Didier Caprion

Melting of model structures: molecular dynamics and Voronoï tessellation

Using classical molecular dynamics simulations combined with Voronoi tessellation we study the geometrical modifications as a function of temperature in two model Frank - Kasper phases: the A15 structure (-tungsten) and the cubic Friauf - Laves structure . We show how the perfect arrangement of disclination lines at 0 K for the crystalline structures evolves through the melting point. In particular, as the temperature is increased, the results permit us to identify the first defect and to show that the initial network of disclination lines survives until the solid - liquid first-order transition has indeed taken place.

Structural and vibrational properties of a soft-sphere glass: Influence of the quenching rate

Abstract Using classical molecular dynamics simulations combined with the Voronoi tessellation we study, at very low temperatures, the phonon spectrum and the geometrical modifications in a model glass as a function of the quenching rate. The shape of the phonon spectrum is not strongly affected by a decrease in the quenching rate, except for the minimum pulsation ωm, corresponding to the first pseudo-Bragg peak wave-vector, which goes through a marked minimum for the critical quenching rate at which the onset of crystallization is detected by the geometrical analysis. This decrease in ωm can be interpreted as an increase in structural order in the glass phase when the cooling rate is lowered.

Influence of the quenching rate and the aging time on the vibrational spectrum of a model glass

The phonon spectrum of a model glass is investigated by classical microcanonical molecular dynamics. The evolution of the shape of the longitudinal and transverse branches is determined as a function of the quenching rate and the aging time. Immediately after the quench, the minimum frequency for both branches, which is located at the pseudo-Bragg peak wave vector, is the lowest for the ideal quenching rate at which the glass is the most stable. When the glass evolves towards a crystalline phase, this minimum frequency increases and then saturates at higher values, as a result of anisotropy.

Numerical investigation of frustration in glassy systems

Abstract A numerical Voronoi tessellation is used to investigate the local order in two classes of model glassy systems: (i) random close packings of 8192 hard spheres with increasing volume fractions, c , built with an efficient algorithm, (ii) super-cooled liquid and glass samples of 1000 atoms at different temperatures, T , obtained after a quench from the liquid state, using classical micro-canonical molecular dynamics (MD) with a simple soft-sphere potential. As a general result, when increasing c in case (i) or when decreasing T in case (ii), the ideal icosahedral order, with five-fold symmetry, appears as an extrapolated situation which cannot be realized due to geometrical frustration.

The Glass Transition in a Simple Model Glass: Numerical Simulations

Abstract In order to improve our understanding of the glass transition we performed constant-energy molecular dynamics simulations on a soft-sphere system quenched from the liquid state to zero temperature. The temperature dependence of static (radial pair distribution function) and dynamic quantities (self-diffusion constant) has been investigated together with the temperature evolution of relevant structural properties deduced from the Delaunay-Voronoi tessellation of these samples. This study permits to shed new light on the structural freezing below the glass transition temperature and the evolution of the local structure through the transition.

Voronoi Tessellation in Model Glass Systems

Numerical Voronoi tessellation is used to investigate the mechanisms of frustration in some model glass systems. First, random packings of 8192 hard spheres of increasing volume fraction c are built in the flat three dimensional space using an efficient computer algorithm. Their Voronoi statistics evolves with c as if the system would like to reach a pure icosahedral order when extrapolating the volume fraction above the Bernal limit cb ≈ 0.645. Second, this study is extended in curved space, the sphere S3 When decurving the space by increasing the number N of spheres, the most compact packings converge to the Bernal packing. For particular N values, the volume fractions exhibit maxima corresponding to narrower histograms for the number of edges of Voronoi polyhedra faces. Third, super-cooled liquid and glass samples of 1000 atoms are generated at different temperatures T after a quench from the liquid state, using classical micro-canonical molecular dynamics with a simple soft-sphere potential. When decreasing T,the ideal icosahedral order appears again as an extrapolated situation which cannot be realized due to geometrical frustration.

Frustration in model glass systems: Numerical investigations

Numerical Voronoi tessellation is used to investigate the mechanisms of frustration in some model glass systems. First, random packings of 8192 hard spheres of increasing volume fraction c are built using an efficient computer algorithm. Their Voronoi statistics evolves with c as if the system would like to reach a pure icosahedral order when extrapolating the volume fraction above the Bernal limit cb≃0.645. Second, super-cooled liquid and glass samples of 1000 atoms are generated at different temperatures T after a quench from the liquid state, using classical micro-canonical molecular dynamics with a simple soft-sphere potential. When decreasing T, the ideal icosahedral order appears again as an extrapolated situation which cannot be realized due to geometrical frustration. Third, a model silica glass of 648 atoms is studied using the potential of van Beest, Kramer and van Santen and a quite similar quenching procedure is performed. As in the soft-sphere case the structural freezing following upon the...