282 research outputs found
Guiding center picture of magnetoresistance oscillations in rectangular superlattices
We calculate the magneto-resistivities of a two-dimensional electron gas
subjected to a lateral superlattice (LSL) of rectangular symmetry within the
guiding-center picture, which approximates the classical electron motion as a
rapid cyclotron motion around a slowly drifting guiding center. We explicitly
evaluate the velocity auto-correlation function along the trajectories of the
guiding centers, which are equipotentials of a magnetic-field dependent
effective LSL potential. The existence of closed equipotentials may lead to a
suppression of the commensurability oscillations, if the mean free path and the
LSL modulation potential are large enough. We present numerical and analytical
results for this suppression, which allow, in contrast to previous quantum
arguments, a classical explanation of similar suppression effects observed
experimentally on square-symmetric LSL. Furthermore, for rectangular LSLs of
lower symmetry they lead us to predict a strongly anisotropic resistance
tensor, with high- and low-resistance directions which can be interchanged by
tuning the externally applied magnetic field.Comment: 12 pages, 9 figure
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Tissue engineering a fetal membrane
The aim of this study was to construct an artificial fetal membrane (FM) by combination of human amniotic epithelial stem cells (hAESCs) and a mechanically enhanced collagen scaffold containing encapsulated human amniotic stromal fibroblasts (hASFs). Such a tissue-engineered FM may have the potential to plug structural defects in the amniotic sac after antenatal interventions, or to prevent preterm premature rupture of the FM. The hAESCs and hASFs were isolated from human fetal amniotic membrane (AM). Magnetic cell sorting was used to enrich the hAESCs by positive ATP-binding cassette G2 selection. We investigated the use of a laminin/fibronectin (1:1)-coated compressed collagen gel as a novel scaffold to support the growth of hAESCs. A type I collagen gel was dehydrated to form a material mimicking the mechanical properties and ultra-structure of human AM. hAESCs successfully adhered to and formed a monolayer upon the biomimetic collagen scaffold. The resulting artificial membrane shared a high degree of similarity in cell morphology, protein expression profiles, and structure to normal fetal AM. This study provides the first line of evidence that a compacted collagen gel containing hASFs could adequately support hAESCs adhesion and differentiation to a degree that is comparable to the normal human fetal AM in terms of structure and maintenance of cell phenotype
Inverse flux quantum periodicity of magnetoresistance oscillations in two-dimensional short-period surface superlattices
Transport properties of the two-dimensional electron gas (2DEG) are
considered in the presence of a perpendicular magnetic field and of a {\it
weak} two-dimensional (2D) periodic potential modulation in the 2DEG plane. The
symmetry of the latter is rectangular or hexagonal. The well-known solution of
the corresponding tight-binding equation shows that each Landau level splits
into several subbands when a rational number of flux quanta pierces the
unit cell and that the corresponding gaps are exponentially small. Assuming the
latter are closed due to disorder gives analytical wave functions and
simplifies considerably the evaluation of the magnetoresistivity tensor
. The relative phase of the oscillations in and
depends on the modulation periods involved. For a 2D modulation
with a {\bf short} period nm, in addition to the Weiss oscillations
the collisional contribution to the conductivity and consequently the tensor
show {\it prominent peaks when one flux quantum passes
through an integral number of unit cells} in good agreement with recent
experiments. For periods nm long used in early experiments, these
peaks occur at fields 10-25 times smaller than those of the Weiss oscillations
and are not resolved
Interacting Generalised Cosmic Chaplygin gas in Loop quantum cosmology: A singularity free universe
In this work we investigate the background dynamics when dark energy is
coupled to dark matter with a suitable interaction in the universe described by
Loop quantum cosmology. Dark energy in the form of Generalised Cosmic Chaplygin
gas is considered. A suitable interaction between dark energy and dark matter
is taken into account in order to at least alleviate (if not solve) the cosmic
coincidence problem. The dynamical system of equations is solved numerically
and a stable scaling solution is obtained. A significant attempt towards the
solution of the cosmic coincidence problem is taken. The statefinder parameters
are also calculated to classify the dark energy model. Graphs and phase
diagrams are drawn to study the variations of these parameters. It is seen that
the background dynamics of Generalised Cosmic Chaplygin gas is completely
consistent with the notion of an accelerated expansion in the late universe.
From the graphs, generalised cosmic Chaplygin gas is identified as a dark fluid
with a lesser negative pressure compared to Modified Chaplygin gas, thus
supporting a 'No Big Rip' cosmology. It has also been shown that in this model
the universe follows the power law form of expansion around the critical point,
which is consistent with the known results. Future singularities that may be
formed in this model as an ultimate fate of the universe has been studied in
detail. It was found that the model is completely free from any types of future
singularities.Comment: 10 pages, 10 figures. arXiv admin note: text overlap with
arXiv:1109.1481, arXiv:1102.275
The research journey: Travels across the idiomatic and axiomatic toward a better understanding of complexity
In this paper, seven researchers reflect on the journeys their research projects have taken when they engage with and synthesize complex problems. These journeys embody an adaptive approach to tackling problems characterized by their interconnectedness and emergence, and that transcend traditional units of analysis such as ecosystems. In this paper we argue that making such a process deliberate and explicit will help researchers better combine different research paradigms such as expert-driven and participant-directed work, thus resulting in both broad explanations and specific phenomenon; research tensions traditionally defined as oppositional must be approached as complimentary. This paper includes researchers. personal journeys as they dealt with the emergent properties of complex problems and participant involvement. This paper argues that that research journey should be more than accidental but is a methodological necessity and should guide the theoretical and practical approaches to complex problems
Global, regional, and national comparative risk assessment of 79 behavioural, environmental and occupational, and metabolic risks or clusters of risks, 1990-2015: A systematic analysis for the Global Burden of Disease Study 2015
Background: The Global Burden of Diseases, Injuries, and Risk Factors Study 2015 provides an up-to-date synthesis of the evidence for risk factor exposure and the attributable burden of disease. By providing national and subnational assessments spanning the past 25 years, this study can inform debates on the importance of addressing risks in context.
Methods: We used the comparative risk assessment framework developed for previous iterations of the Global Burden of Disease Study to estimate attributable deaths, disability-adjusted life-years (DALYs), and trends in exposure by age group, sex, year, and geography for 79 behavioural, environmental and occupational, and metabolic risks or clusters of risks from 1990 to 2015. This study included 388 risk-outcome pairs that met World Cancer Research Fund-defined criteria for convincing or probable evidence. We extracted relative risk and exposure estimates from randomised controlled trials, cohorts, pooled cohorts, household surveys, census data, satellite data, and other sources. We used statistical models to pool data, adjust for bias, and incorporate covariates. We developed a metric that allows comparisons of exposure across risk factors—the summary exposure value. Using the counterfactual scenario of theoretical minimum risk level, we estimated the portion of deaths and DALYs that could be attributed to a given risk. We decomposed trends in attributable burden into contributions from population growth, population age structure, risk exposure, and risk-deleted cause-specific DALY rates. We characterised risk exposure in relation to a Socio-demographic Index (SDI).
Findings: Between 1990 and 2015, global exposure to unsafe sanitation, household air pollution, childhood underweight, childhood stunting, and smoking each decreased by more than 25%. Global exposure for several occupational risks, high body-mass index (BMI), and drug use increased by more than 25% over the same period. All risks jointly evaluated in 2015 accounted for 57·8% (95% CI 56·6–58·8) of global deaths and 41·2% (39·8–42·8) of DALYs. In 2015, the ten largest contributors to global DALYs among Level 3 risks were high systolic blood pressure (211·8 million [192·7 million to 231·1 million] global DALYs), smoking (148·6 million [134·2 million to 163·1 million]), high fasting plasma glucose (143·1 million [125·1 million to 163·5 million]), high BMI (120·1 million [83·8 million to 158·4 million]), childhood undernutrition (113·3 million [103·9 million to 123·4 million]), ambient particulate matter (103·1 million [90·8 million to 115·1 million]), high total cholesterol (88·7 million [74·6 million to 105·7 million]), household air pollution (85·6 million [66·7 million to 106·1 million]), alcohol use (85·0 million [77·2 million to 93·0 million]), and diets high in sodium (83·0 million [49·3 million to 127·5 million]). From 1990 to 2015, attributable DALYs declined for micronutrient deficiencies, childhood undernutrition, unsafe sanitation and water, and household air pollution; reductions in risk-deleted DALY rates rather than reductions in exposure drove these declines. Rising exposure contributed to notable increases in attributable DALYs from high BMI, high fasting plasma glucose, occupational carcinogens, and drug use. Environmental risks and childhood undernutrition declined steadily with SDI; low physical activity, high BMI, and high fasting plasma glucose increased with SDI. In 119 countries, metabolic risks, such as high BMI and fasting plasma glucose, contributed the most attributable DALYs in 2015. Regionally, smoking still ranked among the leading five risk factors for attributable DALYs in 109 countries; childhood underweight and unsafe sex remained primary drivers of early death and disability in much of sub-Saharan Africa.
Interpretation: Declines in some key environmental risks have contributed to declines in critical infectious diseases. Some risks appear to be invariant to SDI. Increasing risks, including high BMI, high fasting plasma glucose, drug use, and some occupational exposures, contribute to rising burden from some conditions, but also provide opportunities for intervention. Some highly preventable risks, such as smoking, remain major causes of attributable DALYs, even as exposure is declining. Public policy makers need to pay attention to the risks that are increasingly major contributors to global burden.
Funding: Bill & Melinda Gates Foundation
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