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Advancements in Understanding Magnetic Topological Insulators
In a significant breakthrough, a research team led by Monash University has revealed insights into the challenges posed by magnetic disorder in quantum anomalous Hall (QAH) insulators. Their findings provide a deeper understanding of how these materials can potentially be utilized in low-energy electronics, highlighting the impact of magnetic fields on restoring topological protection.
The study is detailed in the paper titled “Imaging the Breakdown and Restoration of Topological Protection in Magnetic Topological Insulator MnBi2Te4,” published in Advanced Materials.
“This research outlines a clear direction forward for incorporating magnetic topological insulators (MTIs) into the realm of low-energy topological electronics,” stated Qile Li, a Ph.D. candidate at Monash University and lead author of the study.
The Challenge of Magnetic Disorder
Combining magnetism with topological effects can lead to the manifestation of the QAH effect, which enables the unimpeded flow of electrical currents along one-dimensional edges over substantial distances. However, the reliability of this current flow has been less than satisfactory. Notably, in magnetically doped topological insulators, the QAHE tends to collapse at temperatures exceeding 1 Kelvin, far lower than theoretical predictions.
A promising alternative is presented by intrinsic magnetic topological insulators like MnBi2Te4, which are theorized to maintain more robust QAH effects at elevated temperatures. Experimental results suggest that the QAHE in MnBi2Te4 can persist up to 1.4 K, and intriguingly, can increase to 6.5 K when exposed to stabilizing magnetic fields. This enhances our understanding of the factors that lead to the destabilization of topological protection.
Despite this progress, 6.5 K still falls short of the theoretical limit of 25 K. To unlock practical applications, it remains crucial to further elevate this temperature by gaining insight into the mechanisms that cause topological protection to falter at the surface of these materials.
Exploration of Edge States and Surface Disorder
The Monash-led team employed state-of-the-art techniques in low-temperature scanning tunneling microscopy and spectroscopy (STM/STS) to investigate a five-layer ultra-thin film of MnBi2Te4. This approach allowed them to closely analyze the relationship among surface disorder, local bandgap fluctuations, and chiral edge states.
By examining the properties of the bandgap near crystal defects, as well as in the film’s edge and bulk, the researchers aimed to decipher the breakdown mechanisms of QAHE. They also applied low magnetic fields, which showed potential in restoring the bandgap and QAHE.
Findings on Bandgap Fluctuations
The results from examining five-layer MnBi2Te4 uncovered long-range variations in bandgap energy, fluctuating between 0 (indicating a gapless state) and 70 meV, without correlation to individual surface defects. Crucially, the research demonstrated that gapless edge states, characteristic of QAH insulators, are not isolated but interact with extensive gapless regions within the bulk material.
Moreover, the application of a magnetic field significantly mitigated bandgap fluctuations, elevating the average exchange gap to 44 meV, aligning with theoretical expectations.
“Our findings elucidate the mechanisms underlying the breakdown of topological protection and the efficacy of magnetic fields in reinstating it,” remarked Dr. Mark Edmonds, a corresponding author and FLEET Associate Investigator at Monash University.
For further reading: Qile Li et al, Imaging the Breakdown and Restoration of Topological Protection in Magnetic Topological Insulator MnBi2Te4, Advanced Materials (2024). DOI: 10.1002/adma.202312004
Citation: Overcoming magnetic disorder: Toward low-energy topological electronics (2024, September 11) retrieved 11 September 2024 from https://phys.org/news/2024-09-magnetic-disorder-energy-topological-electronics.html
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