Interstitial muons and hydrogen in crystalline silicon

We have calculated self-consistent total energy surfaces for H+, H° and H2 present interstitially in crystalline Si. We conclude molecular hydrogen is the stable form consistent with the lack of observed electrical and optical activity. Both H+ and H° have complex surfaces, with some features sensitive to lattice distortion. The local minima are too small to give localised states when zero-point energy is included. We discuss our results in relation to earlier theories and to experiments on “normal” and “anomalous” muonium [μ+e-]

Theory of hydrogen in liquid and solid metals

A method for calculating the interatomic forces between isolated hydrogens and their host metal atoms is outlined. The method uses a semiempirical, molecular-orbital approach for a suitable cluster of atoms, with the empirical parameters fitted to experimental potential energy curves for diatomic molecules. Parameters suitable for hydrogen in liquid or solid Li and Na are given. The method is applied to the calculation of solvation energies of hydrogen in liquid Li and Na, where satisfactory agreement with experiment is obtained. Detailed potential energy surfaces are also found for H in solid Na and estimates are made of local mode frequencies, the stability of the tetrahedral sites, lattice relaxation, and effective charges, and atomic radii. Neither the anionic nor the protonic limit is appropriate. It has not proved possible to describe the potential energy surfaces in terms of a sum of twobody and volume-dependent terms alone

INTERSTITIAL MUONS AND HYDROGEN IN DIAMOND AND SILICON

The authors have calculated self-consistent total-energy surface for hydrogen present interstitially as H+, H0 and H2 in crystalline silicon and diamond. The dissimilarities of the two materials are more evident than their similarities, for they show molecular hydrogen to be the stable form in silicon, and atomic hydrogen to be the stable form in diamond in the absence of impurities. The energy surfaces for H0 and H+ are complex, with minima too small to trap the atoms when zero-point energy is taken into account. They discuss their results in relation to other theories and to the normal and anomalous muonium ( mu +e-) experiments

Stability of electronic states of the vacancy in diamond

The vacancy in diamond is a fundamental defect which has been studied theoretically and experimentally for forty years. However, although early theories (Coulson C A and Kearsley M J 1957 Proc. R. Soc. A 241 433) were extremely successful in explaining the nature of the ground state of the neutral defect and the Jahn-Teller distortion expected (Lannoo M and Stoneham A M 1968 J. Phys. Chem. Solids 29 1987), there are still several questions which have not been answered satisfactorily. in particular, the many-electron effects and configuration interaction are vital. They determine not only the order of electronic levels in the vacancy, but also the best-known optical transition. GR1, which cannot be expressed in terms of one-electron levels alone.We bring together much of the derailed recent experimental data on the different charge states and excited states of the vacancy to build up a simple empirical model of the defect. We show that the stability of the states and their photoconductivity, or lack of it, can be reproduced. We can predict that other states of the neutral vacancy, observable by EPR, lie very close above the ground state. and another high-energy optical transition might be detectable

DIMENSION CHANGES DUE TO ALIGNED VK-CENTERS AND H-CENTERS IN IONIC-CRYSTALS

When anisotropic defects are aligned, the dimensions of the host crystal parallel and perpendicular to the defect axis are changed, an effect observed previously for H centres and Vk centres in KCl. The authors have calculated the effect for H centres and Vk centres in several crystal structures. The contribution from long-range Coulomb interactions has been obtained in all cases, with less-detailed estimates of short-range repulsion and covalency effects in special cases. The predictions are in good agreement with experiment for H centres in KCl, but agree only poorly for Vk centres in the same host. The discrepancy appears to arise from modifications of the local repulsive forces near the defect. Measurements of dimension changes show great promise for studies of such interatomic forces

The structure and motion of the self-interstitial in diamond

We have made self-consistent semi-empirical molecular orbital calculations for various possible self-interstitial geometries in diamond, both with and without lattice distortion. Total energies are obtained, not merely the sum of one-electron eigenvalues. The results show that the (100) split interstitial has the lowest formation energy, not the cubic, hexagonal or bond-centred forms favour previously. The nature of the interstitial does not support the local heating model of enhanced diffusion in the presence of recombination or ionisation. A Bourgoin-Corbett mechanism involving negative hexagonal and neutral split interstitials is possible, but the apparent stability of the negative hexagonal interstitial may be an artefact of the calculation. We suggest a local excitation model is appropriate in fourfold-coordinated semiconductors

A COMPARISON OF THE CORE EXCITON AND NITROGEN DONOR IN DIAMOND

We use molecular dynamics with self-consistent quantum chemistry to compare the dynamics of the relaxation processes around the core exciton in diamond with the properties of the nitrogen substitutional atom, which has the same valence electron configuration. Both show a substantial [111] relaxation, associated with the population of an anti-bonding orbital with a neighbouring carbon and not a Jahn-Teller effect. The relaxation into this stable state has no energy barrier in either case, and excites modes of vibration in the frequency range 50-75 meV, with a possible local mode for the core exciton. The lineshapes for the absorption and emission processes in the creation and recombination of the core exciton are obtained from the dynamic calculation

All-electron Exact Exchange Treatment of Semiconductors: Effect of Core-valence Interaction on Band-gap and dd-band Position

Exact exchange (EXX) Kohn-Sham calculations within an all-electron full-potential method are performed on a range of semiconductors and insulators (Ge, GaAs, CdS, Si, ZnS, C, BN, Ne, Ar, Kr and Xe). We find that the band-gaps are not as close to experiment as those obtained from previous pseudopotential EXX calculations. Full-potential band-gaps are also not significantly better for $sp$ semiconductors than for insulators, as had been found for pseudopotentials. The locations of $d$-band states, determined using the full-potential EXX method, are in excellent agreement with experiment, irrespective of whether these states are core, semi-core or valence. We conclude that the inclusion of the core-valence interaction is necessary for accurate determination of EXX Kohn-Sham band structures, indicating a possible deficiency in pseudopotential calculations.Comment: 4 pages 2 fig

Effects of nebulised iloprost on pulmonary function and gas exchange in severe pulmonary hypertension

SummaryNebulised iloprost is established therapy of severe pulmonary hypertension; however, the effects on the bronchoalveolar compartment have not been investigated so far. We studied the short- and long-term effects of nebulised iloprost on pulmonary function tests and gas exchange in 63 patients with severe pulmonary hypertension (idiopathic n=17, chronic thromboembolism n=15, connective tissue disease n=12, congenital heart disease n=11, respiratory diseases n=8). Patients received iloprost in increasing dose up to 140μg iloprost/24h via an ultrasonic nebuliser.Short-term effects were assessed before and after every nebulisation: peak expiration flow decreased in mean by 1.9% (423±98 to 415±98) and percutaneous oxygen saturation increased in mean by 0.7% (90±6 to 91±5) post-nebulisation. There were no significant differences concerning underlying diagnosis or dose of nebulised iloprost. Within 3 months, 9 patients stopped treatment due to non-compliance with frequent nebulisations (n=3), or severe side effects (n=4); 2 patients with additional obstructive lung disease developed bronchoconstriction.Long-term effects were assessed by pulmonary function tests and gas exchange parameters at baseline and after 3 months treatment. There were no significant differences after 3 months therapy neither in FEV1, FVC, TLC, residual volume nor in diffusions capacity, SO2 at rest and during 6min walking test, also in respect of the underlying diseases. However, there was a significant increase in 6min walking distance (6 MWD) after 3 months (246±113 to 294±115m, P<0.05).In conclusion, treatment with nebulised iloprost leads to functional improvement in severe pulmonary hypertension without systematic adverse short- and long-term effects on pulmonary function test or gas exchange. Patients with additional obstructive lung disease might develop bronchoconstriction. Severe side effects leading to discontinuation of treatment occurred in 9% of patients

First-principle study of excitonic self-trapping in diamond

We present a first-principles study of excitonic self-trapping in diamond. Our calculation provides evidence for self-trapping of the 1s core exciton and gives a coherent interpretation of recent experimental X-ray absorption and emission data. Self-trapping does not occur in the case of a single valence exciton. We predict, however, that self-trapping should occur in the case of a valence biexciton. This process is accompanied by a large local relaxation of the lattice which could be observed experimentally.Comment: 12 pages, RevTex file, 3 Postscript figure