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Point defects: The whole picture The electronic states of point defects in semiconductors can be studied via first principles, suitable for large systems with thousands of electrons. The nitrogen-vacancy centre in diamond and the divacancy complex in silicon carbide are promising candidates for quantum applications. The non-radiative decay from their optically allowed excited states is pivotal to initializing and reading their spin. Michel Bockstedte and colleagues now model the electronic states of these defects using a method that relies on many-body theory and is free of empirical parameters. They map the optical transitions, as well as the spin relaxation dynamics and the role of the spin-orbit and electron-phonon coupling interactions involved, in good agreement with experimental data. The method is suitable for large systems, and can therefore be used to model qubits in other semiconductors.

We use scanning tunneling microscopy, photoelectron spectroscopy, and ab initio calculations to investigate the electron-induced dissociation of halogenated benzene molecules adsorbed on ice. Dissociation of halobenzene is triggered by delocalized excess electrons attaching to the ${\ensuremath{\pi}}^{*}$ orbitals of the halobenzenes from where they are transferred to ${\ensuremath{\sigma}}^{*}$ orbitals. The latter orbitals provide a dissociative potential surface. Adsorption on ice sufficiently lowers the energy barrier for the transfer between the orbitals to facilitate dissociation of bromo- and chloro- but not of flourobenzene at cryogenic temperatures. Our results shed light on the influence of environmentally important ice particles on the reactivity of halogenated aromatic molecules.

We investigate the role of electronic-vibrational coupling in resonant electron transport through single-molecule junctions, taking into account that the corresponding coupling strengths may depend on the charge and excitation state of the molecular bridge. In the presence of multiple electronic states, this requires to extend the commonly used model and include vibrationally dependent electron-electron interaction. We use Born-Markov master equation methods and consider selected models to exemplify the effect of the additional interaction on the transport characteristics of a single-molecule junction. In particular, we show that it has a significant influence on local cooling and heating mechanisms, may result in negative differential resistance, and cause pronounced asymmetries in the conductance map of a single-molecule junction.

Silicon carbide with engineered point defects is considered as very promising material for the next generation devices, with applications ranging from electronics and photonics to quantum computing. In this context, we investigate the spin physics of the carbon antisite-vacancy pair that in its positive charge state enables a single photon source. We find by hybrid density functional theory and many-body perturbation theory that the neutral defect possesses a high spin ground state in 4H silicon carbide and provide spin-resonance signatures for its experimental identification. Our results indicate the possibility for the coherent manipulation of the electron spin by optical excitation of this defect at telecom wavelengths, and suggest the defect as a candidate for an alternative solid state quantum bit. Funding Agencies|MTA Lendulet program of Hungarian Academy of Sciences; Knut and Alice Wallenberg Foundation; Swedish Foundation for Strategic Research program SRL [10-0026]; SNIC [001/12-275, 2013/1-331]; US Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-AC02-06CH11357]; Ministry of Education and Science of the Russian Federation [14.Y26.31.0005]; Tomsk State University Academic D. I. Mendeleev Fund Program

The signature of defects in the optical spectra of hexagonal boron nitride (BN) is investigated using many-body perturbation theory. A single BN-sheet serves as a model for different layered BN nanostructures and crystals. In the sheet we embed prototypical defects such as a substitutional impurity, isolated boron and nitrogen vacancies, and the divacancy. Transitions between the deep defect levels and extended states produce characteristic excitation bands that should be responsible for the emission band around 4 eV, observed in luminescence experiments. In addition, defect bound excitons occur that are consistently treated in our ab initio approach along with the “free” exciton. For defects in strong concentration, the coexistence of both bound and free excitons adds substructure to the main exciton peak and provides an explanation for the corresponding feature in cathodo- and photoluminescence spectra., We acknowledge funding by the European Community through e-I3 ETSF project (Contract No. 211956). AR acknowledges funding by the Spanish MEC (FIS2007-65702-C02-01), ACI-promciona project (ACI2009-1036), “Grupos Consolidados UPV/EHU del Gobierno Vasco” (IT-319-07). LW acknowledges funding by the French National Research Agency through Project No. ANR-09-BLAN-0421-01. This work was performed using HPC resources from GENCIIDRIS (Project No. 100063 and Project No. 091827). CA thanks X. Andrade and J. Alberdi Rodriguez for the efficient computer-cluster installation in San Sebastian. This work has been supported by Project No. FIS2010-21282-C02-01 (MCINN) from Spain.

4 páginas, 3 figuras, 2 tablas.-- PACS numbers: 71.35.-y, 71.55.-i, 76.30.Mi, We show that electron correlations control the photophysics of defects in SiC through both renormalization of the quasiparticle band structure and excitonic effects. We consider the carbon vacancy with two possible excitation channels that involve conduction and valence bands. Corrections to the Kohn-Sham ionization levels strongly depend on the defect charge state. Excitonic effects introduce a redshift of 0.23 eV. The analysis reassigns excitation mechanism at the thresholds in photoinduced paramagnetic resonance measurements, We acknowledge funding by the Deutsche Forschungsgemeinschaft (BO1851/2), Spanish MEC (FIS2007-65702-C02-01), ACI-promciona project (ACI2009-1036), and ‘‘Grupos Consolidados UPV/EHU del Gobierno Vasco’’ (IT-319-07), the European Community through e-I3 ETSF project (Contract No. 211956), and support by the Barcelona Supercomputing Center, ‘‘Red Espanola de Supercomputacion.’’

The electronic, structural and vibrational properties of small carbon interstitial and antisite clusters are investigated by ab initio methods in 3C and 4H-SiC. The defects possess sizable dissociation energies and may be formed via condensation of carbon interstitials, e.g. generated in the course of ion implantation. All considered defect complexes possess localized vibrational modes (LVM's) well above the SiC bulk phonon spectrum. In particular, the compact antisite clusters exhibit high-frequency LVM's up to 250meV. The isotope shifts resulting from a_{13}C enrichment are analyzed. In the light of these results, the photoluminescence centers D_{II} and P-U are discussed. The dicarbon antisite is identified as a plausible key ingredient of the D_{II}-center, whereas the carbon split-interstitial is a likely origin of the P-T centers. The comparison of the calculated and observed high-frequency modes suggests that the U-center is also a carbon-antisite based defect., 15 pages, 6 figures, accepted by Phys. Rev. B

Ab initio molecular dynamics simulations are employed to investigate the dominant migration mechanism of the gallium vacancy in gaas as well as to assess its free energy of formation and the rate constant of gallium self-diffusion. our analysis suggests that the vacancy migrates by second nearest neighbour hops. the calculated self-diffusion constant is in good agreement with the experimental value obtained in ^69 GaAs/ ^71 GaAs isotope heterostructures and at significant variance with that obtained earlier from interdiffusion experiments in GaAlAs/GaAs-heterostructures., 15 pages, 4 figures. Z. Phys. Chem, in print