Specific heat of glasses at low temperatures

We have studied the specific heat of nitrate glasses (T&#60;1 K) doped with water, lithium, and nitrite. We have found that there is a basic difference between doped crystals and doped glasses regarding the excess specific heat caused by the presence of the dopants. Dopants in glasses cause a change in the low-temperature specific heat only if the glass transition temperature T<SUB>G</SUB> changes upon admixing. The excess specific heat is found to be inversely proportional to T<SUB>G</SUB>. Similar results have been observed on silica containing sodium and on neutron-irradiated silica, for which the important parameter is the fictive temperature T<SUB>f</SUB>. It is concluded that the frozen-in disorder in the glass, which is measured by T<SUB>G</SUB> (or T<SUB>f</SUB>), is an important factor in determining the density of states of low-energy excitations which are characteristic of the amorphous state