Local electronic and magnetic properties of the doped topological insulators <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow><mml:mi>Bi</mml:mi></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mi>Se</mml:mi></mml:mrow><mml:mn>3</mml:mn></mml:msub><mml:mo>:</mml:mo><mml:mi>Ca</mml:mi></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow><mml:mi>Bi</mml:mi></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mi>Te</mml:mi></mml:mrow><mml:mn>3</mml:mn></mml:msub><mml:mo>:</mml:mo><mml:mi>Mn</mml:mi></mml:math> investigated using ion-implanted <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow/><mml:mn>8</mml:mn></mml:msup><mml:mi>Li</mml:mi></mml:math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>β</mml:mi><mml:mtext>−</mml:mtext><mml:mi>NMR</mml:mi></mml:math>
Ryan M. L. McFadden, Aris Chatzichristos, David Cortie, Derek Fujimoto, Y. S. Hor, Huiwen Ji, Victoria L. Karner, R. F. Kiefl, C. D. P. Levy, Ruohong Li, Iain McKenzie, G. D. Morris, M. R. Pearson, Monika Stachura, R. J. Cava, W. A. MacFarlane
Abstract
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.