Thermally induced two-level systems in glasses and nuclear spin-lattice relaxation

We have measured the ¹⁹F nuclear spin-lattice relaxation rate 1T₁ in a borate glass containing CaF₂. In the range 2.5 KT30 K, 1T₁ exhibits a quadratic temperature dependence not frequently found in other glasses. This behavior is a limiting case predicted by a model of thermally induced two-level systems proposed by Varma, Dynes, and Banavar. The predominant magnetic dipole-dipole interaction prevalent for ¹⁹F nuclei permits a more quantitative comparison with theory than possible with earlier results.