A comprehensive analysis of the (R13xR13)R13.9{\deg} type II structure of silicene on Ag(111)

In this paper, using the same geometrical approach than for the (2R3x2R3) R30{\deg} structure (H. Jamgotchian et al., 2015, Journal of Physics. Condensed Matter 27 395002), for the (R13xR13)R13.9{\deg} type II structure, we propose an atomic model of the silicene layer based on a periodic relaxation of the strain epitaxy. This relaxation creates periodic arrangements of perfect areas of (R13xR13)R13.9{\deg} type II structure surrounded by defect areas. A detailed analysis of the main published experimental results, obtained by Scanning Tunneling Microscopy and by Low Energy Electron Diffraction, shows a good agreement with the geometrical model.Comment: 20 pages, 9 figure

Pattern formation in directional solidification under shear flow. I: Linear stability analysis and basic patterns

An asymptotic interface equation for directional solidification near the absolute stabiliy limit is extended by a nonlocal term describing a shear flow parallel to the interface. In the long-wave limit considered, the flow acts destabilizing on a planar interface. Moreover, linear stability analysis suggests that the morphology diagram is modified by the flow near the onset of the Mullins-Sekerka instability. Via numerical analysis, the bifurcation structure of the system is shown to change. Besides the known hexagonal cells, structures consisting of stripes arise. Due to its symmetry-breaking properties, the flow term induces a lateral drift of the whole pattern, once the instability has become active. The drift velocity is measured numerically and described analytically in the framework of a linear analysis. At large flow strength, the linear description breaks down, which is accompanied by a transition to flow-dominated morphologies, described in a companion paper. Small and intermediate flows lead to increased order in the lattice structure of the pattern, facilitating the elimination of defects. Locally oscillating structures appear closer to the instability threshold with flow than without.Comment: 20 pages, Latex, accepted for Physical Review

The Effect of Convection on Disorder in Primary Cellular and Dendritic Arrays

Directional solidification studies have been carried out to characterize the spatial disorder in the arrays of cells and dendrites. Different factors that cause array disorder are investigated experimentally and analyzed numerically. In addition to the disorder resulting from the fundamental selection of a range of primary spacings under given experimental conditions, a significant variation in primary spacings is shown to occur in bulk samples due to convection effects, especially at low growth velocities. The effect of convection on array disorder is examined through directional solidification studies in two different alloy systems, Pb-Sn and Al-Cu. A detailed analysis of the spacing distribution is carried out, which shows that the disorder in the spacing distribution is greater in the Al-Cu system than in Pb-Sn system. Numerical models are developed which show that fluid motion can occur in both these systems due to the negative axial density gradient or due the radial temperature gradient which is always present in Bridgman growth. The modes of convection have been found to be significantly different in these systems, due to the solute being heavier than the solvent in the Al-Cu system and lighter than it in the Pb-Sn system. The results of the model have been shown to explain experimental observations of higher disorder and greater solute segregation in a weakly convective Al-Cu system than those in a highly convective Pb-Sn system

Influence of grain boundaries and natural convection on microstructure formation in cellular directional solidification of dilute succinonitrile alloys in a cylinder

International audienceThe influence of grain boundaries and natural convection on the birth and development of morphological instability is investigated during the directional solidification of succinonitrile-based alloys (succinonitrile–0.5 wt% acetone and succinonitrile–1.5 wt% water) in a cylindrical crucible. Continuous and in situ observation of the solid–liquid interface by optical methods is carried out through the melt. It is in particular shown that grain boundaries do not always cause morphological instability early and that a dynamical grooving of grains may occur by the propagation of channels, emanating among others from depressions in the solid ridges aside grain boundaries. Then, the accommodation of a grain boundary in the cellular array is presented. As convection in the melt is strong enough in the experiments for solute sweeping over the growth front to be significant, a delay of morphological instability is furthermore observed downstream in regions adjacent to well-oriented solid ridges

In situ and real-time observation of the formation and dynamics of a cellular interface in a succinonitrile-0.5 wt% acetone alloy directionally solidified in a cylinder

International audienceIn situ and real-time observation of the solid-liquid interface during upward Bridgman growth of succinonitrile-0.5 wt% acetone in a cylinder was carried out. This first article concentrates on the study of the emergence and building of nonplanar patterns. By observation under white light in the solidification direction, bright field images and video records of the interface microstructure were obtained, which allowed dynamical phenomena such as the ripening and arrangement of cellular arrays with time. All results show a significant influence of natural convection in the melt. Inparticular, a microstructure gradient was observed at the interface. From a phenomenological model based on a boundary layer approach, its origin can be attributed to a ramp in solute concentration along the growth front created by tangential fluid flow. Statistical analyses of the evolution of primary spacing and array disorder show that the stable arrangement can still be described as a honeycomb disturbed by Gaussian noise. Convection was also found to be responsible for primary spacings smaller than that given by theoretical diffusive models and experiments on thin samples