Broady's perspective on determinism. The paper attempts to extract useful implications for designers working on behaviour-related problems, in terms of analytical or reflective questions to ask during the design process
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