Probing cooperative liquid dynamics with the mean square displacement.
Literature data for picosecond mean square displacements show that the anharmonicity explains only about half of the fragility (with different fractions for different glass formers). The other half must be ascribed to the Adam-Gibbs mechanism of a growing cooperatively rearranging region. One can measure both influences separately by a simultaneous measurement of liquid and crystal in the coexistence region.
Jülich Center for Neutron Science, Forschungszentrum Jülich Postfach 1913, D-52425 Jülich, Federal Republic of Germany.
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Buchenau
Jülich Center for Neutron Science, Forschungszentrum Jülich Postfach 1913, D-52425 Jülich, Federal Republic of Germany.
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Zorn
Laboratorio de Bajas Temperaturas, Departamento de Fisica de la Materia Condensada, Condensed Matter Physics Center (IFIMAC) and Instituto Nicolas Cabrera, Universidad Autonoma de Madrid, Cantoblanco, E-28049 Madrid, Spain.
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Ramos