  Using a distinguishable-particle lattice model based on void-induced
dynamics, we successfully reproduce the well-known linear relation between heat
capacity and temperature at very low temperatures. The heat capacity is
dominated by two-level systems formed due to the strong localization of voids
to two neighboring sites, and can be exactly calculated in the limit of
ultrastable glasses. Similar but weaker localization at higher temperatures
accounts for the glass transition. The result supports the conventional
two-level tunneling picture by revealing how two-level systems emerge from
random particle interactions, which also cause the glass transition. Our
approach provides a unified framework for relating microscopic dynamics of
glasses at room and cryogenic temperatures.

Soft Condensed Matter