Local electronic and magnetic properties of the doped topological insulators Bi2Se3:Ca and Bi2Te3:Mn investigated using ion-implanted 8Li β−NMR

We report $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{NMR}$ measurements in ${\mathrm{Bi}}₂{\mathrm{Se}}₃:\mathrm{Ca}$ and ${\mathrm{Bi}}₂{\mathrm{Te}}₃:\mathrm{Mn}$ single crystals using $⁸\mathrm{Li}⁺$ implanted to depths on the order of 100 nm. Above $\ensuremath{\sim}200\phantom{\rule{4pt}{0ex}}\mathrm{K}$, spin-lattice relaxation reveals diffusion of $⁸\mathrm{Li}⁺$, with activation energies of $\ensuremath{\sim}0.4\phantom{\rule{4pt}{0ex}}\mathrm{eV}$ $(\ensuremath{\sim}0.2\phantom{\rule{4pt}{0ex}}\mathrm{eV})$ in ${\mathrm{Bi}}₂{\mathrm{Se}}₃:\mathrm{Ca}$ $({\mathrm{Bi}}₂{\mathrm{Te}}₃:\mathrm{Mn})$. At lower temperatures, the NMR properties are those of a heavily doped semiconductor in the metallic limit, with Korringa relaxation and a small, negative, temperature-dependent Knight shift in ${\mathrm{Bi}}₂{\mathrm{Se}}₃:\mathrm{Ca}$. From this, we make a detailed comparison with the isostructural tetradymite ${\mathrm{Bi}}₂{\mathrm{Te}}₂\mathrm{Se}$ [McFadden et al., Phys. Rev. B 99, 125201 (2019)]. In the magnetic ${\mathrm{Bi}}₂{\mathrm{Te}}₃:\mathrm{Mn}$, the effects of the dilute Mn moments predominate, but remarkably the $⁸\mathrm{Li}⁺$ signal is not wiped out through the magnetic transition at 13 K, with a prominent critical peak in the spin-lattice relaxation that is suppressed in a high applied field. This detailed characterization of the $⁸\mathrm{Li}⁺$ NMR response is an important step toward using depth-resolved $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{NMR}$ to study the low-energy properties of the chiral topological surface state in the ${\mathrm{Bi}}₂{\mathrm{Ch}}₃$ tetradymite topological insulator.