Amorphous solid

Quantum mechanics

Chemical physics

Materials science

Variety (cybernetics)

Nuclear physics

Computer science

Statistical physics

Specific heat

Universality (dynamical systems)

Physics

Amorphous metal

Nonlinear system

Nanotechnology

Condensed matter physics

Artificial intelligence

Crystallography

Chemistry

Vitrification

Engineering physics

The Journal of Chemical Physics

Glassy solids exhibit a wide variety of generic thermomechanical properties, ranging from universal anomalous specific heat at cryogenic temperatures to nonlinear plastic yielding and failure under external driving forces, which qualitatively differ from their crystalline counterparts. For a long time, it has been believed that many of these properties are intimately related to nonphononic, low-energy quasilocalized excitations (QLEs) in glasses. Indeed, recent computer simulations have conclusively revealed that the self-organization of glasses during vitrification upon cooling from a melt leads to the emergence of such QLEs. In this Perspective, we review developments over the past three decades toward understanding the emergence of QLEs in structural glasses and the degree of universality in their statistical and structural properties. We discuss the challenges and difficulties that hindered progress in achieving these goals and review the frameworks put forward to overcome them. We conclude with an outlook on future research directions and open questions.

Low-energy quasilocalized excitations in structural glasses