Highly-ordered wide bandgap materials for quantized anomalous Hall and magnetoelectric effects

Abstract: An interplay of spin-orbit coupling and intrinsic magnetism is known to give rise to the quantum anomalous Hall and topological magnetoelectric effects under certain conditions. Their realization could open access to low power consumption electronics as well as many fundamental phenomena like image magnetic monopoles, Majorana fermions and others. Unfortunately, being realized very recently, these effects are only accessible at extremely low temperatures and the lack of appropriate materials that would enable the temperature increase is a most severe challenge. Here, we propose a novel material platform with unique combination of properties making it perfectly suitable for the realization of both effects at elevated temperatures. The key element of the computational material design is an extension of a topological insulator (TI) surface by a thin film of ferromagnetic insulator, which is both structurally and compositionally compatible with the TI. Following this proposal we suggest a variety of specific systems and discuss their numerous advantages, in particular wide band gaps with the Fermi level located in the gap.

• The paper demonstrates a novel material platform using a topological insulator with a ferromagnetic insulator thin film to achieve a giant Dirac point gap of up to 77 meV.
• The study confirms a stable, out-of-plane ferromagnetic order via density functional theory, ensuring robust topological properties.
• The paper validates non-zero Chern numbers, underscoring the topological nature of the engineered system for future quantum electronic applications.

Highly-Ordered Wide Bandgap Materials for Quantized Anomalous Hall and Magnetoelectric Effects

The paper presents a novel advancement in the study of quantized anomalous Hall effect (QAHE) and topological magnetoelectric effect (TME) by focusing on the introduction of highly-ordered wide bandgap materials. The research critically addresses the realization of these quantized effects at more practical, elevated temperatures, which could substantially impact the development of low-power electronics.

Core Contributions and Findings

The authors propose the use of a topological insulator (TI) surface extended by a thin film of ferromagnetic insulator (FMI) as a material platform. This configuration aims to overcome the limitation of extremely low temperatures, which has been a major hurdle in practical applications. The proposed structure takes advantage of the compatibility in terms of crystal structure and composition between the TI and FMI. The research is meticulously supported by first-principles calculations which demonstrate the suitable deployment of this concept.

Key findings include:

• Giant Dirac Point Splitting: In systems like MnBi$_2$Te$_4$/Bi$_2$Te$_3$, the Dirac point gap reaches up to 77 meV, a significant increase that validates the potential to observe QAHE at higher temperatures.
• Stable Magnetic Order: The paper details a well-defined ferromagnetic order with an out-of-plane easy axis magnetization, reinforced by the exchange interactions of the FMI film, confirmed through density functional theory (DFT) calculations.
• Chern Number Calculations: The presence of non-zero Chern numbers validates the topological nature of the proposed systems, which is indicative of QAHE. More specifically, for MBT/[Bi$_2$Te$_3$]$_{n\mathrm{QL}}$/MBT systems, a Chern number of -1 is confirmed.

Theoretical and Practical Implications

The research underscores significant implications for both theoretical explorations in topological states of matter and practical developments in electronic materials. The novel material design potentially facilitates a deeper understanding of the intersection between magnetism and topology by providing a platform where none of the conventional limitations, such as inhomogeneous distribution of dopants, prevail. It paves the path for future studies focused on the interplay between spin-orbit coupling and intrinsic magnetism, making new strides in the manipulation of topological insulators.

Furthermore, from a technological perspective, realizing QAHE and TME at higher operational temperatures translates directly into more sustainable and possibly commercially viable electronic devices. As the quantization effects become more experimentally approachable, the paper hints at impending developments in fields like quantum computing, where phenomena such as Majorana fermions could be harnessed.

Future Directions

The findings invoke several avenues for subsequent research. Experimental validation of theoretical predictions using advanced fabrication techniques, like molecular beam epitaxy, stands as an imminent milestone. Additionally, probing the described theoretical models at even wider bandgaps and in other topological insulator systems remains a focal point. Moreover, exploring other candidate materials as FMIs can enhance our understanding of the versatility and applicability of this approach.

In summary, the paper presents a compelling advancement towards the practical realization of QAHE and TME at higher temperatures. It charts a course that could potentially redefine technological applications in various fields through the strategic synergy of topology, magnetism, and materials science.