97,566 research outputs found
Making tracks: electronic excitation roles in forming swift heavy ion tracks
No full textSwift heavy ions cause material modification along their tracks, changes primarily due to their very dense electronic excitation. The available data for threshold stopping powers indicate two main classes of materials. Group I, with threshold stopping powers above about 10 keV nm(-1), includes some metals, crystalline semiconductors and a few insulators. Group II, with lower thresholds, comprises many insulators, amorphous materials and high T-c oxide superconductors. We show that the systematic differences in behaviour result from different coupling of the dense excited electrons, holes and excitons to atomic (ionic) motions, and the consequent lattice relaxation. The coupling strength of excitons and charge carriers with the lattice is crucial. For group II, the mechanism appears to be the self- trapped exciton model of Itoh and Stoneham ( 1998 Nucl. Instrum. Methods Phys. Res. B 146 362): the local structural changes occur roughly when the exciton concentration exceeds the number of lattice sites. In materials of group I, excitons are not self- trapped and structural change requires excitation of a substantial fraction of bonding electrons, which induces spontaneous lattice expansion within a few hundred femtoseconds, as recently observed by laser- induced time- resolved x- ray diffraction of semiconductors. Our analysis addresses a number of experimental results, such as track morphology, the efficiency of track registration and the ratios of the threshold stopping power of various materials
Modelling of silver adhesion on MgO(100) surface with defects
No full textWe show how surface defects (especially F-s(0) and V-s(0) centres) can play a major role in the adhesion of Ag (at 1:4 and 1:1 coverages) on the MgO(100) surface. Our calculations use a periodic (slab) model and an ab initio Hartree-Fock approach with cc posteriori electron correlation corrections. We are able to analyse the interatomic bond populations, effective charges and multipole moments of ions, in combination with the interface binding energy and the equilibrium distances. Both surface defects cause strong redistributions of the electron density which increase the binding energy of metal atoms by more than an order of magnitude. This implies radiation-induced strengthening of metal adhesion on oxide substrates and clarifies defect mechanisms in nucleating film growth. We compare our atomistic predictions with those from simpler methods which might be used for complex technologically interesting systems. There is good general agreement with the image interaction model differences arise partly from different treatments of dispersion and partly from subtle but significant charge redistribution in the Ag. Further, a simple Born-Haber analysis of charge transfer is consistent with the several cases predicted in the atomistic calculations
DEFECT PHENOMENA IN SUPERCONDUCTING OXIDES AND ANALOGOUS CERAMIC OXIDES
No full textIn this review we discuss defect phenomena in superconducting oxides. We survey those aspects of oxide superconductors which relate them most closely to conventional ceramic oxides, concentrating on processes and behaviour related to defects. We also identify areas of difference between two types of oxide.Theoretical modelling of conventional oxides has been extremely effective, and we emphasize that some of these modelling tools can be exploited for the superconducting oxides too. In particular, we stress those methods and ideas that provide a framework for understanding behaviour, those that provide a datasbase of good quantitative experiments and those that provide an established and tested approach to quantitative modelling as a guide to prediction, optimization and extrapolation.Much progress has been made in both theory and experiment, but some problems do remain and these have not been omitted from our discussions. There is potential to exploit past work on defects in oxides, so as to control defect processes and microstructure and hence to enhance performance
STABILITY OF A SELF-TRAPPING HOLE IN ALPHA-QUARTZ
No full textPrevious calculations of self-trapping in quartz adopt quantum chemical methods. However, for certain purposes, for example, when more than a few atoms are involved in a defect process, it would be helpful to use instead the shell model methods which work well for halides. We present the first calculation of the self-trapped hole (STH) in alpha-quartz and other forms of silicon dioxide using the classic defect simulation technique. The calculation suggests that the hole can be self-trapped on oxygen atom with a binding energy of 0.41 eV. The self-trapping is accompanied by a large network distortion, in which the O- ion on which the hole is self-trapped shifts 0.14 angstrom and the nearest-neighbour silicon atoms move 0.4-0.6 angstrom away from the O- ion. These results are similar to those obtained from the ab initio HF Calculation of STH in amorphous SiO2. We have also estimated the effective activation energy of a STH to be 0.12 eV at 180 K though there will also be a significant component of conduction from excitation of the small polaron to the delocalized large-polaron state
STRAIN-INDUCED INTERACTION ENERGIES BETWEEN HYDROGEN-ATOMS IN PALLADIUM
No full textThe authors have made quantitative calculations of the elastic interactions between interstitial hydrogen atoms in Pd metal. These calculations use the Harwell HADES code, and hence go beyond the usual harmonic models. Results have been obtained for several potentials and, where appropriate, agree well with those of previous workers. They find (i) that the absolute values are sensitive to assumptions for the potentials, suggesting caution in the prediction of thermodynamic properties, and (ii) that there are significant few-body terms not included in the usual approaches. These extra terms affect the equilibrium structure, for example by removing the symmetry between fractional occupancies theta and (1- theta ), and may lead to the initial nucleation of metastable structures during hydrogenation. The present results suggest that corner-sharing tetrahedra are favoured
METAL NON-METAL AND OTHER INTERFACES - THE ROLE OF IMAGE INTERACTIONS
No full textThe authors argue that many phenomena associated with metal/nonmetal interfaces and similar situations with a large dielectric constant mismatch can be understood in terms of the image interactions due to charges in the nonmetal. The effects are additional to the traditional interactions, and are especially significant when no reactions between the phases occur. The image-charge concept allows one to rationalise much apparently unrelated information concerning: (a) the systematics of wetting and nonwetting of oxides by liquid metals; (b) the systematics of strong metal-support interaction in catalysis; (c) the spatial variation of stoichiometry in oxides grown on metals; (d) the dependence on thickness of the observed changes in the wetting by water of oxide grown on silicon; (e) some features of radiation-enhanced adhesion; and (f) a number of correlations of behaviour with nonmetal properties in which the precise choice of metal is not critical
Joshua Davis: Author of Spare Parts
No full textCitation: K-State First (2016). Joshua Davis: Author of Spare Parts [Flier]. Manhattan, Kansas: K-State First.Flyer advertising Joshua Davis's author talk at Kansas State University
THEORY OF THE STRUCTURE OF THE SELF-TRAPPED EXCITON IN QUARTZ
No full textQuartz is an insulator with an extremely wide band gap in the vacuum ultra-violet. However, under irradiation from high-energy electrons or X-rays, samples of high purity emit a luminescence band in the blue, corresponding to a Stokes shift of approximately 7 eV. This large Stokes shift has been ascribed to the self-trapping of an exciton in an otherwise perfect lattice owing to the distortion it induces; the authors review the evidence for this assignment, and describe electronic-structure calculations which reveal the structure of the distorted configuration and also explain various experimentally determined properties of the centre. The self-trapping process they postulate is a novel one as it is driven primarily by the electron component of the exciton
ON THE QUANTUM THEORY OF ISOTOPE EFFECTS IN ELECTROMIGRATION AND THERMOMIGRATION OF LIGHT INTERSTITIALS
No full textThe electromigration and thermomigration of light interstitials, such as hydrogen in metals, have been examined in terms of the quantum theory of diffusion of Flynn and Stoneham. The pronounced isotope effect observed in the effective charge Z* is shown to come from the differences in the self trapping distortions of the various isotopes. The qualitative dependence of Z* on isotope, host lattice and temperature are correctly predicted. The interstitial contribution to the reduced heat of transport Q* is calculated directly from the quantum theory
A COMPARISON OF DEFECT ENERGIES IN MGO USING MOTT-LITTLETON AND QUANTUM-MECHANICAL PROCEDURES
No full textThe authors compare the predictions of Mott-Littleton calculations, based on empirical interatomic potentials, with predictions based on self-consistent solutions of the Schrodinger equation for embedded clusters. Simple vacancy and substitutional defects in MgO are modelled using both the classical Mott-Littleton and quantum mechanical methods. Particular attention is paid to the size of the quantum mechanical cluster, the different ways that polarisation is taken into account and the choice of basis set. Results are presented for closed-shell systems only, namely V"Mg and Vo vacancies and for Li'Mg, Na'Mg, AlMg, Fo and Clo substitutional impurities. They find a respectable level of agreement between the quite distinct approaches. This both validates the classical calculations and indicates useful generalisations combining the two approache
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