Nonlinear evolution of step meander during growth of a vicinal surface with no desorption

Step meandering due to a deterministic morphological instability on vicinal surfaces during growth is studied. We investigate nonlinear dynamics of a step model with asymmetric step kinetics, terrace and line diffusion, by means of a multiscale analysis. We give the detailed derivation of the highly nonlinear evolution equation on which a brief account has been given [Pierre-Louis et.al. PRL(98)]. Decomposing the model into driving and relaxational contributions, we give a profound explanation to the origin of the unusual divergent scaling of step meander ~ 1/F^{1/2} (where F is the incoming atom flux). A careful numerical analysis indicates that a cellular structure arises where plateaus form, as opposed to spike-like structures reported erroneously in Ref. [Pierre-Louis et.al. PRL(98)]. As a robust feature, the amplitude of these cells scales as t^{1/2}, regardless of the strength of the Ehrlich-Schwoebel effect, or the presence of line diffusion. A simple ansatz allows to describe analytically the asymptotic regime quantitatively. We show also how sub-dominant terms from multiscale analysis account for the loss of up-down symmetry of the cellular structure.Comment: 23 pages, 10 figures; (Submitted to EPJ B

A cluster partitioning method: determination of density matrices of solids and comparison with X-ray experiments

In this paper we show that 1-electron properties such as Compton profiles and structure factors of crystals can be asymptotically retrieved through cluster-based calculations, followed by an appropriate partition of the 1-electron reduced density matrix (1RDM). This approach, conceptually simple, is checked with respects to both position and momentum spaces simultaneously for insulators and a covalent crystal. Restricting the calculations to small clusters further enables a fair description of local correlation effects in ionic compounds, which improves both Compton profiles and structure factors vs. their experimentally determined counterparts.Comment: 19 pages, 9 figures, 3 tables. Currently submitted to PR

Modulation des réponses des cellules voisines simples et complexes par un contraste différentiel entre le centre et la périphérie des champs récepteurs dans l'aire 17 du chat

Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal

Nonlinear evolution of the step meandering instability of a growing crystal surface

The growth of crystal surfaces, under non-equilibrium conditions, involves the displacement of mono-atomic steps by atom diffusion and atom incorporations into steps. The time-evolution of the growing crystal surface is thus governed by a free boundary value problem [known as the Burton--Cabrera--Franck model]. In the presence of an asymmetry of the kinetic coefficients [Erlich--Schwoebel barriers], ruling the rates of incorporation of atoms at each step, it has been shown that a train of straight steps is unstable to two dimensional transverse perturbations. This instability is now known as the Bales-Zangwill instability (meandering instability). We study the non-linear evolution of the step meandering instability that occurs on a crystalline vicinal surface under growth, in the absence of evaporation, in the limit of a weak asymmetry of atom incorporation at the steps. We derive a nonlinear amplitude equation displaying spatiotemporal coarsening. We characterize the self-similar solutions of this equation.Comment: 6 pages, submitted to Physica D, 200

Du processus aux soins intégrés: Expérience de gestion de projet bottom-up

Le service de chirurgie cardiaque du CHU de Liège a soutenu des recherches visant à développer un programme d’épargne sanguine, enjeu actuel de taille dans ce secteur des soins de santé. Ce projet a évolué vers la création d’un itinéraire clinique chirurgical cardiaque et d’un modèle institutionnel pour le développement d’autres itinéraires cliniques. Une évolution qui permet de déterminer les missions spécifiques de l’institution et ses objectifs stratégiques, et de s’associer aux projets nationaux. L’adhésion multidisciplinaire, soutenue par un leadership médical et infirmier, ainsi que la reconnaissance institutionnelle sont les déterminants de la pérennité de cette démarche bottom-up

Reionisation time fields reconstruction from 21 cm signal maps

During the Epoch of reionisation, the intergalactic medium is reionised by the UV radiation from the first generation of stars and galaxies. One tracer of the process is the 21 cm line of hydrogen that will be observed by the Square Kilometre Array (SKA) at low frequencies, thus imaging the distribution of ionised and neutral regions and their evolution. To prepare for these upcoming observations, we investigate a deep learning method to predict from 21 cm maps the reionisation time field (treion(r)), i.e. the time at which each location has been reionised. treion(r) encodes the propagation of ionisation fronts in a single field, gives access to times of local reionisation or to the extent of the radiative reach of early sources. Moreover it gives access to the time evolution of ionisation on the plane of sky, when such evolution is usually probed along the line-of-sight direction. We trained a convolutional neural network (CNN) using simulated 21 cm maps and reionisation times fields produced by the simulation code 21cmFAST . We also investigate the performance of the CNN when adding instrumental effects. Globally, we find that without instrumental effects the 21 cm maps can be used to reconstruct the associated reionisation times field in a satisfying manner: the quality of the reconstruction is dependent on the redshift at which the 21 cm observation is being made and in general it is found that small scale (<10cMpc/h) features are smoothed in the reconstructed field, while larger scale features are well recovered. When instrumental effects are included, the scale dependance of reconstruction is even further pronounced, with significant smoothing on small and intermediate scales

Drift-Induced Step Instabilities Due to the Gap in the Diffusion Coefficient

On a Si(111) vicinal face near the structural transition temperature, the $1 \times 1$ structure and the $7 \times 7$ structure coexist in a terrace: the $1 \times 1$ structure is in the lower side of the step edge and the $7 \times 7$ structure in the upper side. The diffusion coefficient of adatoms is different in the two structures. Taking account of the gap in the diffusion coefficient at the step, we study the possibility of step wandering induced by drift of adatoms. A linear stability analysis shows that the step wandering always occurs with step-down drift if the diffusion coefficient has a gap at the step. Formation of straight grooves by the step wandering is expected from a nonlinear analysis. The stability analysis also shows that step bunching occurs irrespective of the drift direction if the diffusion in the lower side of the step is faster. The step bunching disturbs the formation of grooves. If step-step repulsion is strong, however, the step bunching is suppressed and the straight grooves appear. Monte Carlo simulation confirms these predictions.Comment: 5 pages, 6 figure

Clinical and radiological results with a 36-mm Cobalt-Chrome prosthetic head, cross-linked Durasul liners associated with Allofit cups: A more than 10-year follow-up period.

peer reviewedHighly cross-linked polyethylene is currently a common articulation surface used for THA. The aim of the present study is in-vivo assessment of highly cross-linked Durasul® polyethylene linear and volumetric wear when associated with a 36-mm prosthetic femoral head. We retrospectively reviewed clinical and radiographic data of 78 patients (81 hips) having primary THAs using Durasul® liner combined with a 36-mm CoCr prosthetic head. All of them were followed for more than 10 years. Patient outcome was assessed with the Harris Hip Score (HHS) preoperatively and at last follow-up. Two-dimensional prosthetic head penetration into polyethylene, three-dimensional wear rates and cup migration were evaluated. the preoperative and last follow-up HHS were 50.43 +/- 10.42 and 97.44 +/- 5.51 respectively. The annual penetration of the prosthetic head into Durasul® liner was 0.029 +/- 0.003 mm. The annual linear penetration and volumetric wear extrapolation rates using Charnley and Ilchmann formulas were 37.84% and 57.76% respectively of that seen with conventional polyethylene liner. At last follow-up, the total loss of material in Durasul® represents only 0.15% of the initial polyethylene mass. We did not observed any significant cup migration in the study group. Results are promising and we believe that these data authorize the continued use of highly cross-linked polyethylene liner associated with a 36-mm prosthetic head for total hip arthroplasties in older patients. More long-term follow-up studies are mandatory before we feel comfortable with the project of using cross-linked polyethylene in young and active patients instead of ceramic-on-ceramic bearings