Three classes of space-time with a uniformly accelerating and nonrotating gravitational source

Three classes of Petrov type-D nonrotating empty spaces, which we denote asA ₊ ^* ,A ₋ ^* , anda ₀ ^* , can be seen to represent, respectively, a gravitational field of a uniformly accelerating source which moves with velocity slower, faster than, or equal to that of light. The first two classes of space-time,A ₊ ^* andA ₋ ^* , approach to the last one,a ₀ ^* , as the acceleration becomes greater and greater. The inertial frame in a flat space-time, relative to which a faster-thanlight particle moving along a spacelike geodesic behaves as an ordinary particle, is given and discussed. The behaviors of Killing horizons in the classes,A ₊ ^* andA ₋ ^* are investigated. Comparisons are made between the analysis by Farhoosh and Zimmerman and ours. Our identification of the acceleration parameter seems to be more appropriate than that by the above authors.     

Laser-assisted chemical beam epitaxy for selective growth

A recent exciting development in the growth of compound semiconductors is the use of light to modify the film growth rate in the irradiation area. We report Ar⁺-laser-assisted CBE of GaAs, InP, GaP and InGaAs to generate various patterned films without lithography. A linked-circle pattern is formed by laser beam scanning and a 0.85 μm pitch corrugation pattern formed by a holographic interference technique. Relationships between the growth rate and substrate temperature for the materials are compared. The mechanism of the growth rate enhancement is revealed to be photolytic decomposition of metalorganic molecules. In the case of InGaAs, laser irradiation above 500°C results in new phenomena of growth rate suppression and composition variation.     

Positron–electron correlation-polarization potential model for positron binding in polyatomic molecules

Positron binding energies (PBEs) of 41 polyatomic molecules were calculated using the positron–electron correlation-polarization potential (CPP) approach and compared with experimentally measured values. In this approach, the short-range positron–electron potential is modeled using the density-functional expression, whereas the long-range potential is approximated by the attractive polarization potential. The positron–electron CPP model based on local-density approximation yields larger PBEs than experimental values; however, the calculated values can be substantially improved by introducing generalized gradient approximation. We also investigated the conformational dependence of PBEs for representative molecules.