109 research outputs found
Softening and Broadening of the Zone Boundary Magnons in Pr0.63Sr0.37MnO3
We have studied the spin dynamics in PrSrMnO above and
below the Curie temperature K. Three distinct new features have been
observed: a softening of the magnon dispersion at the zone boundary for
, significant broadening of the zone boundary magnons as , and
no evidence for residual spin-wave like excitations just above . The
results are inconsistent with double exchange models that have been
successfully applied to higher samples, indicating an evolution of the
spin system with decreasing .Comment: 12 pages, Latex, 3 figure
Exchange coupling in Eu monochalcogenides from first principles
Using a density functional method with explicit account for strong Coulomb
repulsion within the 4f shell, we calculate effective exchange parameters and
the corresponding ordering temperatures of the (ferro)magnetic insulating Eu
monochalcogenides (EuX; X=O,S,Se,Te) at ambient and elevated pressure
conditions. Our results provide quantitative account of the many-fold increase
of the Curie temperatures with applied pressure and reproduce well the
enhancement of the tendency toward ferromagnetic ordering across the series
from telluride to oxide, including the crossover from antiferromagnetic to
ferromagnetic ordering under pressure in EuTe and EuSe. The first and second
neighbor effective exchange are shown to follow different functional
dependencies. Finally, model calculations indicate a significant contribution
of virtual processes involving the unoccupied f states to the effective
exchange.Comment: 4 pages, 6 figure
Influence of defects on the lattice constant of GaMnAs
We study the influence of main compensating defects: As antisites and Mn
interstitials, known to occur in GaMnAs ferromagnetic semiconductor, on its
structural properties. Our experimental results show that there is a balance
between Mn interstitial and As antisite defects, leading to the reduced density
of one type of defect upon increased density of another defect. The significant
differences in the lattice parameters of GaMnAs with the different balance
between these two types of defects were observed. The annealing induced
reduction of GaMnAs lattice constant is inhibited in the samples with large
density of As antisites.Comment: 17 pages, 4 figure
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Integrating Monitoring and Decision Modeling within a Cooperative Framework: Promoting Transboundary Water Management and Avoiding Regional Conflict
Surface and groundwater resources do not recognize political boundaries. Where nature and boundary cross, tension over shared water resources can erupt. Such tension is exacerbated in regions where demand approaches or exceeds sustainable supplies of water. Establishing equitable management strategies can help prevent and resolve conflict over shared water resources. This paper describes a methodology for addressing transboundary water issues predicated on the integration of monitoring and modeling within a framework of cooperation. Cooperative monitoring begins with agreement by international scientists and/or policy makers on transboundary monitoring goals and strategies; it leads to the process of obtaining and sharing agreed-upon information among parties with the purpose of providing verifiable and secure data. Cooperative modeling is the process by which the parties jointly interpret the data, forecast future events and trends, and quantify cause and effect relationships. Together, cooperative monitoring and modeling allow for the development and assessment of alternative management and remediation strategies that could form the basis of regional watershed agreements or treaties. An example of how this multifaceted approach might be used to manage a shared water resource is presented for the Kura River basin in the Caucasus
Vegetative Ecological Characteristics of Restored Reed (Phragmites australis) Wetlands in the Yellow River Delta, China
In this study, we compared ecological characteristics of wetland vegetation in a series of restoration projects that were carried out in the wetlands of Yellow River Delta. The investigated characteristics include plant composition structure, species diversity and community similarity in three kinds of Phragmites australis wetlands, i.e. restored P. australis wetlands (R1, R2, R3 and R4: restored in 2002, 2005, 2007 and 2009, respectively), natural P. australis wetland (N) and degraded P. australis wetland (D) to assess the process of wetlands restoration. The coverage of the R1 was 99%, which was similar to natural wetland. Among all studied wetlands, the highest and lowest stem density was observed in R1 and R2, respectively, Plant height and stem diameter show the same trend as N > R2 > R1 > R3 > D > R4. Species diversity of restored P. australis wetlands became closed to natural wetland. Both species richness and Shannon–Wiener index had similar tendency: increased first and then decreased with restored time. The highest species richness and species diversity were observed in R2, while the lowest values of those parameters were found in natural P. australis wetland. Similarity indexes between restored wetlands and natural wetland increased with the restoration time, but they were still less than 50%. The results indicate that the vegetation of P. australis wetlands has experienced a great improvement after several years’ restoration, and it is feasible to restored degraded P. australis wetlands by pouring fresh water into those wetlands in the Yellow River Delta. However, it is notable that costal degraded P. australis wetland in this region may take years to decades to reach the status of natural wetland
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Studies of the Melting of a 2D Solid
We have made a detailed numerical analysis of neutron diffraction lineshapes across the constant -coverage melting transition of incommensurate monolayers of methane structure factor. We find that in the liquid phase the liquid lineshapes is well fitted with a lorentzian structure factor. While the exact nature of the transition very close to Tm cannot be commented upon, the 'freezing' of the liquid appears to be initially continuous, with an increase in correlation length to hundreds of Angstroms before the appearance of a solid phase
Introduction to the Topical Issue: Training Special Education Faculty for the 21st Century
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