An attempt is described to replace the present definition of the kilogram with the mass of a certain number of silicon atoms. A prerequisite for this is that the Avogadro constant, N A , is determined with a relative uncertainty of better than 2 × 10 −8. For the determination, silicon crystals are used. However, the difficulty arising thereby is the measurement of the average molar mass of natural Si. Consequently, a worldwide collaboration has been launched to produce approximately a 5 kg 28 Si single crystal with an enrichment factor greater than 99.985% and of sufficient chemical purity so that it can be used to determine N A with the targeted relative measurement uncertainty mentioned above. In the following, the first successful tests of all technological steps will be reported (enrichment of SiF 4 , distillation into silane and chemical purification, chemical vapour deposition of polycrystalline 28 Si, floating zone growth of a dislocation-free single crystal) and new equipment for the production of high-purity 28 Si with an enrichment of not less than 99.99% will be described. All steps are well defined by a Technical Road Map (TRM28) and all key results are measured by new mass spectrometric, IR spectroscopic and other chemical and physical methods, such as Hall effect, photoluminescence, laser scattering and x-ray topographic methods (TRM for Analytical Monitoring and Certification, TRM28-AMC). The initial enrichment of the gas is >0.999 95 and the depletion during the entire process is <0.000 05. The isotopic homogeneity is checked by natural Si crystal growth and does, in the enriched sphere, not

The phonon-assisted luminescence in AgBr: I is discussed in the framework of a conventional configuration coordinate model in which both optical and acoustic phonons are coupled to the excited electronic states. We derive the expression for the luminescence intensityusing the coherentphonon description for the displacedoscillator (excited impurity) in a consequent way. As compared to the conventional derivation we gain in simplicity and transparency of the calculation. With the formulae derived, the experimental results concerning the spectral features of the luminescence as well as their dependence on temperature are well reproduced. In particular the importance of the acoustical phonon contribution in determining the correct intensity and its temperature dependence is stressed. Furthermore the question of the interpretation of the experiments in terms of the introduced coherent phonon states is discussed.

Dramatic reductions of the linewidths of well-known deep centers in 28Si reveal "isotopic fingerprints" of the constituents. The approximately 1014 meV Cu center, thought to be either a Cu pair or an isolated Cu, is shown to contain four Cu atoms, and the approximately 780 meV Ag center is shown to contain four Ag. The approximately 944 meV ;{*}Cu center, thought to be a different configuration of a Cu pair, in fact contains three Cu and one Ag, and a new two-Cu two-Ag center is found. The approximately 735 meV center, previously assigned to Fe, actually contains Au and three Cu. This suggests a family of four-atom (Cu, Ag, Au) centers.

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Observation of an electrically tunable exciton g factor in InGaAs/GaAs quantum dots Appl. Phys. Lett. 96, 053113 (2010); 10.1063/1.3309684 Indirect transitions, free and impurity-bound excitons in gallium phosphide: A revisit with modulation and photoluminescence spectroscopyWe report on ultrahigh resolution studies of the bound exciton states associated with the shallow acceptor B and the shallow donor P in highly enriched 28 Si using a tuneable single frequency laser to perform photoluminescence excitation spectroscopy. The linewidths and fine structure of the transitions, which were too narrow to be resolved previously using an available photoluminescence apparatus, are now fully revealed. The P bound exciton transition shows a complicated additional structure, which the Zeeman spectroscopy demonstrates to be a result of the splitting of the donor ground state by the hyperfine interaction between the spin of the donor electron and that of the 31 P nucleus. The 31 P nuclear spin populations can thus be determined, and hopefully modified, by optical means. The predominant Auger recombination channel of these bound excitons is used to observe the same resolved hyperfine transitions in the photocurrent spectrum. This demonstrates that donors in specific electronic and nuclear spin configurations can be selectively photoionized. Possible applications of these results to quantum computing and quantum information systems are discussed.

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