Formation of dislocations with local mechanical damage to a silicon surface at room temperature

A study is made of the formation of dislocations in silicon with local damage to the surface at room temperature and subsequent annealing within the range 1073–1473 K. The damages to the surface are modeled with the use of micro-hardness indentations. Measurements of mean linear dislocation density in a ray of the indentation rosette show that the number of dislocations in the rosette is independent of both the temperature and duration of isothermal annealing. It was found that annealing at 573–773 K leads to partial relaxation of elastic stresses from the indentation due to the formation of sections of silicon with a hexagonal structure near the indentation. Further annealing at high temperatures leads to the disappearance of these sections and the formation of a normal dislocation rosette, with the number of dislocations in the rays corresponding to the case of one-stage annealing. The results are empirical confirmation of the hypothesis of incomplete shear. In accordance with the latter, dislocations are formed during deformation at room temperature, not during subsequent annealing.Zaporozh'e University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 78–82, May, 1992.     

Adaptive walks with noisy fitness measurements

Summary Adaptive walks constitute an optimization technique for searching a space of possible solutions, for example, a space of different molecules. The goal is to find a point in space (a molecule) that is optimal or near-optimal in some property, generally referred to as the fitness, such as its ability to bind to a given receptor. Adaptive walking, an analog of natural selection, is a powerful technique for searching landscapes. However, errors in the measurements will cause errors in the adaptive walks. Mutant molecules of higher fitness may be ignored or mutants of lower fitness may be accepted. To examine the effect of measurement error on adaptive walks, we simulate single-agent hill-climbing walks on NK landscapes of varying ruggedness where Gaussian noise is added to the fitness values to model measurement error. We consider both constant measurement noise and noise whose variance decays exponentially with fitness. We show that fitness-independent noise can cause walks to melt off the peaks in a landscape, wandering in larger regions as the noise increases. However, we also show that a small amount of noise actually helps the walk perform better than with no noise. For walks in which noise decreases exponentially with fitness, the most characteristic behavior is that the walk meanders throughout the landscape until it stumbles across a point of relatively high fitness, then it climbs the landscape towards the nearest peak. Finally, we characterize the balance between selection pressure and noise and show that there are several classes of walk dynamic behavior.     

The Synthesis of 3,5-Diamino-1,2,4-oxadiazoles. 2nd Communication

The 5-amino-3-arylamino-1,2,4-oxadiazoles 2 are conveniently prepared by oxidative cyclization of the arylamidinoureas 10 . The process is also capable of producing a variety of the 5-substituted-amino analogs 32 when the appropriately substituted guanidine 31 is employed as the substrate. Two different types of rearrangement leading to triazol-3-ones accompany cyclization depending on the choice of starting material. The structures of the rearranged products were established by X-ray crystallographic analysis and the reaction mechanisms leading to these unexpected products are discussed.     

Increase and decrease of tunneling by variation of the barrier height

We show, based on some results from classical analysis, that time-dependent perturbations can either increase or decrease the tunneling probability. The results depend on the form of the function T(√V), where T is the transmission coefficient and V the potential. If T(√V) is convex, the perturbation will always result in a decrease of the tunneling; if T(√V) is concave, both increase as well as decrease occur.