Liquid-immersion laser micromachining of GaN grown on sapphire

Liquid-immersion nanosecond-pulsed laser micromachining is introduced as an efficient way for device isolation and rapid prototyping on GaN-on-sapphire wafer. Using deionized water as an ambient medium for laser micromachining, smooth trenches that are free from redeposition can be formed in the GaN layer. Coupled with the large difference between the ablation thresholds and ultraviolet absorption coefficients of GaN and sapphire, the GaN/sapphire interface can be left undamaged after the ablation process. This technique overcomes the limitation of heat accumulation in nanosecond-pulse regime, and offers a cost-effective alternative to ultrashort-pulse laser micromachining. In this report, the advantages offered by liquid immersion are elucidated in terms of improved heat conduction, increased plasma-induced recoil pressure due to water confinement, weakened plasma shielding effect in water, and the collapse of cavitation bubbles. Simulation results show that the reduced fluctuation of temperature profile over time in water could be correlated with the reduced redeposition of Ga from thermal decomposition at the trench sidewalls.     

A new model and a hyper-heuristic approach for two-dimensional shelf space allocation

In this paper, we propose a two-dimensional shelf space allocation model. The second dimension stems from the height of the shelf. This results in an integer nonlinear programming model with a complex form of objective function. We propose a multiple neighborhood approach which is a hybridization of a simulated annealing algorithm with a hyper-heuristic learning mechanism. Experiments based on empirical data from both real-world and artificial instances show that the shelf space utilization and the resulting sales can be greatly improved when compared with a gradient method. Sensitivity analysis on the input parameters and the shelf space show the benefits of the proposed algorithm both in sales and in robustness.     

Structural models with qualitative values

In fields of study from cognition to organizations and social networks , empirical structures have been formally represented in terms of graph theoretical models. When the empirical relationships can be seen as valued, a valued graph or digraph is called for. Values have been conventionally identified with real numbers, but other sorts of entities (most often signs) have been used. In this paper, we demonstrate a general system under which graphs and digraphs with values that are not numbers may be used to represent various important properties and features of empirical structures. Special cases include multiplexity of relationships, formal and informal linkages in organizational structures, systems and their environments, and structural consistency principles. The general system incorporates a matrix methodology which permits the convenient analysis of empirical structures. These cases are also intended to exemplify the ways in which valued relational models may be developed to extend this kind of formalization and its methodology to other areas of substantive interest.     

Bemerkungen zu einer Arbeit von Herrn Carleman

Ohne Zusammenfassung     

On the approximability of the Simplified Partial Digest Problem

In this paper, we analyse the computational complexity of an optimization version of the Simplified Partial Digest Problem (SPDP), which is a mathematical model for DNA mapping based on the results of a simplified partial digest experiment. We prove that recognizing 46.16% of the elements of the DNA map in the error-free simplified partial digest experiment is NP-hard in the strong sense. This implies that the problem of maximizing the number of correct elements of the DNA map in the error-free simplified partial digest experiment is pseudopolynomially non-approximable with the approximation ratio .