RHEED monitoring of AlN epitaxial growth by plasma-assisted molecular beam epitaxy

We have studied the epitaxial growth of AlN layers by plasma-assisted molecular beam epitaxy (MBE) on 6H–SiC substrate. Reflection high-energy electron diffraction (RHEED) was used to monitor the growth by the observation of the 2D–3D growth transition, respectively, in Al- and N-rich conditions. Special attention was given to the elimination of the Al droplets which often form in Al-rich conditions. Different growth procedures are proposed to avoid the appearance of these droplets while keeping a 2D growth. Each of the procedure gives AlN epilayers with identical crystalline quality and low surface roughness as measured, respectively, by X-ray diffraction and atomic force microscopy.     

Non-separable splines and numerical computation of evolution equations by the Galerkin methods

We construct new non-separable splines and we apply the spline sampling approximation to the computation of numerical solutions of evolution equations. The non-separable splines are basis functions which give a fine sampling approximation which enables us to compute numerical solutions by means of the method of lines combined with the Galerkin method. To demonstrate our approach we compute numerical solutions of the Burgers equation and the Kadomtsev–Petviashvili equation.     

Diffusion of Fe in iron tellurides by Mössbauer spectroscopy

The diffusional line broadening of the Mössbauer spectra of iron in β-, δ^′- and ε-phases of iron telluride was studied in the temperature range from 573 to 1023 K. The jump frequencies in the β- and δ^′-phases calculated from Mössbauer line broadening were 1.6×10⁸ and 4.7×10⁶ s^?1 at the temperature of 90% of each peritectic point, respectively. The jump frequency in the ε-phase was too small to measure even at 873 K near the peritectic point being less than 2×10⁶ s^?1. These data indicate that the jump frequency is likely to be dependent on the distance between the iron atom and the nearest neighboring sites.     

Electron paramagnetic resonance spectroscopy in studies of the chemical states of manganese in particulate substances in river waters and of the reduction of manganese by tannery effluents

The chemical states of manganese(II) species in particulate substances in river water, with and without the influence of tannery effluents, were investigated by the use of electron paramagnetic resonance (e.p.r.) spectroscopy. In the e.p.r. spectra of the particulate substances in river water, the six-line signal characteristic of Mn(II) was distinctly observed. The relative intensity of the Mn(II) signal was found to be higher in the lower reaches of a river, and especially below tannery effluent inflows, suggesting that manganese in river water is reduced by natural and artificially added organic compounds. It is shown that the reduction of manganese in river sediments and manganese dioxide by tannery effluents can afford an insoluble Mn(II) species at pH 11 and a soluble Mn(II) species at pH 7.     

On the optimality of algorithms for finite state sequential decision processes

A finite state sequential decision process (sdp) is a model which is able to represent a wide variety of combinatorial optimization problems. Four known and two new algorithms for obtaining optimal policies of three subclasses of sdp's, r-lmsdp, r-pmsdp, and r-psmsdp, are considered, and their optimality in the sense of minimizing the number of evaluations of cost functions associated with state transitions is proved.     

Effects of calcium and zinc on the adsorption of cobalt on γ-Fe2O3

The effects of calcium and zinc on the room-temperature coercivity of γ-Fe₂O₃ particles having cobalt ions adsorbed in 3M NaOH solution at 373K have been studied. When the Ca²⁺ ions are adsorbed on the γ-Fe₂O₃ prior to Co²⁺ ions adsorption, the coercivity of Co-modified γ-Fe₂O₃ significantly increases compared with that of γ-Fe₂O₃ modified only with Co²⁺ ions. In the case of Zn²⁺ ions, the coercivity of Co-modified γ-Fe₂O₃ is the same as that of γ-Fe₂O₃ modified only with Co²⁺ ions. The emission Mössbauer spectrum of⁵⁷Co²⁺ adsorbed on the surfaces of γ-Fe₂O₃ after pretreating with Ca²⁺ ions consists of a typical sextet of Fe³⁺ ions with hyperfine magnetic field, whereas those of γ-Fe₂O₃ modified only with Co²⁺ ions and with both Zn²⁺ and Co²⁺ ions show nonmagnetic components in addition to magnetic one. The effects of Ca²⁺ and Zn²⁺ ions on the adsorption of cobalt on the surface of γ-Fe₂O₃ are discussed from the viewpoint of site preference energy of cations in ferrite and distribution ratio of each cation.     

An iterated local search algorithm for the vehicle routing problem with convex time penalty functions

We propose an iterated local search algorithm for the vehicle routing problem with time window constraints. We treat the time window constraint for each customer as a penalty function, and assume that it is convex and piecewise linear. Given an order of customers each vehicle to visit, dynamic programming (DP) is used to determine the optimal start time to serve the customers so that the total time penalty is minimized. This DP algorithm is then incorporated in the iterated local search algorithm to efficiently evaluate solutions in various neighborhoods. The amortized time complexity of evaluating a solution in the neighborhoods is a logarithmic order of the input size (i.e., the total number of linear pieces that define the penalty functions). Computational comparisons on benchmark instances with up to 1000 customers show that the proposed method is quite effective, especially for large instances.