497 research outputs found
Susceptibility of the 2D S=1/2 Heisenberg antiferromagnet with an impurity
We use a quantum Monte Carlo method (stochastic series expansion) to study
the effects of a magnetic or nonmagnetic impurity on the magnetic
susceptibility of the two-dimensional Heisenberg antiferromagnet. At low
temperatures, we find a log-divergent contribution to the transverse
susceptibility. We also introduce an effective few-spin model that can
quantitatively capture the differences between magnetic and nonmagnetic
impurities at high and intermediate temperatures.Comment: 5 pages, 4 figures, v2: Updated data in figures, minor changes in
text, v3: Final version, cosmetic change
Managing the supercell approximation for charged defects in semiconductors: finite size scaling, charge correction factors, the bandgap problem and the ab initio dielectric constant
The errors arising in ab initio density functional theory studies of
semiconductor point defects using the supercell approximation are analyzed. It
is demonstrated that a) the leading finite size errors are inverse linear and
inverse cubic in the supercell size, and b) finite size scaling over a series
of supercells gives reliable isolated charged defect formation energies to
around +-0.05 eV. The scaled results are used to test three correction methods.
The Makov-Payne method is insufficient, but combined with the scaling
parameters yields an ab initio dielectric constant of 11.6+-4.1 for InP. Gamma
point corrections for defect level dispersion are completely incorrect, even
for shallow levels, but re-aligning the total potential in real-space between
defect and bulk cells actually corrects the electrostatic defect-defect
interaction errors as well. Isolated defect energies to +-0.1 eV are then
obtained using a 64 atom supercell, though this does not improve for larger
cells. Finally, finite size scaling of known dopant levels shows how to treat
the band gap problem: in less than about 200 atom supercells with no
corrections, continuing to consider levels into the theoretical conduction band
(extended gap) comes closest to experiment. However, for larger cells or when
supercell approximation errors are removed, a scissors scheme stretching the
theoretical band gap onto the experimental one is in fact correct.Comment: 11 pages, 3 figures (6 figure files). Accepted for Phys Rev
Long-term tryptophan supplementation decreased the welfare and innate immune status of pikeperch juveniles
CREBBP and WDR 24 Identified as Candidate Genes for Quantitative Variation in Red-Brown Plumage Colouration in the Chicken
Plumage colouration in birds is important for a plethora of reasons, ranging from camouflage, sexual signalling, and species recognition. The genes underlying colour variation have been vital in understanding how genes can affect a phenotype. Multiple genes have been identified that affect plumage variation, but research has principally focused on major-effect genes (such as those causing albinism, barring, and the like), rather than the smaller effect modifier loci that more subtly influence colour. By utilising a domesticâĂâwild advanced intercross with a combination of classical QTL mapping of red colouration as a quantitative trait and a targeted genetical genomics approach, we have identified five separate candidate genes (CREBBP, WDR24, ARL8A, PHLDA3, LAD1) that putatively influence quantitative variation in red-brown colouration in chickens. By treating colour as a quantitative rather than qualitative trait, we have identified both QTL and genes of small effect. Such small effect loci are potentially far more prevalent in wild populations, and can therefore potentially be highly relevant to colour evolution.Funding agencies: Carl Tryggers Stiftelse; Swedish Research CouncilSwedish Research Council; Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS)Swedish Research Council Formas; European Research CouncilEuropean Research Council (ER</p
Measurement with Persons: A European Network
The European âMeasuring the Impossibleâ Network MINET promotes new research activities in measurement dependent on human perception and/or interpretation. This includes the perceived attributes of products and services, such as quality or desirability, and societal parameters such as security and well-being. Work has aimed at consensus about four âgenericâ metrological issues: (1) Measurement Concepts & Terminology; (2) Measurement Techniques: (3) Measurement Uncertainty; and (4) Decision-making & Impact Assessment, and how these can be applied specificallyto the âMeasurement of Personsâ in terms of âMan as a Measurement Instrumentâ and âMeasuring Man.â Some of the main achievements of MINET include a research repository with glossary; training course; book; series of workshops;think tanks and study visits, which have brought together a unique constellation of researchers from physics, metrology,physiology, psychophysics, psychology and sociology. Metrology (quality-assured measurement) in this area is relativelyunderdeveloped, despite great potential for innovation, and extends beyond traditional physiological metrology in thatit also deals with measurement with all human senses as well as mental and behavioral processes. This is particularlyrelevant in applications where humans are an important component of critical systems, where for instance health andsafety are at stake
Long Range Dynamics Related to Magnetic Impurity in the 2D Heisenberg Antiferromagnet
We consider a magnetic impurity in the two-dimensional Heisenberg
antifferomagnet with long range antiferromagnetic order. At low temperature the
impurity magnetic susceptibility has a Curie term () and a
logarithmic correction (). We calculate the correction and
derive related Ward identity for the impurity-spin-wave vertex.Comment: 5 pages, 6 figure
Achieving Paris climate goals calls for increasing ambition of the Kigali Amendment
We assess that full global compliance with the Kigali Amendment to the Montreal Protocol will not provide emission reductions consistent with the 1.5oC target of the Paris Agreement. Following the Montreal Protocolâs start-and-strengthen approach to refrigerant management, fast-tracking hydrofluorocarbon phase-down under the Kigali Amendment would result in additional reductions vital for achieving the Paris climate goals. This would also increase chances of staying below 1.5oC additional warming throughout this century
Density functional theory calculations of defect energies using supercells
Reliable calculations of defect properties may be obtained with density functional theory using the supercell approximation. We systematically review the known sources of error and suggest how to perform calculations of defect properties in order to minimize errors. We argue that any analytical error-correction scheme relying on electrostatic considerations is not appropriate to derive reliable defect formation energies, especially not for relaxed geometries. Instead we propose finite size scaling of the calculated defect formation energies, and compare the use of this with both fully converged and "Gamma" (Î) point only k-point integration. We give a recipe for practical DFT calculations which will help to obtain reliable defect formation energies and demonstrate it using examples from III-V semiconductors
Excitations and spin correlations near the interface of two three-dimensional Heisenberg antiferromagnets
Magnetic excitations and spin correlations near the interface of two
spin- Heisenberg antiferromagnets are considered using the spin-wave
approximation. When the interaction between boundary spins differs essentially
from exchange constants inside the antiferromagnets, quasi-two-dimensional spin
waves appear in the near-boundary region. They eject bulk magnons from this
region, thereby dividing the antiferromagnets into areas with different
magnetic excitations. The decreased dimensionality of the near-boundary modes
leads to amplified nearest-neighbor spin correlations in the interface area.Comment: 6 pages, 5 figure
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