Fate of surface gaps in magnetic topological insulators

TL;DR

This paper investigates how magnon-mediated interactions affect the surface states and gaps in magnetic topological insulators, revealing that these interactions can both enhance and modify the surface gaps depending on the surface orientation.

Contribution

It demonstrates that magnon-mediated electron interactions significantly influence surface gaps, introducing long-range effects that alter the surface state properties in magnetic topological insulators.

Findings

Magnon interactions can enhance surface gaps beyond non-interacting predictions.

On surfaces parallel to magnetic order, magnon interactions induce massless-like gaps.

Magnon-mediated interactions dominate over Coulomb interactions at the surface.

Abstract

In magnetic topological insulators, the surface states can exhibit a gap due to the breaking of time-reversal symmetry. Various experiments, while suggesting the existence of the surface gap, have raised questions about its underlying mechanism in the presence of different magnetic orderings. Here, we demonstrate that magnon-mediated electron-electron interactions, whose effects are not limited to the surfaces perpendicular to the magnetic ordering, can significantly influence surface states and their effective gaps. On the surfaces perpendicular to the spin quantization axis, many-body interactions can enhance the band gap to a degree that surpasses the non-interacting scenario. Then, on surfaces parallel to the magnetic ordering, we find that strong magnon-induced fermionic interactions can lead to features resembling a massless-like gap. These remarkable results largely stem from the fact that magnon-mediated interactions exhibit considerable long-range behavior compared to direct Coulomb interactions among electrons, thereby dominating the many-body properties at the surface of magnetic topological insulators.