Ionic and electronic properties of the topological insulator Bi$_2$Te$_2$Se investigated using $\beta$-detected nuclear magnetic relaxation and resonance of $^8$Li

TL;DR

This study investigates the ionic and electronic properties of the topological insulator Bi$_2$Te$_2$Se using $eta$-detected nuclear magnetic relaxation and resonance of $^8$Li, revealing ion diffusion at high temperatures and complex low-temperature magnetic behavior.

Contribution

It introduces $eta$-detected NMR techniques to probe near-surface properties of Bi$_2$Te$_2$Se, providing new insights into ion diffusion and magnetic phenomena in topological insulators.

Findings

Ion diffusion with activation energy 0.185 eV above 150 K

Observation of Korringa-like relaxation at low temperatures

Anomalous field dependence of resonance shift

Abstract

We report measurements on the high temperature ionic and low temperature electronic properties of the 3D topological insulator Bi$_2$Te$_2$Se using ion-implanted $^8$Li $\beta$-detected nuclear magnetic relaxation and resonance. With implantation energies in the range 5-28 keV, the probes penetrate beyond the expected range of the topological surface state, but are still within 250 nm of the surface. At temperatures above ~150 K, spin-lattice relaxation measurements reveal isolated $^8$Li$^{+}$ diffusion with an activation energy $E_{A} = 0.185(8)$ eV and attempt frequency $\tau_{0}^{-1} = 8(3) \times 10^{11}$ s$^{-1}$ for atomic site-to-site hopping. At lower temperature, we find a linear Korringa-like relaxation mechanism with a field dependent slope and intercept, which is accompanied by an anomalous field dependence to the resonance shift. We suggest that these may be related to a strong contribution from orbital currents or the magnetic freezeout of charge carriers in this heavily compensated semiconductor, but that conventional theories are unable to account for the extent of the field dependence. Conventional NMR of the stable host nuclei may help elucidate their origin.