Nanoscale $\beta$-Nuclear Magnetic Resonance Depth Imaging of Topological Insulators

TL;DR

This paper introduces a non-invasive $eta$-NMR spectroscopy method for nanoscale depth imaging of topological insulators, revealing surface and interface properties crucial for understanding topological phases.

Contribution

It presents a novel $eta$-NMR technique capable of one-dimensional nanoscale imaging of topological insulators, enabling detailed study of surface states and interface coupling.

Findings

Mapped $^8$Li resonance signatures related to TI properties

Detected nanoscale variations in magnetic order and electronic wavefunctions

Provided insights into topological non-trivial characteristics affecting hyperfine interactions

Abstract

Considerable evidence suggests that variations in the properties of topological insulators (TIs) at the nanoscale and at interfaces can strongly affect the physics of topological materials. Therefore, a detailed understanding of surface states and interface coupling is crucial to the search for and applications of new topological phases of matter. Currently, no methods can provide depth profiling near surfaces or at interfaces of topologically inequivalent materials. Such a method could advance the study of interactions. Herein we present a non-invasive depth-profiling technique based on $\beta$-NMR spectroscopy of radioactive $^8$Li$^+$ ions that can provide "one-dimensional imaging" in films of fixed thickness and generates nanoscale views of the electronic wavefunctions and magnetic order at topological surfaces and interfaces. By mapping the $^8$Li nuclear resonance near the surface and 10 nm deep into the bulk of pure and Cr-doped bismuth antimony telluride films, we provide signatures related to the TI properties and their topological non-trivial characteristics that affect the electron-nuclear hyperfine field, the metallic shift and magnetic order. These nanoscale variations in $\beta$-NMR parameters reflect the unconventional properties of the topological materials under study, and understanding the role of heterogeneities is expected to lead to the discovery of novel phenomena involving quantum materials.