Anomalous Spin Response and Virtual-Carrier-Mediated Magnetism in a Topological Insulator

TL;DR

This paper provides a theoretical analysis of the unique spin response in HgTe quantum wells, highlighting how topological properties influence magnetic susceptibility and edge state behavior, with implications for magnetic ordering and g-factors.

Contribution

It offers a novel theoretical insight into the spin susceptibility and magnetic properties of topological insulator quantum wells, emphasizing the role of topological order and carrier dynamics.

Findings

Constant spin susceptibility in topological phase

Decreasing susceptibility with band gap in normal phase

Effective g-factors for bulk and edge states

Abstract

We present a comprehensive theoretical study of the static spin response in HgTe quantum wells, revealing distinctive behavior for the topologically nontrivial inverted structure. Most strikingly, the q=0 (long-wave-length) spin susceptibility of the undoped topological-insulator system is constant and equal to the value found for the gapless Dirac-like structure, whereas the same quantity shows the typical decrease with increasing band gap in the normal-insulator regime. We discuss ramifications for the ordering of localized magnetic moments present in the quantum well, both in the insulating and electron-doped situations. The spin response of edge states is also considered, and we extract effective Lande g-factors for the bulk and edge electrons. The variety of counter-intuitive spin-response properties revealed in our study arises from the system's versatility in accessing situations where the charge-carrier dynamics can be governed by ordinary Schrodinger-type physics, mimics the behavior of chiral Dirac fermions, or reflects the material's symmetry-protected topological order.