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The effect of double laser pulses (DPs) on the ablation process in solids is studied using a hybrid two-temperature model combining a continuum description of the conduction band electrons with a classical molecular dynamics (MD) approach for the ions. The study is concerned with double pulses with delays in the range of 0--50 ps and absorbed laser fluences of 0.5, 1.0, and $1.5\phantom{\rule{0.28em}{0ex}}{\mathrm{J}/\mathrm{m}}^{2}$ [i.e., 1--3 times the ablation threshold for single-pulse ablation (SP)], taking Al as a generic example of simple metals. A detailed analysis, including the assessment of thermodynamic pathways and cavitation rates, leads to a comprehensive picture of the mechanisms active during the different stages of the ablation process initiated by DPs. This study provides an explanation for several phenomena observed in DP ablation experiments. In particular, with respect to SP ablation, crater depths are reduced, which can be explained by the compensation of the rarefaction wave from the first laser pulse with the compression wave from the second pulse, or, at higher fluences and larger delays, by the fact that the target surface is shielded with matter ablated by the first laser pulse. Also, we discuss how smoother surface structures obtained using DPs may be related to features found in the simulations---viz., reduced mechanical strain and peak lattice temperatures. Finally, vaporization appears to be enhanced in DP ablation, which may improve the resolution of emission spectra.

The ion-ion dynamical structure factor and the equation of state of warm dense aluminum in a two-temperature quasi-equilibrium 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 quasi-equilibrium situations. Analyzing the van Hove function, we find that ion-ion correlations vanish in a time significantly smaller than the electron-ion relaxation time so that dynamical properties have a physical meaning for the quasi-equilibrium state. A significant increase in the speed of sound is predicted from the modification of the dispersion relation of the ion acoustic mode as the electron temperature is increased. The two-temperature equation of state including the free energy, internal energy and pressure is also presented.

We develop ion-ion pair potentials for Al, Na and K for densities and temperatures relevant to the warm-densematter (WDM) regime. Furthermore, we emphasize non-equilibrium states where the ion temperature Ti differs from the electron temperature Te. This work focuses mainly on ultra-fast laser-metal interactions where the energy of the laser is almost exclusively transferred to the electron sub-system over femtosecond time scales. This results in a two-temperature system with Te > Ti and with the ions still at the initial room temperature Ti = Tr. First-principles calculations, such as density functional theory (DFT) or quantum Monte Carlo, are as yet not fully feasible for WDM conditions due to lack of finite-T features, e.g. pseudopotentials, and extensive CPU time requirements. Simpler methods are needed to study these highly complex systems. We propose to use two-temperature pair potentials Uii(r, Ti, Te) constructed from linear-response theory using the non-linear electron density n (r) obtained from finite-T DFT with a single ion immersed in the appropriate electron fluid. We compute equilibrium phonon spectra at Tr which are found to be in very good agreement with experiments. This gives credibility to our non-equilibrium phonon dispersion relations which are important in determining thermophysical properties, stability, energy-relaxation mechanisms and transport coefficients. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We study the conductivities $\sigma$ of (i) the equilibrium isochoric state ($\sigma_{\rm is}$), (ii) the equilibrium isobaric state ($\sigma_{\rm ib}$), and also the (iii) non-equilibrium ultrafast matter (UFM) state ($\sigma_{\rm uf}$) with the ion temperature $T_i$ less than the the electron temperature $T_e$. Aluminum, lithium and carbon are considered, being increasingly complex warm dense matter (WDM) systems, with carbon having transient covalent bonds. First-principles calculations, i.e., neutral-pseudoatom (NPA) calculations and density-functional theory (DFT) with molecular-dynamics (MD) simulations, are compared where possible with experimental data to characterize $\sigma_{\rm ic}, \sigma_{\rm ib}$ and $\sigma_{\rm uf}$. The NPA $\sigma_{\rm ib}$ are closest to the available experimental data when compared to results from DFT+MD, where simulations of about 64-125 atoms are typically used. The published conductivities for Li are reviewed and the value at a temperature of 4.5 eV is examined using supporting X-ray Thomson scattering calculations. A physical picture of the variations of $\sigma$ with temperature and density applicable to these materials is given. The insensitivity of $\sigma$ to $T_e$ below 10 eV for carbon, compared to Al and Li, is clarified., Comment: 10 figures

Using the two-temperature model for ultrafast matter (UFM), we compare the equation of state, pair-distribution functions $g(r)$, and phonons using the neutral pseudoatom (NPA) model with results from density-functional theory (DFT) codes and molecular-dynamics (MD) simulations for Al, Li and Na. The NPA approach uses state-dependent first-principles pseudopotentials from an `all-electron' DFT calculation with finite-$T$ XCF. It provides pair potentials, structure factors, the `bound' and `free' states, as well as a mean ionization $\bar{Z}$ unambiguously. These are not easily accessible {\it via} DFT+MD calculations which become prohibitive for $T/T_F$ exceeding $\sim 0.6$, where $T_F$ is the Fermi temperature. Hence, both DFT+MD and NPA methods can be compared up to $\sim 8$ eV, while higher $T$ can be addressed ${\it via}$ the NPA. The high-$T_e$ phonon calculations raise the question of UFM lattice stability and surface ablation in thin UFM samples. The ablation forces in a UFM slab are used to define an "ablation time" competing with phonon formation times in thin UFM samples. Excellent agreement for all properties is found between NPA and standard DFT codes, even for Li where a strongly non-local pseudopotential is used in DFT codes. The need to use pseudopotentials appropriate to the ionization state $\bar{Z}$ is emphasized. The effect of finite-$T$ exchange-correlation functional is illustrated via its effect on the pressure and the electron-density distribution at a nucleus., Comment: 13 apges, 9 figures

Ultrafast laser experiments yield increasingly reliable data on warm dense matter, but their interpretation requires theoretical models. We employ an efficient density functional neutral-pseudoatom hypernetted-chain (NPA-HNC) model with accuracy comparable to ab initio simulations and which provides first-principles pseudopotentials and pair-potentials for warm-dense matter. It avoids the use of (i) ad hoc core-repulsion models and (ii) "Yukawa screening", and (iii) need not assume ionelectron thermal equilibrium. Computations of the x-Ray Thomson scattering (XRTS) spectra of aluminum and beryllium are compared with recent experiments and with density-functional-theory molecular-dynamics (DFT-MD) simulations. The NPA-HNC structure factors, compressibilities, phonons and conductivities agree closely with DFT-MD results, while Yukawa screening gives misleading results. The analysis of the XRTS data for two of the experiments, using two-temperature quasi-equilibrium models, is supported by calculations of their temperature relaxation times., Comment: 12 pages, 11 figures

We investigate, at the quantum-mechanical level, the interaction of ultrashort laser pulses with small (less than ∼4 nm) carbon and silicon nanoclusters. We find that the outer-ionization mechanism—by virtue of which electrons are ejected from the outer shells of the clusters—goes through a peak for very short (fs) pulses, whose position depends exclusively on the duration of the pulse while other parameters, such as laser fluence and cluster size, only affect its intensity. Our results thus establish that Coulomb explosion, an important process for the disintegration of materials under intense laser pulses, is almost entirely determined by the duration of the pulse while other “experimental” parameters have relatively modest effects.

Swift heavy ions interact with electrons in materials and this may yield permanent atomic displacements; the energy transfer mechanisms that bring electronic excitations into atomic motion are not fully understood, and are generally discussed in terms of two theories, viz. Coulomb explosion and heat exchange between excited electrons and atoms, which is limited by electron-phonon coupling. We address this problem for a “generic” material using a semi-classical numerical approach where the dynamics of the evolving electron density is calculated by using molecular dynamics simulations applied to pseudo-electrons. The forces exerted on the nuclei are then used to calculated the trajectories of the nuclei. From the temporal evolution of the atomic kinetic energy, we find that the energy transfer between the electrons and the nuclei can be divided in two parts. First, a Coulomb heating starts the motion of the atoms by giving them a radial speed; this process differs from Coulomb explosion because the atoms are not displaced over interatomic distances. Second, a thermal energy transfer, as described in linear transport theory, takes place. Our study thus confirms the domination of thermal energy exchange mechanisms over Coulomb explosion models.

Laser ablation is a technology widely used in many applications. Understanding in detail the mechanisms that lead to ablation remains a formidable challenge because of the complexity of the processes taking place, the variety of species involved, and the range of length and time scales covered. Atomic-level experimental information is difficult to obtain and must be augmented by theory. In this article, we briefly review the progresses that we have accomplished using a simple two-dimensional molecular-dynamics model, insisting on the importance of considering the thermodynamics of the evolution of the systems in order to understand ablation. Through the identification of the thermodynamic pathways followed by the material after irradiation, our model has provided significant insights on the physical mechanisms leading to ablation. It has been demonstrated in particular that these depend strongly on the fluence, and are actually determined by the effective amount of energy received within different regions of the target. Further, internal or external factors, such as inertial confinement, play a key role in determining the route to ablation – and thus the types and sizes of particles ejected – by constraining the thermodynamical evolution of the system. We have established that, for ultrashort pulses in strongly absorbing materials, ablation proceeds by either spallation, phase explosion or fragmentation; the latter, we demonstrate, is the most important mechanism. For longer pulses, ablation may also proceed by trivial fragmentation.

Molecular-dynamics simulations combined with a two-temperature model are used to study laser ablation in ${\text{Cu}}_{x}{\text{Zr}}_{1\ensuremath{-}x}$ $(x=0.33,0.50,0.67)$ metallic glasses as well as crystalline ${\text{CuZr}}_{2}$ in the ${\text{C11}}_{b}$ $({\text{MoSi}}_{2})$ structure. Ablation thresholds are found to be $430\ifmmode\pm\else\textpm\fi{}10,450\ifmmode\pm\else\textpm\fi{}10,510\ifmmode\pm\else\textpm\fi{}10$, and $470\ifmmode\pm\else\textpm\fi{}10$ ${\mathrm{J}/\mathrm{m}}_{2}$ for a-${\text{Cu}}_{2}\text{Zr}$, a-CuZr, ${\text{a-CuZr}}_{2}$, and ${\text{c-CuZr}}_{2}$, respectively. The larger threshold in amorphous ${\text{CuZr}}_{2}$ results from a weaker electron-phonon coupling and thus longer electron-ion equilibration time. We observe that the velocity of the pressure waves in the amorphous samples is not affected by the fluence, in contrast to the crystal; this is due to differences in the behavior of the shear modulus with increasing pressure. The heat-affected zone in the different systems is characterized in terms of the melting depth as well as inelastic deformations. The melting depth is found to be smaller in the crystal than in the amorphous targets because of its higher melting temperature. The inelastic deformations are investigated in terms of the von Mises shear strain invariant ${\ensuremath{\eta}}^{\text{Mises}}$; the homogeneous nucleation of shear transformation zones is observed in the glass as reported in previous theoretical and experimental studies. The coalescence of the shear transformation zones is also found at higher fluence.

Using molecular-dynamics, we study the thermodynamic evolution of a simple two-dimensional Lennard–Jones system during laser ablation for pulse durations ranging from 200 fs to 400 ps. We briefly review results previously obtained for fs pulses where the evolution of the material was shown to be solely a function of the locally absorbed energy (provided that only thermal effects are important), i.e., is adiabatic. For longer pulses (100 and 400 ps) the situation becomes more complex, as the relaxation path also depends on the position in the target and on the timescale on which expansion occurs. We show that, in contrast to fs pulses, the material ejected following ps laser irradiation does not enter the liquid–vapor metastable region before ablation occurs, hence showing that phase explosion is not the dominant mechanism in this regime. Following on from previous work, we propose that trivial fragmentation is the main ablation mechanism.

The diffusion of nanoclusters on various surfaces is investigated. It is shown that the misfit between the cluster and substrate is the main parameter determining the magnitude of the cluster diffusion on perfect surfaces. The motion of the cluster on non-ideal surfaces is also explored, and shows surprising results.

We use molecular-dynamics simulations to investigate the diffusion of nanoclusters on non-ideal surfaces, containing bond-length disorder, vacancy islands or monoatomic steps. We find that bond-length disorder does not affect diffusion significantly. When vacancy islands are present, it is found that the diffusion path avoids the neighborhood of these defects, which can be explained in terms of an effective repulsion between the defect and the cluster. Indeed, when a step is present, we find a barrier which opposes diffusion, analogous to the well-known Ehrlich–Schwoebel barrier for adatom diffusion.

Structures involving solid particles of nanometric dimensions play an increasingly important role in material sciences. These structures are often characterized through the vibrational properties of their constituent particles, which can be probed by spectroscopic methods. Interpretation of such experimental data requires an extension of continuum elasticity theory down to increasingly small scales. Using numerical simulation and exact diagonalization for simple models, we show that continuum elasticity, applied to disordered system, actually breaks down below a length scale of typically 30 to 50 molecular sizes. This length scale is likely related to the one which is generally invoked to explain the peculiar vibrational properties of glassy systems., 4 pages, 5 figures, LATEX, Europhysics Letters accepted

We address the role of laser pulse fluence on expansion dynamics and size distribution of the nanoparticles produced by irradiating a metallic target with an ultrashort laser pulse in a vacuum, an issue for which contrasting indications are present in the literature. To this end, we have carried out a combined theoretical and experimental analysis of laser ablation of a bulk copper target with ≈50 fs, 800 nm pulses, in an interval of laser fluencies going from few to several times the ablation threshold. On one side, molecular dynamics simulations, with two-temperature model, describe the decomposition of the material through the analysis of the evolution of thermodynamic trajectories in the material phase diagram, and allow estimating the size distribution of the generated nano-aggregates. On the other side, atomic force microscopy of less than one layer nanoparticles deposits on witness plates, and fast imaging of the nanoparticles broadband optical emission provide the corresponding experimental characterization. Both experimental and numerical findings agree on a size distribution characterized by a significant fraction (≈90%) of small nanoparticles, and a residual part (≈10%) spanning over a rather large size interval, evidencing a weak dependence of the nanoparticles sizes on the laser pulse fluence. Numerical and experimental findings show a good degree of consistency, thus suggesting that modeling can realistically support the search for experimental methods leading to an improved control over the generation of nanoparticles by ultrashort laser ablation.

The optical properties of ultrathin InAs impurity layers embedded in bulk InP are investigated. Our calculations are based on a tight-binding description of the electronic structure, with spin-orbit interactions and strain effects included in a consistent manner. It is shown that the energy gap increases with decreasing number of InAs monolayers. In the limit of a single InAs monolayer, the energy gap is found to be 120 meV less than that of bulk InP. Our results are in good agreement with experimental data as far as the heavy-hole–electron transition is concerned. The energy difference between optical transitions is, however, in disagreement with both experiment and effective-mass calculations.

We report here the continuation of our earlier work on the molecular-dynamics simulations of laser ablation of silicon with picosecond laser pulses. A more realistic phenomenon of ablation is observed as, along with a significant expansion of the time scale, carrier diffusion is explicitly taken into account. The motion of approximately 32,000 atoms, contained in a 5 nm ×5 nm ×27 nm surface rectangular box irradiated by a single 308 nm, 10 ps, Gaussian laser pulse, is followed for typically 100 ps of simulation time. Because melting and possibly ablation or desorption of the target following absorption of the laser pulse are described within the thermal annealing model, care is taken not to exceed carrier densities of ∼1022 cm−3. More precisely, the interaction of photons with the target is thought to cause the generation of a dense gas of hot electrons and holes which primarily relaxes through carrier-phonon scattering. Above a characteristic threshold energy of ∼0.30 J/cm2, ejection from the target of big chunks of molten material occurs and the latter are expelled with axial velocities of ∼1000 m/s.

The rate of diffusion of a Cu adatom on the Cu(100) surface is calculated using thermodynamic integration within the transition state theory. The results are found to be in excellent agreement with the essentially exact values from molecular-dynamics simulations. The activation energy and related entropy are shown to be effectively independent of temperature, thus establishing the validity of the Arrhenius law over a wide range of temperatures. Our study demonstrates the equivalence of diffusion rates calculated using thermodynamic integration within the transition state theory and direct molecular-dynamics simulations., 4 pages (revtex), two figures (postscript)

Tight-binding (TB) molecular dynamics (MD) has emerged as a powerful method for investigating the atomic-scale structure of materials – in particular the interplay between structural and electronic properties – bridging the gap between empirical methods which, while fast and efficient, lack transferability, and ab initio approaches which, because of excessive computational workload, suffer from limitations in size and run times. In this short review article, we examine several recent applications of TBMD in the area of defects in covalently bonded semiconductors and the amorphous phases of these materials.

Experimental data on the decay rate of two-dimensional islands on Cu(100) as a function of temperature are reported. The decay is limited by the attachment-detachment process. A comparison of the experimentally observed activation energy for the decay rate with results from first-principles theory renders further support to the understanding that on Cu(100) island decay is due to mass transport via vacancies.

The results of a density-functional-theory study of the formation energies of (100)- and (111)-faceted steps on the Pt(111) surface, as well as of the barrier for diffusion of an adatom on the flat surface, are presented. The step formation energies are found to be in a ratio of 0.88 in favour of the (111)-faceted step, in excellent agreement with experiment; the equilibrium shape of islands should therefore clearly be non-hexagonal. The origin of the difference between the two steps is discussed in terms of the release of stress at the surface through relaxation. For the diffusion barrier, we also find relaxation to be important, leading to a 20% decrease of its energy. The value we obtain, 0.33 eV, however remains higher than available experimental data; possible reasons for this discrepancy are discussed. We find the ratio of step formation energies and the diffusion barrier to be the same whether using the local-density approximation or the generalized-gradient approximation for the exchange-and-correlation energy., Comment: Submitted to Physical Review B; 11 postscript pages including 4 figures; this and related publications available from web sites at http://www.centrcn.umontreal.ca/~lewis and http://www.fhi-berlin.mpg.de/th/th.html

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. As the system relaxes, it reaches deeper energy states with logarithmically growing barriers that need to be unlocked to replenish the heat-releasing events leading to lower-energy configurations.

The binodal and spinodal lines of aluminum have been calculated using Monte Carlo simulations in the Gibbs and NPT ensembles. The interactions among particles are described in terms of two embedded-atom-method potentials, viz. Zope and Mishin and Ercolessi and Adams. We provide estimates for the critical properties of the material and compare them with both experimental values and computational predictions from other models.

We propose a microscopic model of the amorphization of silicon such as that resulting from ion implantation. We demonstrate that amorphization can be induced by the presence of defects provided they form clusters embedded in a defective crystalline matrix. Our results are in striking agreement with transmission-electron microscopy measurements and confirm the superlinear dependence of damage on deposited energy, supporting the view that the crystal-to-amorphous transition proceeds via nucleation and growth.

Using molecular-dynamics simulations and the embedded-atom method, we study the homodiffusion of single adatoms on flat Ag and Au (100) and (111) surfaces. Our results for the (111) surfaces indicate that when the thermal energies of the atoms become larger than the energy barriers, diffusion can no longer be represented by a simple random walk since correlations between successive jumps become important. We present a simple model that takes into account these correlated jumps and reproduces the molecular dynamics data very well. We also demonstrate that knowledge of the energy barriers is not sufficient to determine the preferred mechanism for diffusion on the (100) surface, since the prefactors for the various mechanisms can vary significantly from the value that is usually assumed. The ability of a simple transition-state theory to describe diffusion is also tested. We find, in the cases considered here, that the static barrier is equivalent to the dynamical activation energy and that the prefactor is also well described as long as the relaxation of the substrate remains small. \textcopyright{} 1996 The American Physical Society.

We report a first-principles study of the structure and energetics of the simple and split divacancies in silicon. The formation energies are estimated to be 4.63 and 5.90 eV, respectively. In both cases, relaxation proceeds inwards, and clearly is important, even though the relaxation energies amount to less than about 10% of the unrelaxed formation energies, enough to change the symmetry of the local structure. The binding energy of the divacancy is close to 2 eV. For the simple divacancy, we find the relaxed structure to be of the resonant-bond Jahn-Teller type. We also find, for both the divacancy and the split divacancy, the highest occupied states to lie close to the valence band maximum. \textcopyright{} 1996 The American Physical Society.

Results of molecular dynamics simulations (MDS) of o‐terphenyl, a glass‐forming liquid, are analyzed in terms of the coupling model of relaxation. At low temperatures thermally activated relaxation processes are suppressed, whereby the density–density correlation function, C(t), obtained by MDS is determined entirely by vibrational modes. This enables the low temperature data to be used to deduce the vibrational density of states, g(ω). With g(ω) determined, the vibrational contribution, Cpho(t), is calculated at higher temperatures assuming that g(ω) is independent of temperature. At higher temperatures, relaxation makes its appearance and is modeled here by the fast dynamics of the coupling model. Assuming that vibration and relaxation contribute independently, the density–density self‐correlation function is given by the product Cpho(t)Crel(t), with the relaxation part obtained from the coupling model. There is good overall agreement between the calculated C(t) and the MDS data. Microscopic parameters, including the energy barrier for reorientation of the o‐terphenyl molecule, are extracted from the MDS results.

We use ab initio total-energy minimization methods to study the equilibrium structure of the InP(100) surface, and in particular examine the structure of the surface as a function of In coverage (Θ). We find the low-coverage ( Θ = 0.25), non-dimerized (2 × 4) surface to yield the lowest energy per In atom, and therefore to possibly constitute the ground-state structure of the surface, even though it has not been observed experimentally. Nevertheless, we have found the In-dimerized surfaces at all coverages to be stable. For the fully In-covered surface, we observe the equilibrium structure to be the symmetric (2 × 1) phase, i.e., with the In dimers adopting a symmetric configuration with respect to the surface. For a coverage of Θ = 0.75, we find a symmetric-dimer (2 × 4) reconstruction to provide a stable equilibrium configuration. The dimer bond length, we observe, is relatively insensitive to coverage, and in view of the fact that LDA calculations tend to underestimate distances a little, we predict that the experimental bond-length value should be very close to 2.8 A. For low In coverage surface, our calculations indicate that phosphorus dimers may form.

Silicon/SiO 2 superlattices (SLs) are nanostructured thin films bearing multiple Si/SiO 2 interfaces. In such materials, silicon is walled by its oxide in order to generate multiple quantum wells. Here, the structural and electronic properties of a structurally-relaxed Si/SiO 2 SL model are studied using a first principles approach; the Si/SiO 2 interfaces contain all suboxide Si atoms (Si 1, Si 2, and Si 3) . The valence and conduction band offsets (VBO and CBO) are evaluated from the relative shift between densities of states (DOSs) of Si atoms in bulk SiO 2 and in the SL. The CBO is shown to be reduced compared to the VBO. The DOSs of the three suboxide Si atoms are also calculated. It is shown that there are contributions from all suboxides at the threshold of the gap.

We have studied the coalescence of three-dimensional (3D), quasi-two-dimensional (quasi-2D), and 2D liquid, equal-size Cu and Si nanodroplets in the viscous and inertial regimes using classical molecular-dynamics simulations. At the onset of coalescence, a bridge (of radius $r$) between the droplets forms and develops until the merge is complete. For the 3D and quasi-2D systems, our results show a transition from a viscous-dominated regime at very short time, where $r\ensuremath{\propto}{\ensuremath{\tau}}^{1}$, to a regime dominated by inertial forces at longer time, with $r\ensuremath{\propto}{\ensuremath{\tau}}^{0.5}$, in agreement with theoretical models; the viscous regime is not observed in two dimensions, where only inertial forces seem to be operating. A detailed analysis of the 3D data suggests that the viscous-to-inertial crossover length ${l}_{c}({R}_{0},T)$ (with ${R}_{0}$ being the initial radius of the droplets and $T$ being the temperature) behaves differently in the two systems. While ${l}_{c}\ensuremath{\propto}{R}_{0}^{1/2}$ and depends only weakly on temperature in l-Cu, as theory predicts, ${l}_{c}\ensuremath{\propto}{R}_{0}^{0.96}{T}^{0.41}$ in l-Si. We conclude from these observations and corresponding experimental data that the prefactor for the dependence of $r$ on time in the inertial regime is not ``universal'' and actually depends on system properties, including initial radius, viscosity, and surface tension.

We present the results of a detailed molecular-dynamics study of relaxation in the van der Waals system ortho-terphenyl in the supercooled regime. The molecule is described by a simple rigid three-site model, with interactions between different molecules of the Lennard-Jones form. We find that the long-time (\ensuremath{\alpha}) relaxation, as determined from the intermediate scattering function, is well described by a Kohlrausch law. The zero-time amplitude of this process, often referred to as the ``nonergodicity parameter,'' which can be interpreted as a Debye-Waller factor, is seen to depart from a linear temperature dependence and seems to exhibit a ``singularity'' at a temperature substantially larger than the conventional glass transition temperature, in accord with neutron-scattering data, and as predicted by mode-coupling theory. We find that the observed anharmonic behavior around ${\mathit{T}}_{\mathit{c}}$, as well as the decay of the scattering functions, is equally evident from both orientational and translational correlations. The latter observation indicates that the structural changes that take place during \ensuremath{\alpha} relaxation are neither specifically orientational nor specifically translational. Investigation of the van Hove self-correlation for both center-of-mass and orientational motion reveals the existence of a strong non-Gaussian spatial dependence at intermediate times and of processes, even at low temperatures, that cannot be described as simple vibrational motion, and which consist of rapid reorientations of the molecules about their essentially frozen centers of mass. This motion may be related to the fast secondary mode seen in experiments.

Recent observations on various low-index metallic surfaces of an extraordinary thermal attenuation of diffracted peak intensities suggest that anharmonic effects are important at high temperatures, though such observations have also been interpreted in terms of disordering processes (premelting and roughening). In order to address this problem, we have carried out an extensive molecular-dynamics investigation of the evolution in temperature of the (100) and (110) surfaces of Ni, with the interactions between atoms described using embedded-atom method potentials. We observe both surfaces to suffer an anomalously large thermal expansion, with a concomitant rapid increase of the mean-square amplitudes of vibration; the latter are found in both cases to be larger in the plane of the surface than out of the plane. An analysis of our simulated low-energy electron-diffraction intensities supports the experimental interpretation of the measured ones in terms of enhanced mean-square displacements. Indeed, anharmonicities are found to appear at temperatures well below the onset of disordering, which occurs at 1200 and 1000 K for the (100) and (110) surfaces, respectively (via in-plane diffusion and adatom-vacancy formation). The onset of disordering proceeds differently on the two surfaces: while it involves essentially only the outermost layer on the (100) surface, both the first and second layer participate in the process on the (110) surface. We also calculate the phonon spectra at high-symmetry points of the surface Brillouin zone, which we find to be in good agreement with experimental values at room temperature. We predict, further, that the temperature-dependent frequency shifts are not significantly softened in comparison with bulk phonons, which suggests that surface phonons are affected by anharmonicity in a nontrivial way. Several of the observed properties can be understood in terms of the larger displacements that surface atoms can afford, because of their reduced coordination compared to bulk atoms.

Molecular dynamics simulations of a simple model for ortho-terphenyl are used to investigate the microscopic nature of rotational dynamics in the supercooled regime. A detailed analysis of the van Hove correlation function, as well as direct examination of the orientational motion of selected molecules, show that, at sufficiently low temperatures and on the nanosecond timescale, orientational motion takes place in the near absence of translational motion. The nature of the orientational motion covers a large spectrum of situations, but we find a preponderance of rapid reorientations, i.e., jumps in two- (or multi-) level systems. It is noted, in addition, that the observed relaxation does not manifest itself as a peak in such quantities as the intermediate scattering function and the mean-square displacements. The feature sometimes observed (as in the simulations carried out by the present authors) in these functions is merely an artifact of the model coming from the use of a finite-size molecular dynamics supercell.

We use ab initio total-energy minimization techniques to resolve the lattice strain βh arising from the presence of holes in the valence band of SiGa alloys. We find, in agreement with recent EXAFS results, the lattice to contract upon increasing the proportion of holes (βh < 0). We also find that the valence-band deformation potential av is positive, leading to a violation of Keyes' relation, which asserts that βh and av are equal within a constant positive factor. Our calculations suggest therefore that Keyes' formula does not describe adequately the relation between the two quantities, and also resolve the apparent contradiction between experiment and previous calculations.

We employ ab initio total-energy-minimization techniques to study the relaxation of Ge dimers on Si(100). We find that the dimers adopt an asymmetric configuration, in agreement with recent experimental results [E. Fontes et al., Phys. Rev. Lett. 70, 2790 (1993)]. The average dimer height displacement and tilting angle are found to be 0.79 \AA{} and +19\ifmmode^\circ\else\textdegree\fi{}, respectively. Our calculations predict that the (2\ifmmode\times\else\texttimes\fi{}2) and c(4\ifmmode\times\else\texttimes\fi{}4) reconstructed structures have the same energy, and suggest that either one of these two could be the ground-state structure.

Molecular-dynamics simulations of the ablation of nanocrystalline Al films by ultrashort laser pulses in the low-fluence (no-ionization) regime (0-2.5 times the ablation threshold, F{sub th}) are reported. The simulations employ an embedded-atom method potential for the dynamics of the ions and a realistic two-temperature model for the electron gas (and its interactions with the ion gas), which confers different electronic properties to the monocrystalline solid, nanocrystalline solid, and liquid regions of the targets. The ablation dynamics in three nanocrystalline structures is studied: two dense targets with different crystallite sizes (d=3.1 and 6.2 nm on average) and a d=6.2 nm porous sample. The results are compared to the ablation of monocrystalline Al. Significant differences are observed, the nanocrystalline targets showing, in particular, a lower ablation threshold and a larger melting depth, and yielding pressure waves of higher amplitude than the monocrystalline targets. Furthermore, it is shown that nanocrystalline targets experience no residual stress associated with thermal expansion and lateral constraints, and that little crystal growth occurs in the solid during and after ablation. Laser-induced spallation of the back surface of the films is also investigated; we find, in particular, that the high-strain fracture resistance of nanocrystalline samples is significantly reduced inmore » comparison to the crystalline material.« less

Using molecular-dynamics simulations, we have studied the creation and evolution of damage in crystalline and amorphous silicon following the implantation of energetic keV ions. A method is proposed to identify anomalous atoms based on a weighted combination of local, atomic-scale properties, which applies to both Si phases. For crystalline Si, the passage of the ions causes compact amorphous regions to form, while no evidence for melting is observed. The relaxation of the amorphouslike regions proceeds initially by the rapid recrystallization of smaller clusters and isolated atoms, followed by a long period of steplike changes in the number of defects due to spontaneous annealing of damage pockets at the crystalline-amorphous interface. In amorphous Si, the initial stage of damage annealing (which lasts a few picoseconds) resembles closely that observed in crystalline Si; on larger time scales, however, the damage is found to ``percolate,'' or flow, through the system, inducing damage away from the collision cascade, thus causing an overall ``derelaxation'' of the material.

International audience; We investigate the problem of heat conduction across a molecular junction 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.

We have investigated the response of amorphous silicon (a-Si), in particular crystallization, to external mechanical shear deformations using classical molecular dynamics (MD) simulations and the empirical Environment Dependent Inter-atomic Potential (EDIP) [Phys. Rev. B 56, 8542 (1997)]. In agreement with previous results we find that, at low shear velocity and low temperature, shear deformations increase disorder and defect density. At high temperatures, however, the deformations are found to induce crystallization, demonstrating a dynamical transition associated with both shear rate and temperature. The properties of a-Si under shear deformations and the extent at which the system crystallizes are analyzed in terms of the potential energy difference (PED) between the sheared and non-sheared material, as well as the fraction of defects and the number of particles that possess a crystalline environment., Comment: 8 pages, 12 figures

The results of a detailed molecular dynamics investigation of relaxation in the van der Waals system o-terphenyl are presented. Our calculations give clear evidence for the existence of a fast “relaxation process” on the picosecond time scale. This confirms the neutron-scattering observation of an anomalous decrease of the Debye-Waller factor. The cages formed by their neighbours.

A novel mechanism for the growth on crystalline surfaces under low supersaturation, namely, induced by the presence of point defects, is proposed and studied. More precisely, we use ab initio minimization techniques to calculate the potential-energy surface for a series of adatoms sequentially deposited on the Si(100)2×1 surface in the presence, beneath the top layer, of a vacancy. We find that, indeed, this defect can act as a growth site for the crystal: the energy at the defect is about 1 eV lower than the lowest point of the perfect surface. Adatoms aggregate near the defect and form the nucleus of subsequent growth