Electron-Spin Relaxation by Tunneling States inβ-Al2O3: Na

Spin relaxation measurements on a color center in the conduction plane of $\beta$-${\mathrm{Al}}_2$ ${\mathrm{O}}_3$: Na reveal an exceptionally fast rate with an anomalous temperature and microwave frequency dependence. The data are quantitatively described by a mechanism involving the coupling of a color center to the phonon-induced relaxation of a nearby localized two-level tunneling state. The required density of such states is nearly the same as that used to explain the low-temperature properties of $\beta$-${\mathrm{Al}}_2$ ${\mathrm{O}}_3$: Na.