Resistively detected NMR as a probe of the topological nature of conducting edge/surface states

TL;DR

This paper introduces a resistively detected NMR technique to probe the topological and helical nature of conducting edge and surface states in topological insulators, revealing their spin-momentum locking and potential for device applications.

Contribution

It demonstrates that RDNMR can detect the topological surface states and their helicity, providing a new method to study and manipulate topologically protected electronic states.

Findings

Conductance changes at nuclear resonance frequencies depend on magnetic field orientation.

RDNMR signal sensitivity reveals the helical nature of surface states.

Overhauser field effect dominates conductance change in 3D TIs in quantum Hall regime.

Abstract

Electron spins in edge or surface modes of topological insulators (TIs) with strong spin-orbit coupling cannot be directly manipulated with microwaves due to the locking of electron spin to its momentum. We show by contrast that a resistively detected nuclear magnetic resonance (RDNMR) based technique can be used to probe the helical nature of surface conducting states. In such experiments, one applies a radio frequency (RF) field to reorient nuclear spins that then couple to electronic spins by the hyperfine interaction. The spin of the boundary electrons can thereby be modulated, resulting in changes in conductance at nuclear resonance frequencies. Here, we demonstrate that the conductivity is sensitive to the direction of the applied magnetic field with respect to the helicity of the electrons. This dependence of the RDNMR signal on angle probes the nature of the conductive edge or surface states. In the case of 3D TI in the quantum Hall regime, we establish that the dominant mechanism responsible for the conductance change in a RDNMR experiment is based on the Overhauser field effect. Our findings indicate that the same physics underlying the use of RDNMR to probe TI states also enables us to use RF control of nuclear spins to coherently manipulate topologically protected states which could be useful for a new generation of devices.