Dynamic Nuclear Polarization from Topological Insulator Helical Edge States

TL;DR

This paper investigates how hyperfine interactions in topological insulators can lead to dynamic nuclear polarization, affecting spin transport, through exact calculations and scaling analysis of edge states with nuclear spins.

Contribution

It provides a numerically exact analysis of nuclear polarization in topological insulator edge states, revealing universal scaling laws and implications for spintronics.

Findings

Significant nuclear polarization can occur from weak edge currents.

Universal scaling properties enable extrapolation to larger systems.

Hyperfine interactions influence spin transport in topological insulators.

Abstract

Topological insulators are promising for spintronics and related technologies due to their spin-momentum-locked edge states, which are protected by time-reversal symmetry. In addition to the unique fundamental physics that arises in these systems, the potential technological applications of these protected states has also been driving TI research over the past decade. However, most known topological insulator materials naturally contain spinful nuclei, and their hyperfine coupling to helical edge states intrinsically breaks time-reversal symmetry, removing the topological protection and enabling the buildup of dynamic nuclear spin polarization through hyperfine-assisted backscattering. Here, we calculate scattering probabilities and nuclear polarization for edge channels containing up to $34$ nuclear spins using a numerically exact analysis that exploits the symmetries of the problem to drastically reduce the computational complexity. We then show the emergence of universal scaling properties that allow us to extrapolate our findings to vastly larger and experimentally relevant system sizes. We find that significant nuclear polarization can result from relatively weak helical edge currents, suggesting that it may be an important factor affecting spin transport in topological insulator devices.