Phase diagram of a model for diluted magnetic semiconductors beyond mean-field approximations

A lattice spin-fermion model for diluted magnetic semiconductors (DMS) is investigated numerically, improving on previously used mean-field approximations. Curie temperatures are obtained varying the Mn spin x and hole n densities, and the impurity-hole exchange J in units of the hop-ping t. Optimal values are found in the subtle intermediate regime between itinerant and localized carriers. At intermediate and large J/t, a "clustered" state is observed above the Curie temperature and ferromagnetism is suppressed. Formal analogies between DMS and manganites are discussed.     

Orbital picture of ionization and its breakdown in nanoarrays of quantum dots

We present exact numerical results indicating that ionization could be a useful tool to study electron correlations in artificial molecules and nanoarrays of metallic quantum dots. For nanorings consisting of Ag quantum dots of the type already fabricated, we demonstrate that the molecular orbital picture breaks down even for lowest energy ionization processes, in contrast to ordinary molecules. Our ionization results yield a transition point between localization and delocalization regimes in good agreement with various experimental data.     

The influence of hyperoxia on regional cerebral blood flow (rCBF), regional cerebral blood volume (rCBV) and cerebral blood flow velocity in the middle cerebral artery (CBFVMCA) in human volunteers

Conflicting results reported on the effects of hyperoxia on cerebral hemodynamics have been attributed mainly to methodical and species differences. In the present study contrast-enhanced magnetic resonance imaging (MRI) perfusion measurement was used to analyze the influence of hyperoxia (fraction of inspired oxygen (FiO2) = 1.0) on regional cerebral blood flow (rCBF) and regional cerebral blood volume (rCBV) in awake, normoventilating volunteers (n = 19). Furthermore, the experiment was repeated in 20 volunteers for transcranial Doppler sonography (TCD) measurement of cerebral blood flow velocity in the middle cerebral artery (CBFV(MCA)). When compared to normoxia (FiO2 = 0.21), hyperoxia heterogeneously influenced rCBV (4.95 +/- 0.02 to 12.87 +/- 0.08 mL/100g (FiO2 = 0.21) vs. 4.50 +/- 0.02 to 13.09 +/- 0.09 mL/100g (FiO2 = 1.0). In contrast, hyperoxia diminished rCBF in all regions (68.08 +/- 0.38 to 199.58 +/- 1.58 mL/100g/min (FiO2 = 0.21) vs. 58.63 +/- 0.32 to 175.16 +/- 1.51 mL/100g/min (FiO2 = 1.0)) except in parietal and left frontal gray matter. CBFV(MCA) remained unchanged regardless of the inspired oxygen fraction (62 +/- 9 cm/s (FiO2 = 0.21) vs. 64 +/- 8 cm/s (FiO2 = 1.0)). Finding CBFV(MCA) unchanged during hyperoxia is consistent with the present study's unchanged rCBF in parietal and left frontal gray matter. In these fronto-parietal regions predominantly fed by the middle cerebral artery, the vasoconstrictor effect of oxygen was probably counteracted by increased perfusion of foci of neuronal activity controlling general behavior and arousal.     

Charaktere endlicher Gruppen mit vorgegebenen Schurschen Indizes

Ohne Zusammenfassung     

Non-ohmic behaviour of iodine-doped polyacetylene

We report on the temperature and electric field strength dependence of the conductivity of iodine-doped polyacetylene in the metallic regime. The experimental results are discussed in terms of Sheng model of fluctuation induced tunneling and characteristic dimensions of the tunneling zones between fibres are deduced from a fit between this theory and our experiments.     

Elektronen-Spinpolarisation bei der Photoionisation unpolarisierter Cäsiumatome mit zirkularpolarisiertem Licht

Photoelectrons ejected from cesium atoms by circularly polarized light of certain wavelengths are highly spinpolarized according to Fano's theory. Measurements of the spinpolarization of photoelectrons along the wavelength spectrum are reported. The highest degree of spinpolarization obtained experimentally is 100%. Our results reveal a small deviation from the theoretically predicted spinpolarization. Apart from giving information on the influence of spin-orbit interaction on the photoionization process, the Fano-effect turns out to be a simple and efficient method for producing polarized electron beams. Though not designed as a polarized electron source, our present apparatus yields a beam of 0.3·10^?10 Ampere with a polarization of 81 ± 3% when a broad unfiltered spectral band is irradiated.     

Zur Schurgruppe eines lokalen Körpers

Ohne Zusammenfassung     

Elektrochemische Bildung von Δ1,2-Norbornen

Electrochemical Formation of Δ^1,2-Norbornene The electrochemical reduction of 1,2-dihalogen norbornanes in tetrahydrofuran/furan leads to a mixture of two isomeric cycloadducts 6 and 7. The ratio of these adducts corresponds to those which have been found in reductive bisdehalogenation of 1 and 2 by butyllithium.