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

We report beta-NMR measurements in Bi2Se3:Ca and Bi2Te3:Mn single crystals using Li-8(+) implanted to depths on the order of 100 nm. Above -200 K, spin-lattice relaxation reveals diffusion of Li-8(+) with activation energies of similar to 0.4 eV (similar to 0.2 eV) in Bi2Se3:Ca (Bi2Te3: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 Bi2Se3:Ca. From this, we make a detailed comparison with the isostructural tetradymite Bi2Te2 Se [McFadden et al., Phys. Rev. B 99, 125201 (2019)]. In the magnetic Bi2Te3:Mn, the effects of the dilute Mn moments predominate, but remarkably the Li-8 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 Li-8 NMR response is an important step toward using depth-resolved beta-NMR to study the low-energy properties of the chiral topological surface state in the Bi(2)Ch(3) tetradymite topological insulator.