648 research outputs found
Quasiclassical magnetotransport in a random array of antidots
We study theoretically the magnetoresistance of a
two-dimensional electron gas scattered by a random ensemble of impenetrable
discs in the presence of a long-range correlated random potential. We believe
that this model describes a high-mobility semiconductor heterostructure with a
random array of antidots. We show that the interplay of scattering by the two
types of disorder generates new behavior of which is absent for
only one kind of disorder. We demonstrate that even a weak long-range disorder
becomes important with increasing . In particular, although
vanishes in the limit of large when only one type of disorder is present,
we show that it keeps growing with increasing in the antidot array in the
presence of smooth disorder. The reversal of the behavior of is
due to a mutual destruction of the quasiclassical localization induced by a
strong magnetic field: specifically, the adiabatic localization in the
long-range Gaussian disorder is washed out by the scattering on hard discs,
whereas the adiabatic drift and related percolation of cyclotron orbits
destroys the localization in the dilute system of hard discs. For intermediate
magnetic fields in a dilute antidot array, we show the existence of a strong
negative magnetoresistance, which leads to a nonmonotonic dependence of
.Comment: 21 pages, 13 figure
Electron-electron interaction at decreasing
The contribution of the electron-electron interaction to conductivity is
analyzed step by step in gated GaAs/InGaAs/GaAs heterostructures with different
starting disorder. We demonstrate that the diffusion theory works down to , where is the Fermi quasimomentum, is the mean free
paths. It is shown that the e-e interaction gives smaller contribution to the
conductivity than the interference independent of the starting disorder and its
role rapidly decreases with decrease.Comment: 5 pages, 6 figure
Ab-initio structural, elastic, and vibrational properties of carbon nanotubes
A study based on ab initio calculations is presented on the estructural,
elastic, and vibrational properties of single-wall carbon nanotubes with
different radii and chiralities. We use SIESTA, an implementation of
pseudopotential-density-functional theory which allows calculations on systems
with a large number of atoms per cell. Different quantities like bond
distances, Young moduli, Poisson ratio and the frequencies of different phonon
branches are monitored versus tube radius. The validity of expectations based
on graphite is explored down to small radii, where some deviations appear
related to the curvature effects. For the phonon spectra, the results are
compared with the predictions of the simple zone-folding approximation. Except
for the known defficiencies of this approximation in the low-frequency
vibrational regions, it offers quite accurate results, even for relatively
small radii.Comment: 13 pages, 7 figures, submitted to Phys. Rev. B (11 Nov. 98
Numerical atomic orbitals for linear scaling
The performance of basis sets made of numerical atomic orbitals is explored
in density-functional calculations of solids and molecules. With the aim of
optimizing basis quality while maintaining strict localization of the orbitals,
as needed for linear-scaling calculations, several schemes have been tried. The
best performance is obtained for the basis sets generated according to a new
scheme presented here, a flexibilization of previous proposals. The basis sets
are tested versus converged plane-wave calculations on a significant variety of
systems, including covalent, ionic and metallic. Satisfactory convergence
(deviations significantly smaller than the accuracy of the underlying theory)
is obtained for reasonably small basis sizes, with a clear improvement over
previous schemes. The transferability of the obtained basis sets is tested in
several cases and it is found to be satisfactory as well.Comment: 9 pages with 2 encapsulated postscript figures, submitted to Phys.
Rev.
Graphite and Hexagonal Boron-Nitride Possess the Same Interlayer Distance. Why?
Graphite and hexagonal boron nitride (h-BN) are two prominent members of the
family of layered materials possessing a hexagonal lattice. While graphite has
non-polar homo-nuclear C-C intra-layer bonds, h-BN presents highly polar B-N
bonds resulting in different optimal stacking modes of the two materials in
bulk form. Furthermore, the static polarizabilities of the constituent atoms
considerably differ from each other suggesting large differences in the
dispersive component of the interlayer bonding. Despite these major differences
both materials present practically identical interlayer distances. To
understand this finding, a comparative study of the nature of the interlayer
bonding in both materials is presented. A full lattice sum of the interactions
between the partially charged atomic centers in h-BN results in vanishingly
small monopolar electrostatic contributions to the interlayer binding energy.
Higher order electrostatic multipoles, exchange, and short-range correlation
contributions are found to be very similar in both materials and to almost
completely cancel out by the Pauli repulsions at physically relevant interlayer
distances resulting in a marginal effective contribution to the interlayer
binding. Further analysis of the dispersive energy term reveals that despite
the large differences in the individual atomic polarizabilities the
hetero-atomic B-N C6 coefficient is very similar to the homo-atomic C-C
coefficient in the hexagonal bulk form resulting in very similar dispersive
contribution to the interlayer binding. The overall binding energy curves of
both materials are thus very similar predicting practically the same interlayer
distance and very similar binding energies.Comment: 18 pages, 5 figures, 2 table
The presentation, clinical features, complications, and treatment of congenital dacryocystocele
Purpose To determine the incidence and presenting features of congenital dacryocystocele in the United Kingdom. To report on those cases complicated by dacryocystitis, respiratory compromise, and the treatment undertaken. Methods A prospective observational study of cases of congenital dacryocystocele presenting in the United Kingdom between September 2014 and October 2015. Infants <3 months of age presenting with a cystic swelling in the medial canthal area were included. Cases were identified via the British Ophthalmology Surveillance Unit (BOSU) reporting system. Results A total of 49 cases were reported during the study period. This gives an incidence of 1 in 18 597 live births. There was a 71% response rate to the questionnaire. The average age at presentation was 16.94 days. Dacryocystoceles were unilateral in 91% of cases. Dacryocystitis was a complicating factor in 49% of patients and 17% had respiratory distress. Uncomplicated dacryocystocele responded well to conservative measures in 86%. Surgical intervention was required in 23% of patients. Those cases complicated by dacryocystitis (29%) and nasal obstruction (17%) were more likely to require surgical intervention compared to those with dacryocystocele alone (14%). Digital massage appears to reduce the likelihood of requiring surgical intervention. The mean time to resolution was 19 days. Conclusions Congenital dacryocystocele is a rare presentation in the United Kingdom. Dacryocystitis and respiratory compromise commonly complicate a dacryocystocele. The use of digital massage as an early intervention is advocated and conservative measures may be sufficient in cases of uncomplicated dacryocystocele
New research directions on disparities in obesity and type 2 diabetes
Obesity and type 2 diabetes disproportionately impact U.S. racial and ethnic minority communities and lowâ income populations. Improvements in implementing efficacious interventions to reduce the incidence of type 2 diabetes are underway (i.e., the National Diabetes Prevention Program), but challenges in effectively scalingâ up successful interventions and reaching atâ risk populations remain. In October 2017, the National Institutes of Health convened a workshop to understand how to (1) address socioeconomic and other environmental conditions that perpetuate disparities in the burden of obesity and type 2 diabetes; (2) design effective prevention and treatment strategies that are accessible, feasible, culturally relevant, and acceptable to diverse population groups; and (3) achieve sustainable health improvement approaches in communities with the greatest burden of these diseases. Common features of guiding frameworks to understand and address disparities and promote health equity were described. Promising research directions were identified in numerous areas, including study design, methodology, and core metrics; program implementation and scalability; the integration of medical care and social services; strategies to enhance patient empowerment; and understanding and addressing the impact of psychosocial stress on disease onset and progression in addition to factors that support resiliency and health.This report discusses a workshop convened by the National Institutes of Health to understand how to (1) address socioeconomic and other environmental conditions that perpetuate disparities in the burden of obesity and type 2 diabetes; (2) design effective prevention and treatment strategies that are accessible, feasible, culturally relevant, and acceptable to diverse population groups; and (3) achieve sustainable health improvement approaches in communities with the greatest burden of these diseases.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154507/1/nyas14270_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154507/2/nyas14270.pd
ENIGMA and global neuroscience: A decade of large-scale studies of the brain in health and disease across more than 40 countries
This review summarizes the last decade of work by the ENIGMA (Enhancing NeuroImaging Genetics through Meta Analysis) Consortium, a global alliance of over 1400 scientists across 43 countries, studying the human brain in health and disease. Building on large-scale genetic studies that discovered the first robustly replicated genetic loci associated with brain metrics, ENIGMA has diversified into over 50 working groups (WGs), pooling worldwide data and expertise to answer fundamental questions in neuroscience, psychiatry, neurology, and genetics. Most ENIGMA WGs focus on specific psychiatric and neurological conditions, other WGs study normal variation due to sex and gender differences, or development and aging; still other WGs develop methodological pipelines and tools to facilitate harmonized analyses of "big data" (i.e., genetic and epigenetic data, multimodal MRI, and electroencephalography data). These international efforts have yielded the largest neuroimaging studies to date in schizophrenia, bipolar disorder, major depressive disorder, post-traumatic stress disorder, substance use disorders, obsessive-compulsive disorder, attention-deficit/hyperactivity disorder, autism spectrum disorders, epilepsy, and 22q11.2 deletion syndrome. More recent ENIGMA WGs have formed to study anxiety disorders, suicidal thoughts and behavior, sleep and insomnia, eating disorders, irritability, brain injury, antisocial personality and conduct disorder, and dissociative identity disorder. Here, we summarize the first decade of ENIGMA's activities and ongoing projects, and describe the successes and challenges encountered along the way. We highlight the advantages of collaborative large-scale coordinated data analyses for testing reproducibility and robustness of findings, offering the opportunity to identify brain systems involved in clinical syndromes across diverse samples and associated genetic, environmental, demographic, cognitive, and psychosocial factors
Local Government Revenue Mobilisation in Anglophone Africa
This paper examines opportunities and constraints facing local revenue mobilisation in anglophone Africa, with an emphasis on urban settings. It discusses specific revenue instruments and their effects on economic efficiency, income distribution and accountability. In particular, it addresses political and administrative constraints facing various revenue instruments and factors affecting citizens’ compliance. The analysis is illustrated with examples from across anglophone Africa. A general conclusion emerging from the study is that local revenues mobilised in most local government authorities in Africa are necessary but not sufficient to develop and supply adequate services for the fast-growing population. On this basis, areas for further research on local government revenue mobilisation in Africa are identifiedDfI
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1961
My Ideal Course, Underwater, U.S.A. (page 1) From the Editor (3) Turf Management Club News (3) Quotes from 1961 Seniors (4) The United States Most Western Owned Golf Course: Armed Forces Golf Course, Guam (5) Turf Majors Participate in Horticultural Show (7) Picture - G.C.S.A Scholarships Awarded to Three Turf Seniors (8) Picture - Stockbridge - Majors in turf Management (9) Opportunity and Education (10) The Most Outstanding Turf Senior for the Year - 1961 (11) How We Prepare Our Greens Before Topdressing (12) An Inexpensive Cure for Weeds and Poa Annua (13) Watering (14) Picture - Honorary Members of the Turf Management Club (16) Picture - Graduates of Winter School for Turf Managers - 1961 (17) Welcome Speech by Narry Sperandio (A-1) Handle with Care by Dr. Ellsworth H. Wheeler (A-2) Current Ideas on Green Construction - Panel Discussion (A-4) Automatic Systems for Watering by Robert F. Harper (A-14) History of Golf Course Architecture by Geoffrey S. Cornish (A-22) Effect of Nutrition on Turf Diseases by Dr. Houston B. Couch Turf Disease Control and Use of Fungicides by Dr. R. J. Lukens Trees and Tree Care by Gordon S. King (A-38) Arsenical Toxicity by Dr. C. R. Skogley (A-41) Soil Reaction to Arsenical Compounds by Joseph E. Steckel Brush Control For the Golf Course by Dr. William I. Boyd (A-51) Massachusetts Highway Herbicide Program by Joseph L. Beasley (A-54) General Turf (Alternate Session): Observations on Highway Turf Establishment & Maintenance by E.F. Button (A-62) Pre-emerge Chemicals for the Control of Crabgrass by Dr. John R. Havis, John M. Zak & Joseph Troll (A-70) Root Growth of Turf Grasses as Affected by Different heights of Cut and Nutrient Levels by Evangel J. Bredakis (A-71) The Use of Sod by Daniel Pellegrino (A-72
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