Marc Hayoun

Computer simulation study of surface segregation on Cu3Au

Segregation profiles at the Cu3Au(100) surface have been calculated in the ordered and disordered phases. The calculations are based on two many-body potentials and principally consist of (pseudo) grand-canonical rigid-lattice Monte Carlo (RLMC) simulations. The atomic relaxations are also accounted for in order to determine how vibrational contributions can modify the results of RLMC simulations. The simulations show that (i) below the critical order—disorder temperature Tc, the top layer is strongly enriched in gold (∼ 50% or 75% Au, depending on the potential used) whereas the second layer is almost entirely depleted (∼ 100%), and (ii) above Tc the equilibrium layer composition in Au converges to the bulk value (25%) via a damped oscillatory profile. The depth range of these oscillations decreases with increasing the temperature, in good agreement with recent X-ray scattering experiments. No major changes occur when atomic movements are included in the simulations. These results are compared with those obtained for the (111) surface orientation, and with recently proposed theoretical arguments as well as previous calculations.

Isotope effects in lithium hydride and lithium deuteride crystals by molecular dynamics simulations

Molecular dynamics (MD) simulations have been carried out to study isotope effects in lithium hydride and lithium deuteride crystals. Quantum effects on nuclear motion have been included through a quantum thermal bath (QTB). The interatomic forces were described either within the density functional theory (DFT) in the generalized gradient approximation (GGA) or by the phenomenological approach using the shell model. For both models, the isotopic shift in the lattice parameter can be successfully predicted by QTB-MD simulations. The slope of the experimental isotopic shift in pressure is satisfactorily reproduced by QTB-MD within DFT-GGA, in contrast to both density functional perturbation theory and QTB-MD with the shell model. We have analyzed the reasons for these discrepancies through the vibrational densities of states and the isotopic shifts in bulk modulus. The results illustrate the importance of anharmonic contributions to vibrations and to the isotopic pressure shift between LiH and LiD.

Atomic structure of low-index and (11n) surfaces in ordered Cu3Au

Abstract By computing the surface energy at T = 0 K, we test the relative stability of various among possible atomic configurations of low-index and (11 n ) surfaces in the L1 2 compound Cu 3 Au. The computations rely on two n -body phenomenological potentials and are performed using a classical energy minimization scheme. The energy of (100) and (110) surfaces is at a minimum for a mixed composition terminal layer, in agreement with available experimental results and previous calculations. For (113) and (115) surfaces the minimum energy is obtained when all (100) terraces are mixed. In this case, the height and width of steps and terraces are twice as large as those of a standard surface configuration.

Size-dependent infrared properties of MgO nanoparticles with evidence of screening effect

We have investigated the infrared (IR) absorption properties of MgO nanoparticles (NPs) with the means of molecular dynamics simulations. Several size effects have been observed. We show in particular that the absorption of IR radiation does not occur predominantly through the polariton mode but preferentially through surface modes. This enhanced surface absorption is found to result from the absence of dielectric screening of the first atomic layer of the NPs. We demonstrate concomitantly that a macroscopic description of electrodynamics is inadequate to capture these unusual IR properties.

Competing mechanisms in the atomic diffusion of a MgO admolecule on the MgO(001) surface

The diffusion mechanism of a MgO admolecule on a flat MgO(001) surface has been investigated by equilibrium molecular dynamics simulation. Care has been taken in the choice of the phenomenological interionic potential used. Four distinct mechanisms have been found and the corresponding dynamical barriers determined at high temperature. Some static barriers have also been computed for comparison and all intermediate configurations have been obtained with the same phenomenological potential and also by the DFT-GGA approach. The hopping mechanisms involving the Mg adatom, although dominant, must be combined with the infrequent mechanisms involving displacements of O adatoms in order to provide the mass transport on the surface, which is crucial for crystal growth both in the nucleation and step-flow regimes.

Molecular dynamics study of self-diffusion in a liquid-liquid interface

Abstract The self-diffusion coefficients have been calculated in a model of a planar interface between two almost immiscible liquids. The simulations have been performed by the molecular dynamics method, using Lennard-Jones potentials. The diffusion process is isotropic in the bulk regions of the simulated system, whereas it exhibits anisotropic behaviour in the interfacial region: the transverse diffusion coefficient (parallel to the interface) is higher than the normal one. A qualitative explanation of this behaviour is suggested by comparison with the pressure tensor.

Quantum Thermal Bath for Path Integral Molecular Dynamics Simulation.

The quantum thermal bath (QTB) method has been recently developed to account for the quantum nature of the nuclei by using standard molecular dynamics (MD) simulation. QTB-MD is an efficient but approximate method when dealing with strongly anharmonic systems, while path integral molecular dynamics (PIMD) gives exact results but in a huge amount of computation time. The QTB and PIMD methods have been combined in order to improve the PIMD convergence or correct the failures of the QTB-MD technique. Therefore, a new power spectral density of the random force within the QTB has been developed. A modified centroid-virial estimator of the kinetic energy, especially adapted to QTB-PIMD, has also been proposed. The method is applied to selected systems: a one-dimensional double-well system, a ferroelectric phase transition, and the position distribution of an hydrogen atom in a fuel cell material. The advantage of the QTB-PIMD method is its ability to give exact results with a more reasonable computation time for strongly anharmonic systems.

Low-temperature anharmonicity of barium titanate: A path-integral molecular-dynamics study

We investigate the influence of quantum effects on the dielectric and piezoelectric properties of barium titanate in its (low-temperature) rhombohedral phase, and show the strongly anharmonic character of this system even at low temperature. For this purpose, we perform path-integral molecular-dynamics simulations under fixed pressure and fixed temperature, using an efficient Langevin thermostat-barostat, and an effective hamiltonian derived from first-principles calculations. The quantum fluctuations are shown to significantly enhance the static dielectric susceptibility (~ by a factor 2) and the piezoelectric constants, reflecting the strong anharmonicity of this ferroelectric system even at very low temperature. The slow temperature-evolution of the dielectric properties observed below ~ 100 K is attributed (i) to zero-point energy contributions and (ii) to harmonic behavior if quantum effects are turned off.

Thermodynamics and kinetics of the Schottky defect at terraces and steps on the MgO(001) surface

The Schottky defects at both the flat MgO(001) surface and the monatomic-step edge have been investigated by equilibrium molecular dynamics simulations. The formation enthalpy as a function of the distance between the Mg and O vacancies that form a Schottky defect have been calculated and discussed. We conclude that a step edge is a stable location for a vacancy. The migration mechanism has been elucidated and an intermediate state has been identified. The associated activation enthalpies have been determined in the 700-1100 K temperature range. Both magnesium and oxygen vacancies at the surface are very mobile and can play a role during the crystal growth.

Surface enhanced infrared absorption in dielectric thin films with ultra-strong confinement effects

By formulating a microscopic description of the non-local dielectric constant, we have investigated the mechanisms of infrared absorption in dielectrics thin films by molecular dynamics simulations. We found that light absorption in dielectric slabs does not occur predominantly at the polaritons resonances but through anomalous surface modes extremely confined in space. This demonstrates that any macroscopic description of electrodynamics in dielectrics breaks down at the nanoscale.