The specific heat C//v of Pr^3+ – and Eu^3+ -doped silicate glasses has been determined between 0.1 and 10 K. Below about 5 K, C//v∝T//1+v with v=0.0-0.3. This deopendence is characteristic for the two-level-system (TLS) contribution to the specific heat, C . The values of C and v noticeably vary with the chemical natureand teh concentration of the glass modifies. The number of TLS in samples of almost identical composition is significantly larger for the Pr^3+ doped thanfor the Eu^3+ 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//v. The results do not point to a correlation between the T^1+v and the higher-order terms.
Van de Straat, D. A. Baak, J.