Local electronic and magnetic properties of the doped topological insulators Bi${}_2$Se${}_3$:Ca and Bi${}_2$Te${}_3$:Mn investigated using ion-implanted ${}^8$Li $\beta$-NMR

Abstract

We report $\beta$-detected nuclear magnetic resonance ($\beta$-NMR) measurements in Bi${}_2$Se${}_3$:Ca (BSC) and Bi${}_2$Te${}_3$:Mn (BTM) single crystals using ${}^8$Li${}^{+}$ implanted to depths on the order of 100 nm. Above $\sim 200$ K, spin-lattice relaxation (SLR) reveals diffusion of ${}^8$Li${}^{+}$, with activation energies of $\sim 0.4$ eV ($\sim 0.2$ eV) in BSC (BTM). At lower temperatures, the nuclear magnetic resonance (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 BSC. From this, we make a detailed comparison with the isostructural tetradymite Bi${}_2$Te${}_2$Se (BTS) [McFadden et al., Phys Rev. B 99, 125201 (2019)]. In the magnetic BTM, the effects of the dilute Mn moments predominate, but remarkably the ${}^8$Li signal is not wiped out through the magnetic transition at 13 K, with a prominent critical peak in the SLR that is suppressed in a high applied field. This detailed characterization of the ${}^8$Li NMR response is an important step towards using depth-resolved $\beta$-NMR to study the low-energy properties of the chiral topological surface state (TSS). With the bulk NMR response now established in several Bi${}_2$Ch${}_3$ tetradymite topological insulators (TIs), the prospect of directly probing their chiral TSS using the depth resolution afforded by $\beta$-NMR remains strong.

arXiv comment14 pages, 10 figures

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