Internal stabilization of standard sampled‐data systems

The paper presents necessary and sufficient conditions under which for a standard sampled‐data system there exists a digital controller, that ensures the internal stability of the system. For the case of stabilizability a general expression for the set of all stabilizing controllers is given. The stabilizability conditions are formulated as divisibility of certain polynomials, controllability and observability concepts are not used.     

Jet Cross Sections in e+e− Annihilation

Using an elaborate partial fractioning procedure of all 4-parton matrix elements we calculate 3-jet cross sections in e⁺e⁻ annihilation with mass-cut jet resolution. All subleading contributions from nonsingular terms are included. We give thrust distributions for O(α_s²) 3- and 4-jets. We study integrated cross sections as a function of the cutoff and check the reconstruction of O(α_s²) σ_tot using recently calculated 2-jet cross sections with the same partial fractioning of 4-parton terms.     

Nitrogen as carrier gas for the growth of ZnSe and ZnSe:N in plasma enhanced metalorganic vapor phase epitaxy

For the precursor combination dimethylzinc-triethylamine and ditertiarybutylselenide the use of nitrogen carrier gas was investigated for the metalorganic vapor phase epitaxy (MOVPE) of ZnSe and ZnSe:N. The nitrogen doping was carried out with a separate nitrogen flow which was activated by a dc-plasma. In the photoluminescence spectra of undoped layers grown at 340°C with a VI/II ratio of 0.18 only excitonic emissions, separated into free and donor bound excitons, can be observed. The background carrier concentration was of the order of 1 × 10¹⁶ cm⁻³. The growth rate of epilayers grown in nitrogen is reduced by about 75% in comparison to the value obtained by using hydrogen as carrier gas. The doping can be regulated by the dopant flow and by the total pressure in the reactor. With increasing flow of plasma activated nitrogen and a reduced total pressure, the PL spectra showed broadened DAP emission without excitonic emissions. The electrical and optical properties obtained with nitrogen carrier gas are comparable with the results obtained with hydrogen carrier gas. So far, no p-type conductivity was measured. Therefore, the problem of compensation of p-type conductivity of ZnSe : N doped by dc-plasma enhanced N₂ was not solved by the use of N₂ carrier gas.     

Cartan Geometries and Dynamics

We show how some basic dynamical ideas can be brought to bear on the study of Cartan geometries. In our main results, we give conditions for certain types of Cartan geometries to have constant curvature. We also consider ergodic actions of `higher-rank' semisimple groups on bundles supporting (not necessarily invariant) Cartan connections and show that these are `standard locally homogeneous' actions provided that some noncompact 1-parameter subgroup `preserves' a Cartan geometry.     

Regularized total least squares based on the nonlinear Arnoldi method

A computational approach for solving regularized total least squares problems via a sequence of quadratic eigenvalue problems has recently been introduced by Sima, Van Huffel, and Golub. Combining this approach with the nonlinear Arnoldi method and reusing information from all previous quadratic eigenvalue problems, together with an early update of search spaces we arrive at a very efficient method for large regularized total least squares problems. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)