University of Bordeaux

About: University of Bordeaux is a education organization based out in Bordeaux, France. It is known for research contribution in the topics: Population & Laser. The organization has 28811 authors who have published 55536 publications receiving 1619635 citations. The organization is also known as: UB.

An earlier origin for the Acheulian

Abstract: The Acheulian is one of the first defined prehistoric technocomplexes and is characterized by shaped bifacial stone tools. It probably originated in Africa, spreading to Europe and Asia perhaps as early as 1 million years (Myr) ago. The origin of the Acheulian is thought to have closely coincided with major changes in human brain evolution, allowing for further technological developments. Nonetheless, the emergence of the Acheulian remains unclear because well-dated sites older than 1.4Myr ago are scarce. Here we report on the lithic assemblage and geological context for the Kokiselei 4 archaeological site fromthe Nachukui formation(West Turkana,Kenya) that bears characteristic early Acheulian tools and pushes the first appearance datumfor this stone-age technology back to 1.76Myr ago. Moreover, co-occurrence of Oldowan and Acheulian artefacts at the Kokiselei site complex indicates that the two technologies are not mutually exclusive time-successive components of an evolving cultural lineage, and suggests that the Acheulian was either imported from another location yet to be identified or originated from Oldowan hominins at this vicinity. In either case, the Acheulian did not accompany the first human dispersal from Africa despite being available at the time. This may indicate that multiple groups of hominins distinguished by separate stone-tool-making behaviours and dispersal strategies coexisted in Africa at 1.76Myr ago.

The metathesis reactions: from a historical perspective to recent developments

Abstract: Metathesis is one of the most spectacular recent improvements in synthetic strategies for organic synthesis and polymer science. The historical aspects and modern developments of the metathesis reactions are summarized here. In particular, emphasis is placed on the leading role played by the mechanistic work and proposals of Yves Chauvin and on the history of the efficient catalysts discovered by the groups of R. R. Schrock and R. H. Grubbs. It is pointed out how the Chauvin metathesis mechanism, with formation of a metallacyclobutane, has been generalized to many organometallic reactions that also involve square intermediates comprising a metal atom. Subsequently, the progressive development of ideas by Schrock and Grubbs during the last three decades has brought the field to the forefront of synthetic chemistry. The quest for efficient metathesis catalysts is a success story, starting from organometallic mechanisms, that has now invaded the worlds of organic synthesis and polymer science. Indeed, the Schrock’ and Grubbs’ catalysts and their derivatives are now the most efficient catalysts compatible with functional groups for the metathesis reactions. They considerably shorten synthetic schemes by affording new routes and therefore have changed the way chemists think about synthesis.

A Magnetization Transfer Imaging Study of Normal-Appearing White Matter in Multiple Sclerosis

Abstract: We attempted to define the role of subtle changes in the normal-appearing white matter (NAWM) in the development of disability in multiple sclerosis (MS). Twenty-seven clinically definite MS patients with either relapsing-remitting or chronic-progressive courses and 10 sex- and age-matched controls entered the study. For each patient and control, we studied two NAWM areas in the frontal lobe with magnetization transfer imaging (MTI). For patients, we also calculated the MT ratios (MTRs) for three contiguous areas of NAWM progressively further from "isolated" lesions visible on conventional MRI. Frontal NAWM in MS patients had lower mean MTRs than the frontal white matter of the controls (p = 0.02). MTRs in the NAWM adjacent to isolated lesions increased with distance from them to the cortical gray matter (p = 0.04). This pattern was typical for patients with chronic-progressive MS whose MTRs in the first two regions of NAWM adjacent to lesions were lower than those of the same regions of patients with relapsing-remitting MS. This study confirms that there are alterations in the NAWM of MS patients and suggests that such changes might be relevant to the disability in MS.

Laser-induced microexplosion confined in the bulk of a sapphire crystal: evidence of multimegabar pressures

Abstract: Extremely high pressures ($\ensuremath{\sim}10\text{ }\text{ }\mathrm{TPa}$) and temperatures ($5\ifmmode\times\else\texttimes\fi{}{10}^{5}\text{ }\text{ }\mathrm{K}$) have been produced using a single laser pulse (100 nJ, 800 nm, 200 fs) focused inside a sapphire crystal. The laser pulse creates an intensity over ${10}^{14}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$ converting material within the absorbing volume of $\ensuremath{\sim}0.2\text{ }\text{ }\ensuremath{\mu}{\mathrm{m}}^{3}$ into plasma in a few fs. A pressure of $\ensuremath{\sim}10\text{ }\text{ }\mathrm{TPa}$, far exceeding the strength of any material, is created generating strong shock and rarefaction waves. This results in the formation of a nanovoid surrounded by a shell of shock-affected material inside undamaged crystal. Analysis of the size of the void and the shock-affected zone versus the deposited energy shows that the experimental results can be understood on the basis of conservation laws and be modeled by plasma hydrodynamics. Matter subjected to record heating and cooling rates of ${10}^{18}\text{ }\text{ }\mathrm{K}/\mathrm{s}$ can, thus, be studied in a well-controlled laboratory environment.