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Disorder and Phase Transitions

Glass: a Solid or a Liquid ?

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Glasses are solids in the sense that, in contrast to liquids, they are resistant to shear. However, in contrast to solids, glasses do not have a regular internal order of atoms; they are amorphous. Due to the lack of long-range atomic order, it is still not completely clear if the glass state is thermodynamically different from a strongly supercooled liquid. Large scale computer-simulations of glasses have shown that extended low-dimensional correlations exist within glasses. These structural correlations are responsible for anomalies in the vibrational spectra of glasses and for facilitate relaxational processes.

(C. Oligschleger, H.R. Schober, Phys. Rev. B 59, 811 (1999))


Dynamics of Glasses

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Using computer simulations and analytic methods we undertake studies on models and materials such as Se and CuZr. Cooling from the supercooled melt to glass transition, diffusion and relaxation become strongly correlated. The atoms move in chain-like structures where single atoms typically move only a fraction of the distance to their nearest neighbour. These chain structures persist in glass. Even at temperatures below 1K where tunnelling is observed, systems motion is collective. Collectivity of motion is intimately connected with the dynamic heterogeneity seen in glasses and under cooled melts.

(D. Caprion, J. Matsui, H.R. Schober, Phys. Rev. Lett. 85, 4293 (2000)).


Epitaxial Growth with Long-Range Interactions

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Here our investigations centre on a model for island formation in submonolayer epitaxy in the presence of a long-range elastic interaction between the adsorbed particles. The interaction is caused by the deformation of the underlying substrate and has a repulsive 1/r^3 character. The atoms perform a random walk until they attach themselves irreversibly to an island. With increasing elastic repulsion, island nucleation is hampered and deferred to higher coverage values. We obtain a scaling law combining particle flux, particle mobility and elastic interaction strength.

(F. Gutheim, H. Müller-Krumbhaar, E. Brener. Epitaxial growth with elastic interaction: Submonolayer island formation Phys. Rev. E, vol. 63, 041603 (2001)).



Phase Transitions with Elastic Effects

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Phase transitions of alloys are usually accompanied by strong temperature gradients and lattice misfits between the different components. They can induce elastic deformations which strongly alter growth and coarsen behaviour. Particularly in terms of melting and crystallization processes, new phases appear which are no longer spherical but are of a more complicated, disk-like shape. Strong interactions between adjacent inclusions lead to new physical features.

 

(E. Brener, V. Marchenko, H. Müller-Krumbhaar, and R. Spatschek, Coarsening Kinetics with Elastic Effects, Phys. Rev. Lett. 84, 4914 (2000)).


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