Hubbard model calculation of (2 × 1) reconstructed (111) surfaces of diamond-structure solids

From a Hubbard Hamiltonian for the electrons in the dangling bond orbitals of a (111) surface of diamond-type solids, different superstructures of periodicity (2 × 1) are calculated. The most stable one is an antiferromagnetic ordering of the electron spins, where alternating rows of orbitals show alternating magnetization. This superstructure vanishes in a second order phase transition at a critical temperature, above which the surface shows the unreconstructed (1 × 1) periodicity.     

Transport processes in α-quartz through computer modeling

The charging and discharging processes in a-quartz sandwiched between two blocking contacts are studied by solving numerically the transport equations. Results for the transient current are in good agreement with experiment. The rotation of crystallographic planes upon the application of an electric field is also quantitatively justified through results of the spatial distribution of the electric field.     

Optimal p--cyclic SOR

Summary. Our purpose in this paper is to extend --cyclic SOR theory to consistent singular systems and to apply the results to the solution of large scale systems arising, {\em e.g.,} in queueing network problems in Markov analysis. Markov chains and queueing models lead to structured singular linear systems and are playing an increasing role in the understanding of complex phenomena arising in computer, communication and transportation systems. For certain important classes of singular problems, we develop a convergence theory for --cyclic SOR, and show how to repartition for optimal convergence. Results by Kontovasilis, Plemmons and Stewart on the concept of convergence of SOR in an {\em extended} sense are rigorously analyzed and applied to the solution of periodic Markov chains with period . Received October 20, 1992 / Revised version received September 14, 1993     

High resolution x-ray study of microstructural changes in semiconductors and insulators subject to high electric fields

Using a high-resolution X-ray diffractometer, microstructural changes have been observed in Si-, LiF- and CdS-crystals subject to high d.c. and a.c. fields. At high fields the diffraction peaks are unstable due to inhomogeneous distribution of the electric current in the specimen. The inhomogeneous regions have been directly observed in high-resolution topographs. A correlation between current fluctuations and the diffracted beam intensity has been observed. Changes in peak shapes as a result of field have been observed.     

Zur verkürzung der Analysenzeit durch Erhöhung der Funkenfolgefrequenz im Bereich 50–500 Hz bei der optischen Emissionsspektralanalyse von Stahl und Anregung mit unipolaren Mittelspannungsfunken

The effects of increasing the repetition rate of a spark source for optical emission spectroscopy on analysis time, accuracy and precision were investigated by measuring the distribution of the optical yields of each spark, and the integrated intensities of some 100–1000 sparks.     

Über die thermische Zustandsgieichung in polaren Kristallen

The thermal equation of state is investigated for polar crystals (zinc blende and wurtzite structures). A finite crystal is considered with free charges to compensate the dipole moment of the crystal in equilibrium. The free energy is taken in the quasiharmonic approximation. A simple point-charge model for zinc oxide leads to the thermal expansion coefficients and pyroelectric constants, and to the elastic moduli, piezoelectric moduli and dielectric susceptibilities.     

Zur Reduktionstheorie in allgemeinen Hilbert-Räumen

Ohne ZusammenfassungVon der Naturwissenschaftlichen Fakultät der Universität München als Habilitationsschrift angenommen.     

A new explicit three-level difference scheme for the solution of the heat flow equation

A new explicit three-level difference scheme for the numerical solution of the heat flow equation is proposed. The main features of the new scheme are: i) it is unconditionally stable, ii) it is very highly accurate from the point of view of the truncation error, and iii) its solution converges to the solution of the heat equation even if the time and the distance increments tend to zero independently.     

Photoproduction of eta mesons off protons for 0.75 GeV<E(gamma)<3 GeV

Total and differential cross sections for the reaction p(gamma,eta)p have been measured for photon energies in the range from 750 MeV to 3 GeV. The low-energy data are dominated by the S11 wave which has two poles in the energy region below 2 GeV. Eleven nucleon resonances are observed in their decay into peta. At medium energies we find evidence for a new resonance N(2070)D15 with (M,Gamma)=(2068+/-22, 295+/-40) MeV. At gamma energies above 1.5 GeV, a strong peak in the forward direction develops, signaling the exchange of vector mesons in the t channel.     

Photoproduction of π 0 ω off protons for E γ ≤3 GeV

Differential and total cross-sections for photoproduction of γp→pπ ⁰ ω and γp→Δ ⁺ ω were determined from measurements of the CB-ELSA experiment, performed at the electron accelerator ELSA in Bonn. The measurements covered the photon energy range from the production threshold up to 3GeV.