Introduction to Dislocations Hull, Derek; Bacon, D. J
Materials today (Kidlington, England),
2011., Letnik:
14, Številka:
10
eBook, Journal Article
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In materials science, dislocations are irregularities within the crystal structure or atomic scale of engineering materials, such as metals, semi- conductors, polymers, and composites. Discussing ...this specific aspect of materials science and engineering, Introduction to Dislocations is a key resource for students. The book provides students and practitioners with the fundamental principles required to understand dislocations. Comprised of 10 chapters, the text includes advanced computer modeling and very high- resolution electron microscopy to help readers better understand the structure of atoms close to the core of dislocations. It shows that atomic arrangement has a significant effect on the formation of dislocations and thereby on the properties of solids. The first two chapters of the book present an overview of dislocations. The crystal structures and the various defects and dislocations are discussed, and methods of observation and diagnosis of dislocations are covered. Chapters 3 to 5 discuss the behavior of dislocations and explain how changes in the structure and arrangement of atoms can affect the behavior of dislocations. The three chapters also discuss the mechanical properties of dislocations. The remaining chapters offer a detailed discussion of the mechanisms of dislocations and the mechanical strength of crystalline solids. The book is written for undergraduate- and graduate-level students in both materials science and mechanical engineering. Non-experts and novices working on mechanical properties, mechanisms of deformation and fracture, and properties of materials, as well as industrial and academic researchers, will find this book invaluable. * Long-established academic reference by an expert author team, highly regarded for their contributions to the field. * Uses minimal mathematics to present theory and applications in a detailed yet easy-to-read manner, making this an understandable introduction to a complex topic. * Unlike the main competition, this new edition includes recent developments in the subject and up-to-date references to further reading and research sources.
The linking of North and South America by the Isthmus of Panama had major impacts on global climate, oceanic and atmospheric currents, and biodiversity, yet the timing of this critical event remains ...contentious. The Isthmus is traditionally understood to have fully closed by ca. 3.5 million years ago (Ma), and this date has been used as a benchmark for oceanographic, climatic, and evolutionary research, but recent evidence suggests a more complex geological formation. Here, we analyze both molecular and fossil data to evaluate the tempo of biotic exchange across the Americas in light of geological evidence. We demonstrate significant waves of dispersal of terrestrial organisms at approximately ca. 20 and 6 Ma and corresponding events separating marine organisms in the Atlantic and Pacific oceans at ca. 23 and 7 Ma. The direction of dispersal and their rates were symmetrical until the last ca. 6 Ma, when northern migration of South American lineages increased significantly. Variability among taxa in their timing of dispersal or vicariance across the Isthmus is not explained by the ecological factors tested in these analyses, including biome type, dispersal ability, and elevation preference. Migration was therefore not generally regulated by intrinsic traits but more likely reflects the presence of emergent terrain several millions of years earlier than commonly assumed. These results indicate that the dramatic biotic turnover associated with the Great American Biotic Interchange was a long and complex process that began as early as the Oligocene–Miocene transition.
Significance The formation of the Isthmus of Panama, which linked North and South America, is key to understanding the biodiversity, oceanography, atmosphere, and climate in the region. Despite its importance across multiple disciplines, the timing of formation and emergence of the Isthmus and the biological patterns it created have been controversial. Here, we analyze molecular and fossil data, including terrestrial and marine organisms, to show that biotic migrations across the Isthmus of Panama began several million years earlier than commonly assumed. An earlier evolution of the Isthmus has broad implications for the mechanisms driving global climate (e.g., Pleistocene glaciations, thermohaline circulation) as well as the rich biodiversity of the Americas.
Plants have a long evolutionary history, during which mass extinction events dramatically affected Earth's ecosystems and its biodiversity. The fossil record can shed light on the diversification ...dynamics of plant life and reveal how changes in the origination–extinction balance have contributed to shaping the current flora.
We use a novel Bayesian approach to estimate origination and extinction rates in plants throughout their history. We focus on the effect of the ‘Big Five’ mass extinctions and on estimating the timing of origin of vascular plants, seed plants and angiosperms.
Our analyses show that plant diversification is characterized by several shifts in origination and extinction rates, often matching the most important geological boundaries. The estimated origin of major plant clades predates the oldest macrofossils when considering the uncertainties associated with the fossil record and the preservation process.
Our findings show that the commonly recognized mass extinctions have affected each plant group differently and that phases of high extinction often coincided with major floral turnovers. For instance, after the Cretaceous–Paleogene boundary we infer negligible shifts in diversification of nonflowering seed plants, but find significantly decreased extinction in spore-bearing plants and increased origination rates in angiosperms, contributing to their current ecological and evolutionary dominance.
Quantum algorithms offer a dramatic speedup for computational problems in material science and chemistry. However, any near-term realizations of these algorithms will need to be optimized to fit ...within the finite resources offered by existing noisy hardware. Here, taking advantage of the adjustable coupling of gmon qubits, we demonstrate a continuous two-qubit gate set that can provide a threefold reduction in circuit depth as compared to a standard decomposition. We implement two gate families: an imaginary swap-like (iSWAP-like) gate to attain an arbitrary swap angle, θ, and a controlled-phase gate that generates an arbitrary conditional phase, ϕ. Using one of each of these gates, we can perform an arbitrary two-qubit gate within the excitation-preserving subspace allowing for a complete implementation of the so-called Fermionic simulation (fSim) gate set. We benchmark the fidelity of the iSWAP-like and controlled-phase gate families as well as 525 other fSim gates spread evenly across the entire fSim (θ, ϕ) parameter space, achieving a purity-limited average two-qubit Pauli error of 3.8 × 10−3 per fSim gate.
We assess and develop techniques to remove contaminants when calculating the 3D galaxy power spectrum. We separate the process into three separate stages: (i) removing the contaminant signal, (ii) ...estimating the uncontaminated cosmological power spectrum and (iii) debiasing the resulting estimates. For (i), we show that removing the best-fitting contaminant (mode subtraction) and setting the contaminated components of the covariance to be infinite (mode deprojection) are mathematically equivalent. For (ii), performing a quadratic maximum likelihood (QML) estimate after mode deprojection gives an optimal unbiased solution, although it requires the manipulation of large
$N_{\rm mode}^2$
matrices (N
mode being the total number of modes), which is unfeasible for recent 3D galaxy surveys. Measuring a binned average of the modes for (ii) as proposed by Feldman, Kaiser & Peacock (FKP) is faster and simpler, but is sub-optimal and gives rise to a biased solution. We present a method to debias the resulting FKP measurements that does not require any large matrix calculations. We argue that the sub-optimality of the FKP estimator compared with the QML estimator, caused by contaminants, is less severe than that commonly ignored due to the survey window.
We study the feasibility of detecting weak lensing spatial correlations between supernova (SN) Type Ia magnitudes with present (Dark Energy Survey, DES) and future (Large Synoptic Survey Telescope, ...LSST) surveys. We investigate the angular auto-correlation function of SN magnitudes (once the background cosmology has been subtracted) and cross-correlation with galaxy catalogues. We examine both analytical and numerical predictions, the latter using simulated galaxy catalogues from the MICE Grand Challenge Simulation. We predict that we will be unable to detect the SN auto-correlation in DES, while it should be detectable with the LSST SN deep fields (15 000 SNe on 70 deg super( 2)) at ...6... level of confidence (assuming 0.15 mag of intrinsic dispersion). The SN-galaxy cross-correlation function will deliver much higher signal to noise, being detectable in both surveys with an integrated signal to noise of ~100 (up to 30 arcmin separations). We predict joint constraints on the matter density parameter (...) and the clustering amplitude (...) by fitting the auto-correlation function of our mock LSST deep fields. When assuming a Gaussian prior for ..., we can achieve a 25 per cent measurement of ... from just these LSST supernovae (assuming 0.15 mag of intrinsic dispersion). These constraints will improve significantly if the intrinsic dispersion of SNe Ia can be reduced. (ProQuest: ... denotes formulae/symbols omitted.)
We explore the potential of using intensity mapping surveys (MeerKAT, SKA) and optical galaxy surveys (DES, LSST) to detect H i clustering and weak gravitational lensing of 21 cm emission in auto- ...and cross-correlation. Our forecasts show that high-precision measurements of the clustering and lensing signals can be made in the near future using the intensity mapping technique. Such studies can be used to test the intensity mapping method, and constrain parameters such as the H i density
$\Omega _{\rm H\,\small {I}}$
, the H i bias
$b_{\rm H\,\small {I}}$
and the galaxy-H i correlation coefficient
$r_{\rm H\,\small {I}-g}$
.
We demonstrate a significant enhancement in the sensitivity of split ring resonator terahertz metamaterial dielectric sensors by the introduction of etched trenches into their inductive-capacitive ...gap area, both through finite element simulations and in experiments performed using terahertz time-domain spectroscopy. The enhanced sensitivity is demonstrated by observation of an increased frequency shift in response to overlaid dielectric material of thicknesses up to 18 µm deposited on to the sensor surface. We show that sensitivity to the dielectric is enhanced by a factor of up to ∼2.7 times by the incorporation of locally etched trenches with a depth of ∼3.4 µm, for example, and discuss the effect of the etching on the electrical properties of the sensors. Our experimental findings are in good agreement with simulations of the sensors obtained using finite element methods.
Methods in historical biogeography have revolutionized our ability to infer the evolution of ancestral geographical ranges from phylogenies of extant taxa, the rates of dispersals, and biotic ...connectivity among areas. However, extant taxa are likely to provide limited and potentially biased information about past biogeographic processes, due to extinction, asymmetrical dispersals and variable connectivity among areas. Fossil data hold considerable information about past distribution of lineages, but suffer from largely incomplete sampling. Here we present a new dispersal–extinction–sampling (DES) model, which estimates biogeographic parameters using fossil occurrences instead of phylogenetic trees. The model estimates dispersal and extinction rates while explicitly accounting for the incompleteness of the fossil record. Rates can vary between areas and through time, thus providing the opportunity to assess complex scenarios of biogeographic evolution. We implement the DES model in a Bayesian framework and demonstrate through simulations that it can accurately infer all the relevant parameters. We demonstrate the use of our model by analysing the Cenozoic fossil record of land plants and inferring dispersal and extinction rates across Eurasia and North America. Our results show that biogeographic range evolution is not a time-homogeneous process, as assumed in most phylogenetic analyses, but varies through time and between areas. In our empirical assessment, this is shown by the striking predominance of plant dispersals from Eurasia into North America during the Eocene climatic cooling, followed by a shift in the opposite direction, and finally, a balance in biotic interchange since the middle Miocene. We conclude by discussing the potential of fossil-based analyses to test biogeographic hypotheses and improve phylogenetic methods in historical biogeography.
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