Ion-ion dynamic structure factor, acoustic modes, and equation of state of two-temperature warm dense aluminum.

The ion-ion dynamical structure factor and the equation of state of warm dense aluminum in a two-temperature quasiequilibrium state, with the electron temperature higher than the ion temperature, are investigated using molecular-dynamics simulations based on ion-ion pair potentials constructed from a neutral pseudoatom model. Such pair potentials based on density functional theory are parameter-free and depend directly on the electron temperature and indirectly on the ion temperature, enabling efficient computation of two-temperature properties. Comparison with ab initio simulations and with other average-atom calculations for equilibrium aluminum shows good agreement, justifying a study of quasiequilibrium situations.

Isochoric, isobaric, and ultrafast conductivities of aluminum, lithium, and carbon in the warm dense matter regime.

We study the conductivities σ of (i) the equilibrium isochoric state σ_{is}, (ii) the equilibrium isobaric state σ_{ib}, and also the (iii) nonequilibrium ultrafast matter state σ_{uf} with the ion temperature T_{i} less than the electron temperature T_{e}. Aluminum, lithium, and carbon are considered, being increasingly complex warm dense matter systems, with carbon having transient covalent bonds. First-principles calculations, i.

Dominant structural defects in amorphous silicon.

The nature of disorder in amorphous silicon (a-Si) is explored by investigating the spatial arrangement and energies of coordination defects in a numerical model. Spatial correlations between structural defects are examined on the basis of a parameter that quantifies the probability for two sites to share a bond. Pentacoordinated atoms are found to be the dominant coordination defects.

Understanding subtle changes in medium-range order in amorphous silicon.

Based on a detailed study of the radial distribution function (RDF) of a model for amorphous silicon (a-Si), we address the relation between short-range rearrangements and an increase in medium-range order induced by thermal relaxation. Recent experimental measurements have shown that a small peak appears in the RDF around 4.7 Å upon annealing, along with other subtle changes, and this is attributed to ordering among the dihedral angles.

Replenish and relax: explaining logarithmic annealing in ion-implanted c-Si.

We study ion-damaged crystalline silicon by combining nanocalorimetric experiments with an off-lattice kinetic Monte Carlo simulation to identify the atomistic mechanisms responsible for the structural relaxation over long time scales. We relate the logarithmic relaxation, observed in a number of disordered systems, with heat-release measurements. The microscopic mechanism associated with this logarithmic relaxation can be described as a two-step replenish and relax process.

Cross-correlations between phonon modes in anharmonic oscillator chains: role in heat transport.

We have computed current-current correlation functions in chains of anharmonic oscillators described by various models (FPU-β, FPU-αβ, ϕ4), considering both the total current and the currents associated with individual phonon modes, which are important in view of the Green-Kubo relation for heat conductivity. Our simulations show that, contrary to the common hypothesis, there are, under some circumstances, significant correlations between neighboring modes. These cross-mode correlations are the dominant contribution to the conductivity in the low anharmonicity regime.

Comment on "The local structure of amorphous silicon".

Treacy and Borisenko (Reports, 24 February 2012, p. 950) argue from reverse Monte Carlo modeling of electron diffraction and fluctuation electron microscopy data that amorphous silicon is paracrystalline and not described by a continuous random network. However, their models disagree with high-resolution x-ray measurements and other evidence, whereas the agreement with fluctuation electron microscopy is at best qualitative.

Heat conduction across molecular junctions between nanoparticles.

We investigate the problem of heat conduction across molecular junctions connecting two nanoparticles, both in vacuum and in a liquid environment, using classical molecular dynamics simulations. In vacuum, the well-known result of a length independent conductance is recovered; its precise value, however, is found to depend sensitively on the overlap between the vibrational spectrum of the junction and the density of states of the nanoparticles that act as thermal contacts. In a liquid environment, the conductance is constant up to a crossover length, above which a standard Fourier regime is recovered.

Critical heat flux around strongly heated nanoparticles.

We study heat transfer from a heated nanoparticle into surrounding fluid using molecular dynamics simulations. We show that the fluid next to the nanoparticle can be heated well above its boiling point without a phase change. Under increasing nanoparticle temperature, the heat flux saturates, which is in sharp contrast with the case of flat interfaces, where a critical heat flux is observed followed by development of a vapor layer and heat flux drop.

Effect of elastic interactions on coarsening in elastically inhomogeneous multiphase systems.

We investigate the effect of interactions between inclusions on the coarsening behavior of elastically inhomogeneous multiphase systems with lattice misfit using a recently introduced two-dimensional multiscale model based on the classical time-dependent density-functional theory. We show that spontaneous shape changes are very efficient in limiting the impact of the interactions on the chemical potential of inclusions. For this reason, the interactions between isolated pairs of inclusions are unable to significantly affect coarsening.

Kinetic faceting and anomalous coarsening in elastically inhomogeneous multiphase systems.

With a view of finding a route toward microstructural stability in alloys, we numerically study the impact of elastic inhomogeneities on the growth of inclusions in multiphase systems. We show that growth can proceed either continuously at rough interfaces, or in a layer-by-layer fashion following an elastically induced kinetic faceting process. In the former case, the chemical potential of the inclusions is a smooth function of size, while in the latter case, elasticity increases the barrier for nucleation of new terraces on the facets, leading to an oscillatory behavior of the chemical potential and hence a strong resistance against coarsening, opening up the possibility to stabilize the structure.

Interface properties and microstructural stabilization in elastically inhomogeneous multiphase systems.

Using a two-dimensional multiscale model based on the classical time-dependent density-functional theory for lattice systems we recently introduced, we numerically study the impact of elastic inhomogeneities on the growth of isolated inclusions in multiphase alloys. We demonstrate that the coupling between the overall interface structure (as determined by the shape of the inclusions) and the local, atomic-scale structure can be very large, and is able to significantly affect the behavior of inclusions during growth. Elasticity is shown to have a strong influence on the local energetics at interfaces, leading to shape modulations and kinetic faceting.

Multiscale model for microstructure evolution in multiphase materials: Application to the growth of isolated inclusions in presence of elasticity.

We present a multiscale model based on the classical lattice time-dependent density-functional theory to study microstructure evolution in multiphase systems. As a first test of the method, we study the static and dynamic properties of isolated inclusions. Three cases are explored: elastically homogeneous systems, elastically inhomogeneous systems with soft inclusions, and elastically inhomogeneous systems with hard inclusions.

Stable fourfold configurations for small vacancy clusters in silicon from ab initio calculations.

Using density-functional-theory calculations, we have identified new stable configurations for tri-, tetra-, and pentavacancies in silicon. These new configurations consist of combinations of a ring hexavacancy with three, two, or one interstitial atoms, respectively, such that all atoms remain fourfold. As a result, their formation energies are lower by 0.

Short-pulse laser ablation of solids: from phase explosion to fragmentation.

The mechanisms of laser ablation in silicon are investigated close to the threshold energy for pulse durations of 500 fs and 50 ps. This is achieved using a unique model coupling carrier and atom dynamics within a unified Monte Carlo and molecular-dynamics scheme. Under femtosecond laser irradiation, isochoric heating and rapid adiabatic expansion of the material provide a natural pathway to phase explosion.

Ablation of solids under femtosecond laser pulses.

We study the basic mechanisms leading to ablation by femtosecond laser pulses using molecular dynamics and a simple two-dimensional Lennard-Jones model. We demonstrate that the ablation process involves three different mechanisms as a function of deposited energy. In particular, it can result from mechanical fragmentation, which does not require the system to cross any metastability or instability line.