Local electromigration model for crystal surfaces

We analyze the dynamics of crystal surfaces in the presence of electromigration. From a phase field model with a migration force which depends on the local geometry, we derive a step model with additional contributions in the kinetic boundary conditions. These contributions trigger various surface instabilities, such as step meandering, bunching and pairing on vicinal surfaces. Experiments are discussed

Dewetting of solid films with substrate mediated evaporation

The dewetting dynamics of an ultrathin film is studied in the presence of evaporation - or reaction - of adatoms on the substrate. KMC simulations are in good agreement with an analytical model with diffusion, rim facetting, and substrate sublimation. As sublimation is increased, we find a transition from the usual dewetting regime where the front slows down with time, to a sublimation-controlled regime where the front velocity is approximately constant. The rim width exhibits an unexpected non-monotonous behavior, with a maximum in time.Comment: 6 pages, 6 figure

Preliminary results of a network of trials related to sugarcane nutrition in Reunion Island

IN REUNION ISLAND, sugarcane is grown under highly contrasting climatic conditions, from the sea level to the highlands (up to 1000 m altitude), with very different soil types. Taking this diversity into account, a network of agronomic trials was implemented in 2005 on grower farms across the island. Four main fertilisation or amelioration techniques were tested: i) reduction of soil acidity using mill ash compared to liming materials, Mag lime and Physiolith; ii) sustainable nutrient management based on soil analysis, iii) effect of a slow release fertiliser-polymer-coated granular urea and iv) effect of splitting nitrogen application. In each trial, the traditional grower practices were used as control. Outcomes of the trials included: correction of soil acidity using mill ash with a sugar yield 10 to 23% higher than the control; a better sugar yield per ha using sustainable nutrient management (in one of two trials); a sugar yield loss using a reduced rate (-30%) of polymer-coated granular urea; a positive impact on ratoon yield when nitrogen application was split into two events. Such a network of trials should facilitate a faster adoption of research recommendations by the growers as it allows a direct exchange of information between sugarcane farmers and agronomists and addresses growers' issues. (Résumé d'auteur

Divide-and-Conquer Method for Instanton Rate Theory

Ring-polymer instanton theory has been developed to simulate the quantum dynamics of molecular systems at low temperatures. Chemical reaction rates can be obtained by locating the dominant tunneling pathway and analyzing fluctuations around it. In the standard method, calculating the fluctuation terms involves the diagonalization of a large matrix, which can be unfeasible for large systems with a high number of ring-polymer beads. Here we present a method for computing the instanton fluctuations with a large reduction in computational scaling. This method is applied to three reactions described by fitted, analytic and on-the-fly ab initio potential-energy surfaces and is shown to be numerically stable for the calculation of thermal reaction rates even at very low temperature

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

Double symmetry breaking and 2D quantum phase diagram in spin-boson systems

The quantum ground state properties of two independent chains of spins (two-levels systems) interacting with the same bosonic field are theoretically investigated. Each chain is coupled to a different quadrature of the field, leading to two independent symmetry breakings for increasing values of the two spin-boson interaction constants $\Omega_C$ and $\Omega_I$. A phase diagram is provided in the plane ($\Omega_C$,$\Omega_I$) with 4 different phases that can be characterized by the complex bosonic coherence of the ground states and can be manipulated via non-abelian Berry effects. In particular, when $\Omega_C$ and $\Omega_I$ are both larger than two critical values, the fundamental subspace has a four-fold degeneracy. Possible implementations in superconducting or atomic systems are discussed

Scattering rates and lifetime of exact and boson excitons

Although excitons are not exact bosons, they are commonly treated as such provided that their composite nature is included in effective scatterings dressed by exchange. We here \emph{prove} that, \emph{whatever these scatterings are}, they cannot give both the scattering rates $T_{ij}^{-1}$ and the exciton lifetime $\tau_0$, correctly: A striking factor 1/2 exists between $\tau_0^{-1}$ and the sum of $T_{ij}^{-1}$'s, which originates from the composite nature of excitons, irretrievably lost when they are bosonized. This result, which appears as very disturbing at first, casts major doubts on bosonization for problems dealing with \emph{interacting} excitons

Atomic step motion during the dewetting of ultra-thin films

We report on three key processes involving atomic step motion during the dewetting of thin solid films: (i) the growth of an isolated island nucleated far from a hole, (ii) the spreading of a monolayer rim, and (iii) the zipping of a monolayer island along a straight dewetting front. Kinetic Monte Carlo results are in good agreement with simple analytical models assuming diffusion-limited dynamics.Comment: 7 pages, 5 figure