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CNR: Alamanacco della Scienza


N. 13 - 3 ago 2011
ISSN 2037-4801

International info   a cura di Cecilia Migali


Why is chaos warmer than order?    


Glasses are remarkably different from crystals at low temperature. They accumulate more heat and conduct less.  This anomaly is related to a particular ensemble of atomic motions called the 'boson peak', which is universally observed for all glasses.  Its nature, however, remained unknown for more than 50 years.  Because of this lengthy research period, the boson peak has been called the last puzzle of solid state physics.  Most models explain the boson peak by additional vibrational modes created by the chaos of the atomic positions, while others attribute it to sound waves.

Now, an international team of scientists, including researchers of Cnr-Institute of materials (Iom) and Perugia University, combined efforts at the European Synchrotron Radiation Facility (Esrf) in Grenoble (France), to find a definitive solution to this problem. Their results have just been published in the scientific journal Physical Review LettersAleksandr Chumakov, leading author for the publication and scientist at beamline ID18,commented, "All decisive X-ray data were taken at the Esrf. But we also crucially relied upon German, American, Italian, and Polish Institutes with expertise  in sample preparation, high pressure experiments, sound velocity measurements  and ab initio calculations".

The researchers compared atomic motions in a glass and a crystal using the nuclear inelastic scattering technique which determines an exact number of vibrational states.  The results show that around the boson peak, the number of states in a glass is exactly the same as the number of sound wave states in the crystal. Furthermore, application of pressure causes a gradual transformation of the boson peak towards a particular (van Hove) singularity of the crystal created by sound waves with a period equal to the crystal periodicity. These observations unambiguously identify the boson peak with sound waves.  Thus, more heat can be stored in a glass not because chaos allows for more vibrations, but because it changes sound waves. Unexpectedly, but in accordance with the general theory of knowledge, the sound-wave nature of glass anomalies revealed here is a return to the earliest of ideas, but with the addition of a new level of understanding. 



Fonte: Lucia Comez, Istituto Officina dei Materiali, tel. + 39 075 5852729, email comez@fisica.unipg.it

Fonte: Claus Habfast, European Synchrotron Radiation Facility (Esrf), tel. +33 666 662 384, email claus.habfast@esrf.fr