Simulations Reveal Quantum Tunneling Events in Glass
The idea that glass flows like a liquid over long timescales may be a myth, but that doesn’t mean that its atoms are completely frozen. Even at very low temperatures, glasses can shift between similar configurations as a result of quantum tunneling of atoms. Now, Dmytro Khomenko, at Columbia University, and colleagues show by using numerical simulations that the prevalence of such tunneling events depends on the thermal history of the glass. Their findings explain why a glass’s low-temperature properties, like its specific heat, are so sensitive to how the glass was prepared.
Because glass has a disordered structure, its atoms can adopt an infinite number of distinct configurations. At very low temperatures, the energy barriers separating alternative arrangements are too high to overcome. But if two similar configurations have nearly the same energy, atoms can switch places by quantum tunneling. Such two-level systems (TLSs) are present in all glasses formed by rapidly cooling a liquid, explaining why their thermodynamic properties differ from those of crystalline solids. But glasses that are formed layer-by-layer using vapor deposition have still different properties, suggesting they have fewer TLSs.
The study from Khomenko and colleagues puts this idea to the test. They used a recently developed algorithm that can explore the energy landscape of glasses prepared with an unprecedentedly wide range of thermal histories. They found that the density of TLSs is lower in more kinetically stable glasses. And since glasses formed through a slow, layer-by-layer approach are more kinetically stable than those cooled rapidly from a melt, they have fewer TLSs. The researchers also found that, while TLSs typically include just a few atoms, clusters of a hundred or more atoms can sometimes be involved.
This research is published in Physical Review Letters.
Marric Stephens is a Corresponding Editor for Physics based in Bristol, UK.