We are used to thinking of glass as a fragile and common material, but glass is still one of the greatest enigmas for physics. In crystals, atoms are arranged in geometric order, while chaos reigns in glass. This disorder generates unique properties, especially near absolute zero, where the glass behaves very differently from crystals. A study conducted by the Department of Physics of the University of Trento in collaboration with the European Synchrotron Radiation Facility (ESRF) in Grenoble and other European research centres sheds new light on this mystery. The working group analysed the so-called ultra-stable glasses, which are produced with advanced techniques that make them perfect candidates for the title of 'ideal glass'. The first author of the article published in Physical Review X is Irene Festi, who worked on the project for her PhD thesis at the Department of Physics of the University of Trento. Giacomo Baldi, professor of Experimental Physics of Matter and head of the Laboratory of Structure and Dynamics of Complex Systems at the same Department of UniTrento, is the scientific coordinator of the study.
The challenge of 'ultra-stable' glass. In recent years, research has focused on this type of material, obtained from organic molecules now used in various applications, from pharmaceutical tablets to OLED for TVs. Unlike traditional glasses, which are produced by rapidly cooling a molten liquid, ultra-stable glasses are built "molecule by molecule" by depositing vapours on a temperature-controlled surface. The result is a denser, more stable material whose thermal properties are similar to those of a crystal. At low temperature, crystals behave predictably: the atoms can only vibrate around their equilibrium position. In glass, in addition to vibrations, tiny atomic rearrangements can take place, small 'jumps' from one configuration to another. The central question of the study was: if an ultra-stable glass imitates a crystal from the thermal point of view, does its microscopic vibrations also change? The research investigated whether this thermal similarity depended on a different vibration of the atoms. The result is surprising, as Giacomo Baldi comments: "Despite different thermal properties, the basic vibrations (sound waves) between normal and ultra-stable glass remain almost identical. This contradicts years of computer simulations and suggests that vibrational modes are less sensitive to disorder than previously thought."
X-ray atomic vibrations. To analyse these glasses – and this is the most innovative part of the study – the research team used an X-ray spectrometer capable of measuring atomic vibrations up to an extraordinarily low frequency of a few tens of gigahertz, exploiting an energy resolution of one part per billion, ten times higher than previous methods. This experiment was made possible thanks to collaboration with the Grenoble synchrotron, one of the most important fourth-generation synchrotrons currently in operation in the world. It's like trying to distinguish the buzz of a mosquito at a rock concert.
The results. The study revealed something unexpected: while vibrations at intermediate frequencies decrease drastically when the glass becomes more stable, vibrations at even lower frequencies remain practically identical to those of an ordinary glass. "This means – explains Professor Baldi – that even if almost all the defects of a glass are eliminated, making it ultra-stable, residual vibrations remain, and this seems to be an intrinsic and ineliminable characteristic of the glassy state. This demonstrates that these vibrations are not due to localised defects, as previously thought, but are linked to the way in which sound waves disperse and damp through the disordered structure of the material."
Possible application scenarios. Although this is basic research, this study could have a potential impact on high-tech sectors. First of all, in consumer electronics. The material of the study is used in the Oled monitors of televisions and smartphones. Understanding how the atoms in these thin films vibrate improves heat conduction and stability over time to optimize energy savings and screen life. Another sector is that of pharmaceuticals. Many medicines are encapsulated in organic glassy matrices. "Because the glasses are out of equilibrium, they can deteriorate over time ('aging'). If the glass changes structure, drug release into the body within a certain time since the ingestion may vary and be ineffective. Studying atomic stability helps to create capsules that do not 'age'", says Baldi. The University of Trento has been working on this topic for a very long time, contributing with many cutting-edge studies, as you can see from the seventeenth edition of the workshop on complex systems which took place in Andalo in March. A conference that is considered as a reference event of international relevance by the physics community.
