Abstract

The intense electronic excitations and ionizations released in the wake ofswift heavy ions can induce important structural modifications in solids.It is well known that the irradiation of many crystalline materials withswift heavy ions generally leads to structural disorder which spans frompoint defect creation up to total amorphization. However, there is nowgrowing evidence that the structural changes induced by the irradiationwith swift heavy ions are not limited to damage creation but can alsocomprise crystalline-to-crystalline phase transitions. More recently,these effects were also enriched by the finding that swift heavy ions caninduce epitaxial recrystallization of predamaged layers produced bynuclear collisions.From the fundamental point of view, two types of models, either based onthe ‘Coulomb explosion’ concept or the ‘thermal spike’ mechanism areusually proposed to account for swift heavy ion induced atomic movements.Since for both models the timescales of the involved processes are quiteinaccessible to usual characterization techniques, it is very difficult toidentify the actual mechanism. Nevertheless, the recent study ofirradiation effects in the twin oxides zirconia and hafnia, which sharemany physical and chemical properties, offered the unique opportunity toimpose drastic constraints on the models. This comparative study allowedto demonstrate that the thermal spike is the appropriate mechanism whichgoverns the transition from the monoclinic to the tetragonal phase inzirconia and hafnia rather than the Coulomb explosion process.In this contribution, these different types of swift heavy ion inducedphase transitions in solids (i.e., crystalline-to-amorphous,crystalline-to-crystalline and amorphous-to-crystalline transformations)will be illustrated with recent experiments performed at the GANILaccelerator and the mechanisms leading to these effects will be discussedand interpreted within a single view based on the thermal spike process.

Abstract

Coarse grained hydrodynamic theories have so far been successful indescribing patterns formed in in vitro mixtures of active polar filaments(eg Actin, Microtubules) and motors (Myosin, Dyenin, Kinesin). Thereexists little evidence of their validity in vivo.In this talk I will present a theoretical stdy of the dynamics of activepolar filaments on a two dimensional substrate. We determine that thesteady states of the equations of motion of these filaments aregenerically composed of inward pointing asters in different arrangements.The asters display an interesting dynamics in the presence of noise- theybreak up and reform with characteristic residence times, whosedistribution depends on the noise strength.I will further present evidence that the results are applicable to thepatterning of molecules on the cell surface. The evidence is based onanisotropy experiments of GPI-anchored proteins, a class of proteins thatassociate with Actin.

Abstract

The Indus script is one of the major undeciphered scripts ofthe ancient world. I will discuss the difficulties associated withdeciphering this script and present a theoretical framework, based onMarkov chains, within which the syntax of the script can be exploredsystematically. I will present key results from the Markov chain analysiswhich provide evidence in favour the hypothesis that the script was usedto represent natural language. The use of other probabilistic methods foranalysis corpora of linear sequences will also be briefly outlined.

Abstract

Abstract