Initial stages of MnAs/GaAs(0 0 1) epitaxy studied by RHEED azimuthal scans

We study the nucleation phase of molecular beam epitaxy of (hexagonal) MnAs on (cubic) GaAs (0 0 1) using reflection high-energy electron diffraction (RHEED) azimuthal scans. The nucleation proceeds from a non-reconstructed initial stage through randomly oriented small nuclei and two orientation stages to the final single-phase epitaxial orientation. The fascinatingly complex nucleation process contains elements of both Volmer-Weber and Stranski-Krastanov growth. The measurement demonstrates the potential of high-resolution RHEED techniques to assess details of the surface structure during epitaxy.     

Direct soft photon production inK + p andπ + p interactions at 250 GeV/c

Inclusive production of direct soft photons is studied inK ⁺ p andK ⁺ π interactions at 250 GeV/c. Total cross sections, Feynman-x and transverse momentum distributions of direct γ's are presented. The measured cross sections are several times larger than expected from QED inner bremsstrahlung, indicating the presence of an anomalous soft photon source. The model of Lichard and Van Hove, based on the “cold quark-gluon plasma” picture, agrees with the data.     

Scattering loss in Nd-doped silica based optical fibers

Rare-earth doped fibers suffer from relatively high attenuation in comparison to conventional communication fibers. In order to improve the properties of such fibers, understanding of the mechanisms involved in scattering effects is of great importance. The effect of Nd-doping, Al-codoping and of the drawing temperature on the scattering effect was investigated for several different optical fibers. To this end a measuring setup was realized which allows absolute measurements of the scattering coefficient at different positions along a fiber. Also separation of total attenuation and scattering effects is possible. The presented results indicate scattering effects as primary source for increased attenuation compared with undoped fibers.     

Chemometric investigations on the representativity of soil sampling

Summary The characterization of the heavy metal contents of a soil area is carried out by narrow screening sampling and subsequent analysis. It is possible by means of the univariate variance analysis to detect significant alterations within the tested area. The position and extent of multivariate changes of the features can be characterized more exactly with the help of multivariate statistical methods such as the combination of cluster and discriminant analytical methods or the principal component analysis. The maximum possible distance between two sampling points is determined by calculation of the autocorrelation function of the features. The number of soil samples, which is necessary for precisely determining the heavy metal contents of the investigated area, can be derived therefrom. With the help of the multivariate autocorrelation function the distance for the representative sampling of the multivariate loads by heavy metals can be described.     

Microscopical studies at cadmium impurities in compound semiconductors

Molecule-like defect complexes have been studied at Cd acceptors in the III–V semiconductors GaAs, GaP, InP, InAs, and InSb by PAC spectroscopy. After hydrogen plasma treatment, the formation of Cd-H complexes was observed in all these compounds. Additionally, the formation and stability of Cd-S and Cd-Se pairs in GaAs have been analyzed.     

Asymptotic behavior of positive solution branches of elliptic problems with linear part at resonance

We consider Dirichlet problems for semilinear elliptic equations whose linear part is at resonance. For various types of nonlinearities we provide sufficient conditions for the solvability of these problems for certain types of forcing terms. Depending upon the dimension of the independent variable and the shape of the domain, we obtain multiple existence results and show that in some cases infinitely many solutions may exist.Dedicated to Klaus Kirchgässner on the occasion of his 60th birthdayThe authors' work on this project has been supported by several institutions and thanks are due to the University of Utah for granting Schaaf a one year's visiting professorship during 1989–1990, to the DFG and the University of Stuttgart for providing Schmitt with the opportunity to visit that institution during the summer semester 1989. Final preparation was supported by a grant from NSF.     

The optimal size of the exchange hole and reduction to one-particle Hamiltonians

We use the concept of the exchange hole introduced by Slater to bound the energy of atoms, molecules, and other systems interacting by Coulomb forces from below by one-particle Hamiltonians with an effective screening potential and an exchange hole around each electron. Interestingly enough the optimal size of the exchange hole is smaller than Slater proposed: the best lower bound is obtained when the exchange hole carries charge 1/2 instead of 1. To highlight the quality of our estimate we show that the Dirac exchange energy with a slightly different constant bounds the exchange–correlation energy from below, an estimate previously derived by Lieb and later improved by Lieb and Oxford. Acknowledgements The authors thank Walter Farkas and Christian Hainzl for helpful discussions. Financial support by the European Union under its IHP network program, grant HPRN-CT-2002-00277, is gratefully acknowledged.© By the authorsThe author A.M. Klaus Müller is deceased