Excitons associated with miniband dispersion in (InGa)As---GaAs strained layer superlattices

Photoluminescence excitation (PLE) spectroscopy has been used to characterise miniband formation in (InGa)-As---GaAs superlattices with nominally 50 Å wide wells and barriers between 200 Å and 50 Å. The nominal composition of the alloy layers was 0.06. The observed exciton features are consistent with theoreical predictions of both parity allowed and forbidden transitions, at the mini-Brillouin zone centre and edge, including transitions associated with M₁ critical points in the superlattice bandstructure. Furthermore, as the GaAs thickness is varied we monitor changes in shape of the PLE spectra in the region of the first free electron to heavy-hole subband continuum, brought about by the electron-hole Coulomb interaction within the miniband. We also report PLE measurements on a structure which has been designed specifically to maximise the possibility of revealing a Δn = 0 exciton resonance below the saddle point.     

Combined-load testing of the space-shuttle insulation at 1100°C

The Space Shuttle's insulation coating has been mechanically characterized at temperatures to 1100°C. A creep/combined-load test machine was fabricated, and a capacitive sensor system was devised to assist in the alignment and installation of fragile, thin-walled cylindrical specimens in the test frame. A biaxial strain sensor is described which features low cross-talk, excellent long-term stability and high resolution. Ceramic fixtures and silicate-coated refractory metals are used where temperatures exceed 600°C, and specimen heating is simplified by attaching the transducer to the inside of test cylinders.     

Testing the space-shuttle thermal-protection coating

A light-weight insulation material and its protective glassy coating will protect thespace shuttle from temperatures as high as 1250°C (2300°F). The critical performance characteristics of the brittle coating are investigated using testing techniques developed to accommodate these extreme environments and the delicate material. These include an ultimate-strain test-specimen geometry which circumvents problems created by flawed edges, as well as a tension specimen preparation and loading system with which premature failures due to excessive bending moment are avoided. Additionally, an elevated-temperature mechanical strain transducer—useable at more than 870°C (1600°F)—is described. Potential alterations to this sensor are discussed which would make it functional at up to 1600°C (3000°F).     

The influence of random interactions and decision heuristics on norm evolution in social networks

In this paper we explore the effect that random social interactions have on the emergence and evolution of social norms in a simulated population of agents. In our model agents observe the behaviour of others and update their norms based on these observations. An agent’s norm is influenced by both their own fixed social network plus a second random network that is composed of a subset of the remaining population. Random interactions are based on a weighted selection algorithm that uses an individual’s path distance on the network to determine their chance of meeting a stranger. This means that friends-of-friends are more likely to randomly interact with one another than agents with a higher degree of separation. We then contrast the cases where agents make highest utility based rational decisions about which norm to adopt versus using a Markov Decision process that associates a weight with the best choice. Finally we examine the effect that these random interactions have on the evolution of a more complex social norm as it propagates throughout the population. We discover that increasing the frequency and weighting of random interactions results in higher levels of norm convergence and in a quicker time when agents have the choice between two competing alternatives. This can be attributed to more information passing through the population thereby allowing for quicker convergence. When the norm is allowed to evolve we observe both global consensus formation and group splintering depending on the cognitive agent model used.     

Strain and minibands in InGaAs---GaAs superlattices

Interband transmission and reflectivity measurements are performed on InGaAs-GaAs superlattices in magnetic fields up to 16 Tesla. The in-plane dispersion shows the effect of strain decoupling between the heavy and light hole subbands as their energy separation is increased. Significant interwell coupling is demonstrated by a reduction in the exciton binding energy and the presence of a finite miniband dispersion in the growth direction. The effects of both electron and holes saturating at the top of the miniband is studied.     

Near-ultraviolet photolysis of beta-phenylpyruvic acid generates free radicals and results in DNA damage

Ultraviolet A (UVA) light (315-400 nm) is ubiquitously found in our environment and constitutes about 95% of the total solar UV; all UVC and most UVB being absorbed by the stratospheric ozone layer. Compared with UVB and C, UVA does not show any direct effect on biological systems. Indirect effects of UVA, however, have been recognised overwhelmingly and this includes photosensitization of biological and non-biological compounds and production of free radicals many of which include oxygen and are hence known as reactive oxygen species or ROS. Several types of free radicals have been identified although their impacts on various macro- and micro-biomolecules are yet to be fully elucidated. beta-Phenylpyruvic acid is ubiquitously found in eukaryotic cells as a metabolite of phenylalanine, which is subsequently converted to phenyllactate and/or to 2-hydroxyphenylacetate and mandelate. In patients suffering from phenylketonuria the hydroxylation of phenylalanine to tyrosine is defective due to lack of phenylalanine hydroxylase. These result in accumulation and excretion of this compound in the urine. Here we present evidence that photolysis of beta-phenylpyruvic acid by a skin tanning lamp, emitting 99% UVA (315-400 nm) and 1% UVB (290-315 nm) generates carboxyl radicals (CO(2)(*)) and also possibly causes direct electron transfer (or type 1) reactions. Electron paramagnetic resonance was used to detect the free radicals. To determine the biological effects of this photolytic reaction, T7 was exposed to these photolytic reactive agents and found to lead to high levels of phage inactivation. Damage to DNA and/or components such as tail fibre proteins may be involved in T7 inactivation. In addition, our unpublished data suggest that certain phenylketonuria cell lines are more sensitive to PPA+NUV, lending importance to photolytic studies of this agent.     

Zeros of the Whittaker function associated to Coulomb waves

The zeros in the complex z plane of the Whittaker function W_c/z,µ(z),closely related to spherical waves in the quantum-mechanicalCoulomb problem, are investigated for varying real values ofthe parameters c and µ