Correlation between density of tunneling states and fragility of glasses

Evidence of a correlation between the density of tunneling states of two-level systems and the fragility for a variety of glasses is presented. More fragile glasses tend to have a higher excess low-temperature (T < I K) specific heat and thus a higher density of tunneling states. This correlation is consistent with a picture which connects the fragility of glasses to the topology on their potential energy hypersurfaces in configuration space.