61 research outputs found
Ab initio many-body calculations on infinite carbon and boron-nitrogen chains
In this paper we report first-principles calculations on the ground-state
electronic structure of two infinite one-dimensional systems: (a) a chain of
carbon atoms and (b) a chain of alternating boron and nitrogen atoms. Meanfield
results were obtained using the restricted Hartree-Fock approach, while the
many-body effects were taken into account by second-order M{\o}ller-Plesset
perturbation theory and the coupled-cluster approach. The calculations were
performed using 6-31 basis sets, including the d-type polarization
functions. Both at the Hartree-Fock (HF) and the correlated levels we find that
the infinite carbon chain exhibits bond alternation with alternating single and
triple bonds, while the boron-nitrogen chain exhibits equidistant bonds. In
addition, we also performed density-functional-theory-based local density
approximation (LDA) calculations on the infinite carbon chain using the same
basis set. Our LDA results, in contradiction to our HF and correlated results,
predict a very small bond alternation. Based upon our LDA results for the
carbon chain, which are in agreement with an earlier LDA calculation
calculation [ E.J. Bylaska, J.H. Weare, and R. Kawai, Phys. Rev. B 58, R7488
(1998).], we conclude that the LDA significantly underestimates Peierls
distortion. This emphasizes that the inclusion of many-particle effects is very
important for the correct description of Peierls distortion in one-dimensional
systems.Comment: 3 figures (included). To appear in Phys. Rev.
Hypomaturation Amelogenesis Imperfecta due to WDR72 Mutations: A Novel Mutation and Ultrastructural Analyses of Deciduous Teeth
Can forest management based on natural disturbances maintain ecological resilience?
Given the increasingly global stresses on forests, many ecologists argue that managers must maintain ecological resilience: the capacity of ecosystems to absorb disturbances without undergoing fundamental change. In this review we ask: Can the emerging paradigm of natural-disturbance-based management (NDBM) maintain ecological resilience in managed forests? Applying resilience theory requires careful articulation of the ecosystem state under consideration, the disturbances and stresses that affect the persistence of possible alternative states, and the spatial and temporal scales of management relevance. Implementing NDBM while maintaining resilience means recognizing that (i) biodiversity is important for long-term ecosystem persistence, (ii) natural disturbances play a critical role as a generator of structural and compositional heterogeneity at multiple scales, and (iii) traditional management tends to produce forests more homogeneous than those disturbed naturally and increases the likelihood of unexpected catastrophic change by constraining variation of key environmental processes. NDBM may maintain resilience if silvicultural strategies retain the structures and processes that perpetuate desired states while reducing those that enhance resilience of undesirable states. Such strategies require an understanding of harvesting impacts on slow ecosystem processes, such as seed-bank or nutrient dynamics, which in the long term can lead to ecological surprises by altering the forest's capacity to reorganize after disturbance
SYMBA: An end-to-end VLBI synthetic data generation pipeline: Simulating Event Horizon Telescope observations of M 87
Context. Realistic synthetic observations of theoretical source models are essential for our understanding of real observational data. In using synthetic data, one can verify the extent to which source parameters can be recovered and evaluate how various data corruption effects can be calibrated. These studies are the most important when proposing observations of new sources, in the characterization of the capabilities of new or upgraded instruments, and when verifying model-based theoretical predictions in a direct comparison with observational data. Aims. We present the SYnthetic Measurement creator for long Baseline Arrays (SYMBA), a novel synthetic data generation pipeline for Very Long Baseline Interferometry (VLBI) observations. SYMBA takes into account several realistic atmospheric, instrumental, and calibration effects. Methods. We used SYMBA to create synthetic observations for the Event Horizon Telescope (EHT), a millimetre VLBI array, which has recently captured the first image of a black hole shadow. After testing SYMBA with simple source and corruption models, we study the importance of including all corruption and calibration effects, compared to the addition of thermal noise only. Using synthetic data based on two example general relativistic magnetohydrodynamics (GRMHD) model images of M 87, we performed case studies to assess the image quality that can be obtained with the current and future EHT array for different weather conditions. Results. Our synthetic observations show that the effects of atmospheric and instrumental corruptions on the measured visibilities are significant. Despite these effects, we demonstrate how the overall structure of our GRMHD source models can be recovered robustly with the EHT2017 array after performing calibration steps, which include fringe fitting, a priori amplitude and network calibration, and self-calibration. With the planned addition of new stations to the EHT array in the coming years, images could be reconstructed with higher angular resolution and dynamic range. In our case study, these improvements allowed for a distinction between a thermal and a non-thermal GRMHD model based on salient features in reconstructed images
Supplementary Material for: Expression of the Sodium/Calcium/Potassium Exchanger, NCKX4, in Ameloblasts
Transcellular calcium transport is an essential activity in mineralized tissue formation, including dental hard tissues. In many organ systems, this activity is regulated by membrane-bound sodium/calcium (Na<sup>+</sup>/Ca<sup>2+</sup>) exchangers, which include the NCX and NCKX [sodium/calcium-potassium (Na<sup>+</sup>/Ca<sup>2+</sup>-K<sup>+</sup>) exchanger] proteins. During enamel maturation, when crystals expand in thickness, Ca<sup>2+</sup> requirements vastly increase but exactly how Ca<sup>2+</sup> traffics through ameloblasts remains uncertain. Previous studies have shown that several NCX proteins are expressed in ameloblasts, although no significant shifts in expression were observed during maturation which pointed to the possible identification of other Ca<sup>2+</sup> membrane transporters. NCKX proteins are encoded by members of the solute carrier gene family, Slc24a, which include 6 different proteins (NCKX1–6). NCKX are bidirectional electrogenic transporters regulating Ca<sup>2+</sup> transport in and out of cells dependent on the transmembrane ion gradient. In this study we show that all NCKX mRNAs are expressed in dental tissues. Real-time PCR indicates that of all the members of the NCKX group, NCKX4 is the most highly expressed gene transcript during the late stages of amelogenesis. In situ hybridization and immunolocalization analyses clearly establish that in the enamel organ, NCKX4 is expressed primarily by ameloblasts during the maturation stage. Further, during the mid-late maturation stages of amelogenesis, the expression of NCKX4 in ameloblasts is most prominent at the apical poles and at the lateral membranes proximal to the apical ends. These data suggest that NCKX4 might be an important regulator of Ca<sup>2+</sup> transport during amelogenesis
Oxygen Scavenging from Air in Package Headspaces by Singlet Oxygen Reactions in Polymer Media
- …