Vacuum birefringence in strong inhomogeneous electromagnetic fields

Birefringence is one of the fascinating properties of the vacuum of quantum electrodynamics (QED) in strong electromagnetic fields. The scattering of linearly polarized incident probe photons into a perpendicularly polarized mode provides a distinct signature of the optical activity of the quantum vacuum and thus offers an excellent opportunity for a precision test of nonlinear QED. Precision tests require accurate predictions and thus a theoretical framework that is capable of taking the detailed experimental geometry into account. We derive analytical solutions for vacuum birefringence which include the spatio-temporal field structure of a strong optical pump laser field and an x-ray probe. We show that the angular distribution of the scattered photons depends strongly on the interaction geometry and find that scattering of the perpendicularly polarized scattered photons out of the cone of the incident probe x-ray beam is the key to making the phenomenon experimentally accessible with the current generation of FEL/high-field laser facilities.

Reconsidering the Origins of Forsbergh Birefringence Patterns

In 1949, P. W. Forsbergh Jr. reported spontaneous spatial ordering in the birefringence patterns seen in flux-grown BaTiO3 crystals [1], under the transmission polarized light microscope [2]. Stunningly regular square-net arrays were often only found within a finite temperature window and could be induced on both heating and cooling, suggesting genuine thermodynamic stability. At the time, Forsbergh rationalized the patterns to have resulted from the impingement of ferroelastic domains, creating a complex tessellation of variously shaped domain packets. However, evidence for the intricate microstructural arrangement proposed by Forsbergh has never been found. Moreover, no robust thermodynamic argument has been presented to explain the region of thermal stability, its occurrence just below the Curie Temperature and the apparent increase in entropy associated with the loss of the Forsbergh pattern on cooling. As a result, despite decades of research on ferroelectrics, this ordering phenomenon and its thermodynamic origin have remained a mystery. In this paper, we re-examine the microstructure of flux-grown BaTiO3 crystals, which show Forsbergh birefringence patterns. Given an absence of any obvious arrays of domain polyhedra, or even regular shapes of domain packets, we suggest an alternative origin for the Forsbergh pattern, in which sheets of orthogonally oriented ferroelastic stripe domains simply overlay one another. We show explicitly that the Forsbergh birefringence pattern occurs if the periodicity of the stripe domains is above a critical value. Moreover, by considering well-established semiempirical models, we show that the significant domain coarsening needed to generate the Forsbergh birefringence is fully expected in a finite window below the Curie Temperature. We hence present a much more straightforward rationalization of the Forsbergh pattern than that originally proposed, in which exotic thermodynamic arguments are unnecessary.

Influence of molten-state annealing on the phase structure and crystallization behaviour of high impact polypropylene copolymer

The phase structure evolution of high impact polypropylene copolymer (IPC) during molten-state annealing and its influence on crystallization behaviour were studied. An entirely different architecture of the IPC melt was observed after being annealed, and this architecture resulted in variations of the crystallization behaviour. In addition, it was found that the core-shell structure of the dispersed phase was completely destroyed and the sizes of the dispersed domains increased sharply after being annealed at 200 degrees C for 200 min. Through examination of the coarseness of the phase morphology using phase contrast microscopy (PCM), it was found that a co-continuous structure and an abnormal 'sea-island' structure generally appeared with an increase in annealing time. The original matrix PP component appeared as a dispersed phase, whereas the copolymer components formed a continuous 'sea-island' structure. This change is ascribed to the large tension induced by solidification at the phase interface and the great content difference between the components. When the temperature was reduced the structure reverted to its original form. With increasing annealing time, the spherulite profiles became more defined and the spherulite birefringence changed from vague to clear. Overall crystallization rates and nucleation densities decreased, but the spherulite radial growth rates remained almost constant, indicating that molten-state annealing mainly affects the nucleation ability of IPC, due to a coarsened microstructure and decreased interface area. (C) 2011 Elsevier Ltd. All rights reserved.

Combining histology, stable isotope analysis and ZooMS collagen fingerprinting to investigate the taphonomic history and dietary behaviour of extinct giant tortoises from the Mare aux Songes deposit on Mauritius

Taphonomic research of bones can provide additional insight into a site's formation and development, the burial environment and ongoing post-mortem processes. A total of 30 tortoise (Cylindraspis) femur bone samples from the Mare aux Songes site (Mauritius)were studied histologically, assessing parameters such as presence and type of microbial alteration, inclusions, staining/infiltrations, the degree of microcracking and birefringence. The absence of microbial attack in the 4200 year old Mare aux Songes bones suggests the animals rapidly entered the soil whole-bodied and were sealed anoxically, although they suffered frombiological and chemical degradation (i.e. pyrite formation/oxidation, mineral dissolution and staining) related to changes in the site's hydrology. Additionally, carbon and nitrogen stable isotopeswere analysed to obtain information on the animals' feeding behaviour. The results show narrowly distributed δ13C ratios, indicating a terrestrial C3 plant-based diet, combined with a wide range in δ15N ratios. This is most likely related to the tortoises' drought-adaptive ability to change their metabolic processes, which can affect the δ15N ratios. Furthermore, ZooMS collagen fingerprinting analysis successfully identified two tortoise species (C. triserrata and C. inepta) in the bone assemblage,which,when combined with stable isotope data, revealed significantly different δ15N ratios between the two tortoise species. As climatic changes around this period resulted in increased aridity in the Mascarene Islands, this could explain the extremely elevated δ15N ratio in our dataset. The endemic fauna was able to endure the climatic changes 4200 years ago, although human arrival in the 17th century changed the original habitat to such an extent that it resulted in the extinction of several species. Fortunately we are still able to study these extinct tortoises due to the beneficial conditions of their burial environment, resulting in excellent bone preservation.