No abstract available

[en] Light induced electron transfer of metal complexes has been studied extensively during the last decade. This interest was stimulated by attempts to develop an artificial photosynthesis for the conversion and chemical storage of solar energy. Even if this goal has not yet been achieved photochemical redox processes of coordination compounds are now much better understood. In this review the various possibilities of photoinduced electron transfer are discussed and illustrated by selected samples. A distinction is made between intra- and intermolecular electron transfer which may occur as a direct optical transition or by an excited state electron transfer mechanism. (author). 52 refs.; 1 fig

[en] First-principles electronic structure and positron state calculations for transition metal carbides and nitrides are performed. Perfect NaCl-structures as well as structures with metal or carbon/nitrogen vacancies are considered. The positron affinities and lifetimes are determined. The trends are discussed and the results are compared with recent positron lifetime measurements for group IV and V refractory metal carbides and in hexagonal WC. The present analysis indicates, contradictory to an earlier interpretation, that positrons are trapped and annihilated both at carbon and metal vacancies. The concentration of metal vacancies seen by positrons is probably very low, below the sensitivity limit of the other experimental methods. Experimental data for WC may be well understood if a Bloch-like state in the lattice with carbon vacancies is employed as an initial positron state in the trapping model. (orig.)

No abstract available

[en] The neutron activation analysis technique has been used for the analysis of uranium in pitchblende (from IAEA), monasite, xenotime and zircon samples. The samples were activated in Reaktor TRIGA Puspati and the activity of ²³⁹Np was measured with a hyper-pure Germanium detector. The results are expressed in percent. (author)

[en] The remarkably strong Coulomb interaction in atomically thin transition metal dichalcogenides (TMDs) results in an extraordinarily rich many-particle physics including the formation of tightly bound excitons. Besides optically accessible bright excitonic states, these materials also exhibit a variety of dark excitons. Since they can even lie below the bright states, they have a strong influence on the exciton dynamics, lifetimes, and photoluminescence. While very recently, the presence of dark excitonic states has been experimentally demonstrated, the origin of these states, their formation, and dynamics have not been revealed yet. Here, we present a microscopic study shedding light on time- and energy-resolved formation and thermalization of bright and dark intra- and intervalley excitons as well as their impact on the photoluminescence in different TMD materials. We demonstrate that intervalley dark excitons, so far widely overlooked in current literature, play a crucial role in tungsten-based TMDs giving rise to an enhanced photoluminescence and reduced exciton lifetimes at elevated temperatures. (paper)

[en] Highlights: • Design of Chern insulating phases in honeycomb lattices. • We present a description of how our three groups, working on two different honeycomb lattice materials classes, learned and designed two examples of transition metal oxide nanolayers whose ground states are predicted to be Chern insulators (quantum anomalous Hall insulators). The main features or properties that guided our search are compared and contrasted between the two classes of materials. - Abstract: The search for robust examples of the magnetic version of topological insulators, referred to as quantum anomalous Hall insulators or simply Chern insulators, so far lacks success. Our groups have explored two distinct possibilities based on multiorbital 3d oxide honeycomb lattices. Each has a Chern insulating phase near the ground state, but materials parameters were not appropriate to produce a viable Chern insulator. Further exploration of one of these classes, by substituting open shell 3d with 4d and 5d counterparts, has led to realistic prediction of Chern insulating ground states. Here we recount the design process, discussing the many energy scales that are active in participating (or resisting) the desired Chern insulator phase.

No abstract available

[en] A review of the electron-stimulated-desorption (ESD) literature shows that many of the features of ESD that are difficult to rationalize within the model of Menzel, Gomer, and Redhead can easily be interpreted using the Auger decay model, which has recently been developed to explain ESD from transition-metal oxide surfaces. Specifically, the Auger model helps to explain the charge state of the desorbing species, the high-energy (approx. 30--40 eV) onset behavior that is seen, the differences in thresholds for positive and neutral desorbates, ESD cross-section and isotope-effect data, and the high kinetic energies of desorbing particles. The success of the Auger picture for ionically bonded surfaces suggests a number of new applications of ESD, including the deduction of reaction paths in surface chemistry and the study of the evolution of surface oxides

[en] A method of reproducibly making thin film superconductors of transition metal interstitial compounds, for example, niobium nitride (NbN), having an optimum transition temperature (T/sub c/) of at least about 15⁰K to 16⁰K, comprises the steps of reactively depositing on a substrate the compounds by sputtering techniques in a system which is initially decontaminated by heating to about 400⁰C while at a pressure of about 5 x 10^-10 Torr, the deposition of the metal compound being carried out in an atmosphere of nitrogen and/or a hydrocarbon gas while the substrate is at a temperature of about 450⁰C to about 1000⁰C. 9 claims, 4 figures