6533b82afe1ef96bd128cb6a

RESEARCH PRODUCT

Understanding the glass transition and the amorphous state of matter: can computer simulation solve the challenge?

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description

The glass transition of supercooled fluids is one of the big puzzles of condensed matter physics, because there occurs a dramatic slowing down (the viscosity η can increase from about η = 1 Poise at the melting transition to η 10 13 Poise at the glass transition temperature T g ), but one hardly sees any accompanying change in the static structure. Theoretical concepts are very controversial - e.g., the Gibbs-di Marzio theory attributes glassy freezing to an underlying entropy catastrophe (the entropy of the supercooled fluid would fall below the crystal entropy at the Kauzmann temperature T 0 T g . Computer simulations offer the advantage that atomistically detailed information on structure and dynamics of well-defined models can readily be obtained, including quantities that are not accessible in experiments. But they have the disadvantage that a limited range of relaxation times (typically about 6 to 7 decades only) is accessible, and thus it is mostly the regime T > T c that can be studied. Using coarse-grained models for short polymer chains, such as the bond-fluctuation model on the lattice or a beadspring model in the continuum, nevertheless useful information has been obtained: e.g., it could be shown that the configurational entropy S stays nonvanishing, although the strong decrease of S that does occur can account for the increase of the relaxation time in accord with the Gibbs-Adam theory. Also the mode coupling theory is compatible with the relaxation, although difficulties remain. These difficulties are traced to the limited range of T where idealized mode coupling theory applies (rounding of singularities very close to T c is not fully understood). Finally, molecular dynamics simulations for a realistic model of fluid SiO 2 are presented to show that simulations can contribute to a better understanding of the properties of real amorphous materials.