The density of low energy states in vitreous silica: Specific heat and thermal conductivity down to 25 mK

Abstract

We have extended the specific heat and thermal conductivity measurements down to 25 mK for two types of vitreous silica of different content of OH groups. Attributing the specific heat to two-level systems, we show that the density of states does not become constant, not even at energies below 100 mK. Though the specific heat depends on the content of OH groups, the thermal conductivity is scarely influenced by these impurities.