MOVPE growth of InP/GaInAs and GaAs/GaInP heterostructures for electronic transport applications

The influence of the layer structure on the electrical properties in (i) modulation doped InP/GaInAs quantum well samples, and, (ii) GaAs/GaInP resonant tunneling diodes has been investigated. The results reveal the importance of the different scattering processes. In particular, the influence of interface roughness scattering has been evaluated for both types of structures.     

Pressure-induced intermetallic valence transition in BiNiO3

The valence state change of BiNiO3 perovskite under pressure has been investigated by a powder neutron diffraction study and electronic-state calculations. At ambient pressure, BiNiO3 has the unusual charge distribution Bi(3+)(0.5)Bi(5+)(0.5)Ni(2+)O3 with ordering of Bi(3+) and Bi(5+)charges on the A sites of a highly distorted perovskite structure. High-pressure neutron diffraction measurements and bond valence sum calculations show that the pressure-induced melting of the charge disproportionated state leads to a simultaneous charge transfer from Ni to Bi, so that the high-pressure phase is metallic Bi(3+)Ni(3+)O3. This unprecedented charge transfer between A and B site cations coupled to electronic instabilities at both sites leads to a variety of ground states, and it is predicted that a Ni-charge disproportionated state should also be observable.     

On column generation approaches for approximate solutions of quadratic programs in intensity-modulated radiation therapy

This paper deals with numerical behavior of a recently presented column generation approach for optimization of so called step-and-shoot radiotherapy treatment plans. The approach and variants of it have been reported to be efficient in practice, finding near-optimal solutions by generating only a low number of columns. The impact of different restrictions on the columns in a column generation method is studied, and numerical results are given for quadratic programs corresponding to three patient cases. In particular, it is noted that with a bound on the two-norm of the columns, the method is equivalent to the conjugate-gradient method. Further, the above-mentioned column generation approach for radiotherapy is obtained by employing a restriction based on the infinity-norm and non-negativity. The column generation method has weak convergence properties if restricted to generating feasible step-and-shoot plans, with a “tailing-off” effect for the objective values. However, the numerical results demonstrate that, like the conjugate-gradient method, a rapid decrease of the objective value is obtained in the first few iterations. For the three patient cases, the restriction on the columns to generate feasible step-and-shoot plans has small effect on the numerical efficiency.     

Chemical and spectroscopic methods for investigating polymer degradation and stabilization: Oxidative degradation of a hydrogenated nitrile rubber

The oxidation of a hydrogenated butadiene-acrylonitrile (2:1 mole ratio) rubber has been examined by infrared (IR) spectroscopy and the use of γ-initiation to give a controlled initiation step. The resolution and sensitivity of the IR technique was enhanced by the use of chemical derivatization in which carboxylic acids were measured as acid fluorides, hydroperoxides as nitrates and alcohols as nitrites. In all of these compounds, their spectra are sensitive to the precise substituents (primary, secondary or tertiary). Oxidation produced hydroperoxides from both tert. H-C-C=N and sec. H-C-H groups (≈︁1:1.1 mole ratio) as well as chain-end carboxylic acids [-CH₂-C(=O)OH]. From a comparison with measurements of peroxyl radicals produced at low temperature, average kinetic chain lengths for the various oxidation products were estimated and compared with these from other ethylene copolymers.