Perspective: Thermodynamics of structural glasses

According to the conventional wisdom, a (liquid to) glass transition takes place when a liquid, upon cooling, falls out of equilibrium because the observation time becomes less than the structural relaxation time of the liquid. The external time constraint prevents the system from exploring its entire configurational space. The freezing of the slow configurational changes that occur on a time scale longer than the observation time partitions the configurational space into mutually inaccessible components and the system becomes trapped in each component of the partition with a probability that is equal to the probability with which it was exploring that component before partitioning. A glass is such a broken-ergodic system. The volume of the configurational space accessible to a glass is reduced resulting in a loss of configurational entropy during a glass transition. This view of a real glass transition, called the disordered solid (or the broken-ergodic) view, is consistent with all observed features of glass transition such as absence of latent heat and a drop in the heat capacity. Thermodynamics of glassy state based on this and the traditional view are compared. It is concluded that the broken-ergodic view is fundamentally sound but not the traditional view.