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M. J. Stott

Publications and source records attributed to M. J. Stott.

6 recordsLinked to original sources
arXiv2006

Pressure induced structural and dynamical changes in liquid Si. An ab-initio study

The static and dynamic properties of liquid Si at high-pressure have been studied using the orbital free ab-initio molecular dynamics method. Four thermodynamic states at pressures 4, 8, 14 and 23 GPa are considered. The calculated static structure shows qualitative agreement with the available experimental data. We analize the remarkable structural changes occurring between 8 and 14 GPa along with its effect on several dynamic properties.

cond-mat.mtrl-sci↗
arXiv2005

Structural and dynamical properties of liquid Si. An orbital-free molecular dynamics study

Several static and dynamic properties of liquid silicon near melting have been determined from an orbital free {\em ab-initio} molecular dynamics simulation. The calculated static structure is in good agreement with the available X-ray and neutron diffraction data. The dynamical structure shows collective density excitations with an associated dispersion relation which closely follows recent experimental data. It is found that liquid silicon can not sustain the propagation of shear waves which can be related to the power spectrum of the velocity autocorrelation function. Accurate estimates have also been obtained for several transport coefficients. The overall picture is that the dynamic properties have many characteristics of the simple liquid metals although some conspicuous differences have been found.

cond-mat.mtrl-sci↗
arXiv2002

Dynamic structure in a molten binary alloy by ab initio Molecular Dynamics: Crossover from Hydrodynamics to the Microscopic Regime

The dynamic structure factor of the 7Li0.61Na0.39 liquid alloy at T=590 K has been calculated by ab initio molecular dynamics simulations using 2000 particles. For small wavevectors, 0.15 <= q/A-1 <= 1.6, we find clear side peaks in the partial dynamic structure factors. Whereas for q <= 0.25 A-1 the peak frequencies correspond to the hydrodynamic sound dispersion of the binary alloy, for greater q values we obtain two modes with phase velocities above and below the hydrodynamic sound. A smooth transition between hydrodynamic sound and the two collective modes is shown to take place in the range 0.25 <= q/A-1 <= 0.35. The mass ratio in this system, mNa/mLi = 3, is the smallest one so far for which the fast mode is observed. We also predict that inelastic X-ray scattering experiments would be able to detect the slow mode, and explain why the inelastic neutron scattering experiments [P.R. Gartyrell-Mills et al, Physica B 154, 1 (1988)] do not show any of these modes.

cond-mat.dis-nn↗
arXiv2000

Solution of Poisson's equation for finite systems using plane wave methods

Reciprocal space methods for solving Poisson's equation for finite charge distributions are investigated. Improvements to previous proposals are presented, and their performance is compared in the context of a real-space density functional theory code. Two basic methodologies are followed: calculation of correction terms, and imposition of a cut-off to the Coulomb potential. We conclude that these methods can be safely applied to finite or aperiodic systems with a reasonable control of speed and accuracy.

physics.comp-ph↗
arXiv2000

Melting in large sodium clusters: An orbital-free molecular dynamics study

The melting-like transition in sodium clusters Na_N, with N=55, 92, and 142 is studied by using constant-energy molecular dynamics simulations. An orbital-free version of the Car-Parrinello technique is used which scales linearly with system size allowing investigation of the thermal behaviour of large clusters. The ground state isomer of Na_142 (an uncomplete three-shell icosahedron) melts in two steps: the first one (at approx. 240 K) is characterized by the high mobility of the atoms located on the cluster surface; the second, homogeneous melting (at approx. 270 K), involves diffusive motion of all the atoms across the cluster. For the case of Na_92, the icosahedral structure has a larger number of surface vacancies, and melts in two well separated steps, surface melting at approx. 130 K and homogeneous melting at approx. 240 K. Na_55, a complete two-shell icosahedron, melts in a single stage at approx. 190 K. Our results on homogeneous melting for Na_142 and Na_92 are in excellent agreement with recent experimental determinations of melting temperatures and latent heats. However, the experimentally observed enhancement of the melting temperature around N=55 is not reproduced by the calculations.

physics.atm-clus↗