Li tunneling effects on spin-lattice relaxation rates of a color center in CaO:Li

Low-temperature spin-lattice relaxation rates of the ${[{F}_{\mathrm{Li}}]}^{0}$ center in single crystals of CaO:Li at $X$- and $\mathrm{Ku}$-band microwave frequencies exhibit an anisotropy in the functional form of the temperature dependence: from $T$ in the [100] direction to csch ($\frac{25}{T}$) in the [111] direction. A model involving longitudinal and transverse tunneling of a Li ion adjacent to the trapped electron can qualitatively explain this and most of the other characteristics of the relaxation data, with the exception of the hyperfine dependence.