LOW-TEMPERATURE SPECIFIC-HEAT OF RARE-EARTH-DOPED SILICATE-GLASSES

The specific heat C-upsilon of Pr3+- and Eu3+-doped silicate glasses h as been determined between 0.1 and 10 K. Below about 5 K, C-upsilon pr oportional to T-1+upsilon with upsilon=0.0-0.3. This dependence is cha racteristic for the two-level-system (TLS) contribution to the specifi c heat, C-upsilon(TLS). The values of C-upsilon(TLS) and upsilon notic eably vary with the chemical nature and the concentration of the glass modifiers. The number of TLS in samples of almost identical compositi on is significantly larger for the Pr3+ doped than for the Eu3+ doped and the undoped glass. Above 5 K, the T-3-phonon term and the T-5 term associated with soft localized vibrations also contribute to C-upsilo n. The results do not point to a correlation between the T-1+upsilon a nd the higher-order terms.