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Waterloo physicist Michel Gingras and post-doctoral fellow Jeffrey Rau have succeeded in describing the mixed frozen and dynamical behaviour of glasses by adapting a theoretical model originally designed to study a class of magnetic materials called spin ices.
While condensed matter physicists have understood the behaviour of ideal gases and crystalline structures for nearly 100 years, they continue to struggle with the heterogeneous nature of glasses, which exhibit both frozen solid and dynamic liquid behaviour at the molecular level.
Glasses are not just windows and beverage containers – they include any solid without an ordered atomic crystalline structure such as metal alloys, plastics and polymers.
“As a glass-forming liquid is rapidly cooled, its ability to reach the minimum energy state becomes frustrated. The system eventually jams and forms a solid, without organizing into a perfect crystalline structure. Why? What is the fundamental nature of a glass?” says Gingras, the Canada Research Chair in Condensed Matter Theory and Statistical Mechanics at the University of Waterloo.
“Spin ices” were serendipitously discovered in the late 1990s as a result of seemingly unrelated condensed matter research. This new class of magnetic materials was found to display exotic properties closely mimicking some of common water ice.
Water molecules consists of two hydrogen (H) atoms and one oxygen (O) atom, hence the well-known chemical formula H2O. As water molecules freeze into a solid at 0 C, the oxygen atoms spatially organize perfectly periodically and thus form a regular crystalline structure. Water’s hydrogen atoms, on the other hand, have numerous ways to position themselves to hold the water molecules together in an ice solid.
In spin ice materials, the configuration of the atomic magnetic moments, or spins, mimic (“map”) the hydrogen atom positions in water ice. Spin ices have attracted much attention from physicists because they provide a much simpler platform than water ice to study experimentally and theoretically materials with a vast number of configurations - in this case, the positions of hydrogen atoms - that can have an identical minimum energy.
By slightly increasing the complexity of the current spin ice model, Gingras and Rau found a way to endow spin ice with a sort of slushy magnetic state – a mixed magnetic phase akin to the slushy drink containing both solid and liquid.
So in their model, spin slush behaves like a “glass,” with predominant regions of “stiff” magnetic moments that are “frozen”, surrounded by dynamic “liquid” regions where the magnetic moments fluctuate very rapidly.
“We study magnetism as a way to address broad and fundamental questions about how collective behaviour happens in nature,” says Gingras. “Simplified models of magnetic systems may ultimately prove an exquisite way to study and unravel the fundamental aspects of the heterogeneous dynamics of glasses. Such research may, perhaps eventually, inspire the chemical synthesis of novel glasslike materials of significant potential for applications or devices”
Gingras and Rau’s work was supported by the Natural Sciences and Engineering Research Council of Canada, the Canadian Foundation for Advanced Research, the Canada Research Chair Program and the Perimeter Institute for Theoretical Physics.