Assessing the validity of theoretical results

We present recent theoretical results for the V₃ and Au₄ clusters. Calculations of the V₃ doublet system indicate that the 6-311+G(d) basis set is sufficiently flexible to provide reliable minimum energy structures and vibrational frequencies, that these structures and frequencies are insensitive to spin contamination of the wave function when the BPW91 functional is used, and that changing to the B3LYP functional may result in very different structures and frequencies. A computationally less expensive scalar relativistic treatment of Au₄ clusters gives structural properties that are in good agreement with those obtained using a four-component method. Received 30 November 2000     

Cluster-embedding method to simulate large cluster and surface problems

In order to describe processes which are localized on a surface or inside the bulk of a solid, molecular calculations of an inner cluster may be adequate as long as the effect of the outer environment is taken into account via an embedding-method. Using a relativistic density functional method for the self-consistent cluster calculation we have developed a new cluster-embedding scheme here. As an example we have studied the adsorption of Al on an Al(100) surface and we get significant agreement with different methods. This indicates that this embedding-method is reliable enough to simulate an unlimited solid. Received 29 November 2000     

Manipulation of Cu adatoms on anisotropic Cu surfaces using scanning tunneling microscopy

The physical origin of tip-induced motion of Cu adatoms on anisotropic Cu surfaces is investigated by means of total energy calculations which are based on three different semi-empirical potentials. The calculations show that for certain tip–adatom distances the activation barrier for the adatom to move towards the tip disappears completely, whereas the barrier in the opposite direction increases and the adatom experiences an attractive force towards the tip. The general trends do not depend on the shape and chemical nature of the tip, but quantitatively there appear differences.     

The influence of Cu and Nb on the crystallization products of annealed FeCuNbSiB

Depending on different Nb and Cu concentration, we studied the formation of nano- and microcrystalline phases in differently annealed samples of FeCuNbSiB. First, the kinetics of crystallization was explored by measuring the temperature-dependent electrical resistivity and magnetization. After this, we collected Mössbauer spectra and did an X-ray diffraction analysis of annealed samples at chosen temperatures to discover the different crystalline phases. It was found how the onset of crystallization had been shifted towards other temperatures. The analysis of Mössbauer spectra shows that building up the well-known DO₃ structure of Fe₃Si is disturbed in the alloy without Cu. At higher Nb content, we resolve two different crystallization steps in the formation of Fe₃Si and a new crystalline phase, probably being a structure like Fe₂₃B₆.     

Sound attenuation in forests

Many measurements of sound attenuation rates in forests have been made but there is little in common in the measuring procedures used or the results obtained. Consequently there is a considerable divergence of opinion on the effectiveness of vegetation as a noise control measure. In this paper the factors controlling the transmission of sound through vegetation are examined and the attenuation rates achieved in pine plantations are presented.     

High brightness 735 nm tapered lasers – optimisation of the laser geometry

High brightness tapered laser diodes with different resonator geometries were fabricated and analysed. The devices consist of an index-guided straight section and a gain-guided tapered section. Lasers with a total length L = 2 and 4 mm and different length of the ridge waveguide L _RW (500 μm ≤ L _RW ≤ 1250 μm for L = 2 mm and 500 μm ≤ L _RW ≤ 2000 μm for L = 4 mm) were processed to study the influence of the straight section on the spatial mode filtering. The power–voltage–current-characteristics, the beam waist, the far field, and the beam propagation factor M ² were measured. From the experiments, it can be stated that the lasers with a small L _RW reach higher output powers compared to those with larger L _RW. Concerning the beam quality the length L _RW should exceed a minimal value to guarantee efficient spatial mode filtering. Devices optimised concerning maximum output power and excellent beam quality reach a beam propagation factor smaller than 2.1 at an output power P = 2 W.