An empirical potential for silicon under conditions of strong electronic excitation

We present an empirical potential developed for silicon under conditions of strong electronic excitation. We show the essentially athermal nature of the melting transition when the electronic temperature is extremely high. The resulting liquid is shown to be distinct from ordinary liquid silicon. For less intense excitations, we determine the thermal melting temperature and demonstrate the possible existence of a regime where ordinary thermodynamic melting can occur but at a reduced temperature T(m). We show laser-induced softening of the lattice can lead to lattice cooling for very short time scales (similar to 100 fs), an effect never before recognized

Thermodynamics and kinetics of silicon under conditions of strong electronic excitation

We present a detailed analysis of a recently-developed empirical potential to describe silicon under conditions of strong electronic excitation. The parameters of the potential are given as smooth functions of the electronic temperature T(e), with the dependence determined by fitting to finite-temperature density-functional theory calculations. We analyze the thermodynamics of this potential as a function of the electronic temperature T(e) and lattice temperature T(ion). The potential predicts phonon spectra in good agreement with finite-temperature density-functional theory, including the previously predicted lattice instability. We predict that the melting temperature T(m) decreases strongly as a function of T(e). Electronic excitation has a strong effect on the rate of crystallization from the melt. In particular, high T(e) results in very slow kinetics for growing crystal from the melt, due mainly to the fact that diamond becomes much less stable as T(e) increases. Finally, we explore annealing amorphous Si (a-Si) below T(m), and find that we cannot observe annealing of a-Si directly at high T(e). We hypothesize that this is also due to the decreased stability of the diamond structure at high T(e)

Computational methodology for analysis of the Soret effect in crystals: Application to hydrogen in palladium

Different computational methodologies to compute thermodiffusion of hydrogen in palladium are explored. It is found that diffusion occurs rapidly enough to directly observe thermodiffusion in the presence of an applied temperature gradient. This provides an unequivocal result that hydrogen moves from low to high temperatures, corresponding to a negative value for the reduced heat of transport Q*\u27 approximate to -0.3 eV, which can be used to validate other methods. Further simulations using the Green-Kubo formulae and a recently developed constrained-dynamics approach are found to be in agreement with direct simulation results. In particular, in each of the three methods used, the value of Q*\u27 is found to be in the range between -0.3 and -0.2 eV. We show how to correctly define and compute the partial-enthalpy term for hydrogen which is key to obtaining accurate results. The results provide important foundational and numerical validation for the constrained-dynamics approach. The advantage of the constrained-dynamics method is that it can be applied to thermodiffusion in materials where diffusion does not occur on a molecular-dynamics time scale. Finally, we show that the empirical potential predicts behavior that is not in agreement with experiment. In particular, experiments are reported to show hydrogen diffusing from high to low temperatures corresponding to a positive value for Q*\u27

Dissipation and adhesion hysteresis between (010) forsterite surfaces using molecular-dynamics simulation and the Jarzynski equality

Dissipation and adhesion are important in many areas of materials science, including friction and lubrication, cold spray deposition, and micro-electromechanical systems (MEMS). Another interesting problem is the adhesion of mineral grains during the early stages of planetesimal formation in the early solar system. Molecular-dynamics (MD) simulation has often been used to elucidate dissipative properties, most often in the simulation of sliding friction. In this paper, we demonstrate how the reversible and irreversible work associated with interactions between planar surfaces can be calculated using the dynamical contact simulation approach based on MD and empirical potentials. Moreover, it is demonstrated how the approach can obtain the free-energy $\Delta A(z)$ as a function of separation between two slabs using the Jarzynksi equality applied to an ensemble of trajectories which deviate significantly from equilibrium. Furthermore, the dissipative work can also be obtained using this method without the need to compute an entire cycle from approach to retraction. It is expected that this technique might be used to efficiently compute dissipative properties which might enable the use of more accurate approaches including density-functional theory. In this paper, we present results obtained for forsterite surfaces both with and without MgO-vacancy surface defects. It is shown that strong dissipation is possible when MgO-vacancy defects are present. The mechanism for strong dissipation is connected to the tendency of less strongly-bound surface units to undergo large displacements including mass transfer between the two surfaces. Systems with strong dissipation tend to exhibit a long-tailed distribution rather than the Gaussian distribution often anticipated in near-equilibrium applications of the JE.Comment: 26 pages, 15 figure

Connection between partial pressure, volatility, and the Soret effect elucidated using simulations of non-ideal liquid mixtures

Building on recent simulation work, it is demonstrated using molecular-dynamics (MD) simulations of two-component liquid mixtures that the chemical contribution to the Soret effect in two-component non-ideal fluid mixtures arises due to differences in how the partial pressures of the components respond to temperature and density gradients. Further insight is obtained by reviewing the connection between activity and deviations from Raoult's law in the measurement of the vapor pressure of a liquid mixture. A new parameter gamma, defined in a manner similar to the activity coefficient, is used to characterize differences deviations from ``ideal'' behavior. It is then shown that the difference gamma2-gamma1 is predictive of the sign of the Soret coefficient and is correlated to its magnitude. We hence connect the Soret effect to the relative volatility of the components of a liquid mixture, with the more volatile component enriched in the low-density, high-temperature region, and the less volatile component enriched in the high-density, low-temperature region. Because gamma is closely connected to the activity coefficient, this suggests the possibility that measurement of partial vapor pressures might be used to indirectly determine the Soret coefficient. It is proposed that the insight obtained here is quite general and should be applicable to a wide range of materials systems. An attempt is made to understand how these results might apply to other materials systems including interstitials in solids and multicomponent solids with interdiffusion occurring via a vacancy mechanism.Comment: 21 pages, 7 Figures, to be submitted to J. Chem Physic

Density functional theory study of water adsorption at reduced and stoichiometric ceria (111) surfaces

We study the structure and energetics of water molecules adsorbed at ceria (111) surfaces for 0.5 and 1.0 ML coverages using density functional theory. The results of this study provide a theoretical framework for interpreting recent experimental results on the redox properties of water at ceria (111) surfaces. In particular, we have computed the structure and energetics of various absorption geometries at the stoichiometric ceria (111) surface. We find that single hydrogen bonds between the water and the oxide surface are favored in all cases. At stoichiometric surfaces, the water adsorption energy depends rather weakly on coverage. We predict that the observed coverage dependence of the water adsorption energy at stoichiometric surfaces is likely the result of dipole-dipole interactions between adsorbed water molecules. When oxygen vacancies are introduced in various surface layers, water molecules are attracted more strongly to the surface. We find that it is very slightly energetically favorable for adsorbed water to oxidized the reduced (111) surface with the evolution of H-2. In the event that water does not oxidize the surface, we predict that the effective attractive water-vacancy interaction will result in a significant enhancement of the vacancy concentration at the surface in agreement with experimental observations. Finally, we present our results in the context of recent experimental and theoretical studies of vacancy clustering at the (111) ceria surface

Analysis of simulation methodology for calculation of the heat of transport for vacancy thermodiffusion

Computation of the heat of transport Q*(a) in monatomic crystalline solids is investigated using the methodology first developed by Gillan [J. Phys. C: Solid State Phys. 11, 4469 (1978)] and further developed by Grout and coworkers [Philos. Mag. Lett. 74, 217 (1996)], referred to as the Grout-Gillan method. In the case of pair potentials, the hopping of a vacancy results in a heat wave that persists for up to 10 ps, consistent with previous studies. This leads to generally positive values for Q*(a) which can be quite large and are strongly dependent on the specific details of the pair potential. By contrast, when the interactions are described using the embedded atom model, there is no evidence of a heat wave, and Q*(a) is found to be negative. This demonstrates that the dynamics of vacancy hopping depends strongly on the details of the empirical potential. However, the results obtained here are in strong disagreement with experiment. Arguments are presented which demonstrate that there is a fundamental error made in the Grout-Gillan method due to the fact that the ensemble of states only includes successful atom hops and hence does not represent an equilibrium ensemble. This places the interpretation of the quantity computed in the Grout-Gillan method as the heat of transport in doubt. It is demonstrated that trajectories which do not yield hopping events are nevertheless relevant to computation of the heat of transport Q*(a)

Interfacial phonon scattering in semiconductor nanowires by molecular-dynamics simulation

We use molecular-dynamics simulations of vibrational wave packets to study the scattering of longitudinal-acoustic modes from interfaces in semiconductor nanowires of varying diameters. The energy transmission coefficient at the interface is found to depend strongly on both the nanowire diameter and the frequency of the incident wave. By analyzing the scattering events, we determine the selection rules for nanowire scattering that can be understood in terms of the representations of the point-group symmetry of the nanowire. Using such symmetry arguments, we predict that the presence of gaps in the phonon spectrum of thin high-symmetry nanowires will result in a complete reflection of phonons at the interfaces. We discuss the implications of our results for interfacial scattering in real systems, including Si/Ge superlattice nanowires

Atomic-scale simulation of the thermodiffusion of hydrogen in palladium

We report molecular-dynamics simulations of Pd:H to elucidate transport properties, with special focus placed on determining the temperature dependence of the heat of transport Q*. Simulation results are analyzed using the Green-Kubo approach. It is found that Q* describing the thermodiffusion of hydrogen increases linearly with temperature. By contrast, the reduced heat of transport Q*\u27 = Q* - h(2), with h(2) the partial enthalpy of hydrogen, is approximately temperature independent. By computing separately the potential, kinetic, and virial contributions to Q*, it is possible to understand key features of the thermodiffusion process. In particular, the sum of the kinetic and potential energy of hydrogen atoms is increased above that of an average hydrogen atom by an amount comparable to the migration energy during a successful hop. However, the virial term in the energy flux is less than what would be expected based on the average local stress contribution due to the hydrogen atoms. Detailed calculations show that the relevant component of the stress tensor due to a hopping hydrogen atom exhibits a minimum at the transition state. Hence, while Q* has significant positive contributions due to the excited nature of the hopping hydrogen atom, the reduced heat of transport Q*\u27 can still be negative. The results here present important insight into the failure of simple kinetic theories of thermodiffusion, and provide a new perspective that can be tested on other systems