Abstract. We report results from density-functional plane-wave pseudopotential calculations for carbon and silicon self-interstitials in cubic silicon carbide (3C-SiC). Several initial ionic configurations are used in the search for the global totalenergy minimum including tetragonal, split [100] and split [110] geometries. Neutral carbon interstitials are found to have several nearly degenerate total-energy minima configurations in split-interstitial geometries, with formation energies rangingbesides higher metastable ones -from 6.3 to 6.7 eV in stoichiometric SiC. In contrast, the neutral silicon interstitials have a clear single minimum total-energy configuration at the tetrahedral configuration with carbon nearest neighbours, exhibiting a formation energy of 6.0 eV. The split interstitial in the [110] direction at silicon site and the tetrahedral configuration with silicon nearest neighbours are metastable and have significantly higher formation energies. The present calculations indicate that the carbon interstitial introduces deep levels in the band gap while the silicon interstitial at the tetrahedral site behaves like a shallow donor.

We have used positron annihilation spectroscopy to study vacancy-type defects in strained phosphorus doped Si 1Ϫx Ge x layers grown on Si substrates and irradiated with 2-MeV protons. The results show that the dominant defect in the SiGe layer after irradiation is the E center, the vacancy-phosphorus pair. When the sample is annealed at 150-175°C, the dominant defect species in the SiGe layer changes into a complex consisting of a vacancy, a phosphorus dopant, and a germanium atom (V-P-Ge complex͒. Furthermore, we observe that the total concentration of vacancy-type defect complexes before and after annealing remains approximately constant. We thus conclude that the V-P-Ge complex is formed when a migrating E center encounters a Ge atom and forms the V-P-Ge complex. The V-P-Ge complex anneals out at 200°C. The 50°C higher annealing temperature of the V-P-Ge complex corresponds to about 0.1-0.2 eV larger binding energy than that of the V-P pair. By ab initio calculations, we reproduce this value and confirm that the V-P pair is more stable when neighbored by a germanium atom.

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The microscopic structure of a silicon vacancy is studied theoretically using first-principles supercell calculations. Both the standard Kohn-Sham local-density approximation (LDA) scheme and the generalized Kohn-Sham screened-exchange local-density approximation (sX-LDA) scheme are used. The latter approximation is expected to improve the description of electronic levels in the gap region substantially, while providing accurate total energies and bond lengths. The present LDA calculations are in line with the earlier corresponding calculations of the silicon vacancy,predicting an inward relaxation of the nearest neighbours of the vacant site. The LDA calculations also predict the Jahn-Teller distortions and negative effective-U effects for charged vacancies, qualitatively in agreement with the experimental results and the Watkins model. In contrast to LDA results, the present sX-LDA calculations predict an outward relaxation and sp 2 type hybridization for the ions surrounding the vacancy. This somewhat surprising result is explained by the removal of the systematic overbinding associated with LDA.

Hurricane Ophelia was a category 3 hurricane which underwent extratropical transition and made landfall in Europe as an exceptionally strong post-tropical cyclone in October 2017. In Ireland, Ophelia was the worst storm in 50 years and resulted in significant damage and even loss of life. In this study, the different physical processes affecting Ophelia's transformation from a hurricane to a mid-latitude cyclone are studied. For this purpose, we have developed software that uses OpenIFS model output and a system consisting of a generalized omega equation and vorticity equation. By using these two equations, the atmospheric vertical motion and vorticity tendency are separated into the contributions from different physical processes: vorticity advection, thermal advection, friction, diabatic heating, and the imbalance between the temperature and vorticity tendencies. Vorticity advection, which is often considered an important forcing for the development of mid-latitude cyclones, is shown to play a small role in the re-intensification of the low-level cyclone. Instead, our results show that the adiabatic upper-level forcing was strongly amplified by moist processes, and thus, the diabatic heating was the dominant forcing in both the tropical and extratropical phases of Ophelia. Furthermore, we calculated in more detail the diabatic heating contributions from different model parameterizations. We find that the temperature tendency due to the convection scheme was the dominant forcing for the vorticity tendency during the hurricane phase, but as Ophelia transformed into a mid-latitude cyclone, the microphysics temperature tendency, presumably dominated by large-scale condensation, gradually increased becoming the dominant forcing once the transition was complete. Temperature tendencies caused by other diabatic processes, such as radiation, surface processes, vertical diffusion, and gravity wave drag, were found to be negligible in the development of the storm.