Ion-Implanted $^8$Li Nuclear Magnetic Resonance in Highly Oriented Pyrolytic Graphite

TL;DR

This study uses beta-detected NMR to investigate ultra-dilute $^{8}$Li$^{+}$ in highly oriented pyrolytic graphite, revealing limited lithium mobility and unusual relaxation behavior linked to paramagnetic centers.

Contribution

It provides new insights into lithium behavior in graphite, showing limited mobility and identifying the relaxation mechanisms associated with paramagnetic centers.

Findings

Li$^+$ is not appreciably mobile up to 400 K

Fast relaxation persists down to cryogenic temperatures

Relaxation attributed to paramagnetic centers, not impurities or ferromagnetism

Abstract

We report $\beta$-detected nuclear magnetic resonance of ultra-dilute $^{8}$Li$^{+}$ implanted in highly oriented pyrolytic graphite (HOPG). The absence of motional narrowing and diffusional spin-lattice relaxation implies Li$^+$ is not appreciably mobile up to 400 K, in sharp contrast to the highly lithiated stage compounds. However, the relaxation is remarkably fast and persists down to cryogenic temperatures. Ruling out extrinsic paramagnetic impurities and intrinsic ferromagnetism, we conclude the relaxation is due to paramagnetic centers correlated with implantation. While the resulting effects are not consistent with a Kondo impurity, they also differ from free paramagnetic centers, and we suggest that a resonant scattering approach may account for much of the observed phenomenology.