Researchers have made a significant breakthrough in understanding the mechanisms behind the breakdown and restoration of topological protection in magnetic topological insulators (MTIs), paving the way for the development of low-energy topological electronics.
Magnetic Disorder: The Achilles’ Heel of Topological Insulators
One of the most important challenges to harnessing the topological properties of quantum anomalous Hall (QAH) insulators is overcoming magnetic disorder. Magnetic doping of topological insulators can lead to the non-trivial surface states theoretically predicted for ideal bismuth-based materials but breaks the topological protection, potentially at temperatures orders of magnitude less than expected from theory.
This led to the discovery of a new class of exotic materials called intrinsic magnetic topological insulators (MTIs) like MnBi2Te4. These materials exhibit non-trivial topology and inherent magnetism, which implied the possibility of stronger QAHE at elevated temperature. But in MnBi2Te4, too, the QAHE was a weak effect that appeared only up to 6.5 K — far short of the 25 K expected theoretically.
Understanding the detailed processes that lead to topological protection breaking down at the surface of these materials is therefore key in order to take them towards practical applications. That is where the Monash-led research team came in, and systematically probed the effect of surface disorder and local fluctuations on the bandgap energy, affecting chirality-induced edge states using advanced microscopy techniques.
Unveiling the Breakdown and Restoration of Topological Protection
By using low-temperature scanning tunneling microscopy and spectroscopy (STM/STS), Tian et al. probed the ultra-thin film MnBi2Te4 with an eye towards identifying the origins of QAHE breakdown.
Their results found that deep within the film were long-range fluctuations (from 0, gapless to 70 meV) in bandgap energy not associated with individual surface defects. It implies that the gapless edge state, essentially topological protected only on one side but not another side (opposite to opposite), is indeed deeply connected with bulk extended-gapless regions and hence loses its topological protection.
Crucially, they determined that the application of a small magnetic field nearly eliminated the bandgap variation with an average exchange gap of 44 meV, in good agreement with predicted values. It shows that the topological protection can be regained by adding a field that stabilizes magnetic order, at temperatures much lower than the magnetic transition of the material.
Conclusion
These results shed light on the dynamical nature of topological protection in magnetic topological insulators. The new knowledge is important for the development of low-energy topological electronics in which one can utilize the exotic properties of these materials concretely. Researchers have overcome the challenge of magnetic disorder in a new study that paves the way for very large pure spin currents, essential for the realization of highly energy-efficient topological insulator-based quantum anomalous Hall effect device technologies.
