Symmetry breaking of ionic semiconductors under pressure: the case of InAs.

The NaCl-to-Cmcm phase transition and the Cmcm structure of InAs under high pressure are studied by x-ray diffraction. The lattice parameters and fractional coordinates are given as a function of pressure. We propose a mechanism responsible for this type of symmetry breaking under pressure. We show that the ppσ interactions do not play a major role in the stabilization of the NaCl structure. Consequently the NaCl-to-Cmcm transition occurs only in compounds with a large charge transfer. General conclusions on the behavior of III-V semiconductors under pressure are drawn.

New phase transition of solid bromine under high pressure.

Solid bromine has been studied by x-ray absorption spectroscopy experiments up to a maximum pressure of 75 GPa. The data analysis of the extended fine structure reveals that the intramolecular distance first increases, reaching its maximum value at 25+/-5 GPa. From this value the intramolecular distance abruptly begins to decrease evidencing a nonpreviously observed phase transformation taking place at 25+/-5 GPa. A maximum variation of 0.08 A is observed at 65+/-5 GPa where again a phase transition occurs. This last transformation could correspond with the recently observed change to an incommensurate modulated phase. We discuss the possible generalization of the observed new phase transition at 25+/-5 GPa to the case of the other halogens.

Nucleation and growth of single-walled nanotubes: the role of metallic catalysts.

We present a review of experimental and theoretical results on the nucleation and growth of single-walled nanotubes, with particular emphasis on the growth of nanotube bundles emerging from catalyst particles obtained from evaporation-based elaboration techniques. General results are first discussed. Experiments strongly suggest a root-growth process in which carbon, dissolved at high temperatures in catalytic particles, segregates at the surface at lower temperatures to form tube embryos and finally nanotubes through a nucleation and growth process. A theoretical analysis of the reasons carbon does not always form graphene sheets to wrap the particles suggests analogies with other surface or interface instabilities, in particular, with those found in epitaxial growth. In the second part, detailed experimental results for nickel-rare earth metal catalysts are presented. By using various electron microscopy techniques, it is shown that carbon and the rare earth metal co-segregate at the surface of the particle and form carbide platelets, providing nucleation sites for nanotubes growing in directions perpendicular to the surface. A simple theoretical model is then presented in which the role of the rare earth metal is just to transfer electrons from metal to carbon. The graphene sheet is shown to become unstable; pentagons and heptagons are favored, which can explain the occurrence of local curvatures and of tube embryos. Finally, a brief discussion of some recent atomistic models is given.

Regulation of vascular endothelial growth factor by the Wnt and K-ras pathways in colonic neoplasia.

Angiogenesis is not restricted to advanced stages of tumor development but is also observed in benign precursor lesions. Vascular endothelial growth factor (VEGF) is a key regulator of tumor angiogenesis, but the genetic mechanisms controlling its expression in premalignant lesions are poorly described. The Wnt signaling pathway, which is commonly mutated in benign colonic adenomas, was found to strongly up-regulate VEGF. A T-cell factor-4-binding element at -805 bp in the VEGF promoter is an important mediator of this effect. Signaling through the K-ras oncogene, also frequently mutated in benign colonic polyps, up-regulated VEGF in a phosphatidylinositol 3-kinase-dependent manner. Furthermore, K-ras activation appeared to enhance Wnt signaling, which suggests a unique interaction between these two pathways. These studies thus identify VEGF as a novel target of the Wnt pathway in early colonic neoplasia and serve to underscore the importance of angiogenesis in premalignant disease.