Topological axion states in magnetic insulator MnBi$_2$Te$_4$ with the quantized magnetoelectric effect (1808.08014v5)

Abstract: Topological states of quantum matter have attracted great attention in condensed matter physics and materials science. The study of time-reversal-invariant (TRI) topological states in quantum materials has made tremendous progress in both theories and experiments. As a great success, thousands of TRI topological materials are predicted through sweeping search. Richer exotic phenomena are expected to appear in magnetic topological materials because of varied magnetic configurations, but this study falls much behind due to the complex magnetic structures and transitions. Here, we predict the tetradymite-type compound MnBi$_2$Te$_4$ and its related materials host interesting magnetic topological states. The magnetic ground state of MnBi$_2$Te$_4$ is an antiferromagnetic phase which leads to an antiferromagetic topological insulator state with a large topologically non-trivial energy gap ($\sim$0.2~eV). It is the parent state for the axion state, which has gapped bulk and surface states, and quantized topological magnetoelectric effect. The ferromagnetic phase of MnBi$_2$Te$_4$ leads to an ideal minimal type-II Weyl semimetal with two Weyl points accompanied by one hole-type and one electron-type Fermi pocket at the Fermi level, which has never been discovered elsewhere. We further present a simple and unified continuum model to capture the salient topological features of this kind of materials.

• The paper demonstrates that MnBi₂Te₄ exhibits an antiferromagnetic topological insulator state with a substantial ~0.2 eV energy gap and quantized magnetoelectric effect.
• It employs first-principles calculations to reveal gapped bulk and surface states, validating the presence of topological axion dynamics.
• The study highlights potential applications in quantum computing and spintronics by exploring controllable magnetic phase transitions in MnBi₂Te₄ and related materials.

Analysis of Topological Axion States in MnBi$_2$Te$_4$

The paper of magnetic insulators and their topological properties, particularly those with magnetic ordering, is a rapidly evolving area within condensed matter physics. The paper by Zhang et al. explores the intrinsic magnetic topological states in the compound MnBi$_2$Te$_4$, emphasizing its role as a potential host for an antiferromagnetic (AFM) topological insulator state with the quantized magnetoelectric effect. By deploying first-principles calculations, the authors present a compelling theoretical framework to predict new topologically nontrivial magnetic states in MnBi$_2$Te$_4$ and related materials, potentially offering insights into a diverse array of exotic quantum phenomena.

Key Findings

MnBi$_2$Te$_4$ emerges as a promising platform for exploring magnetic topological phenomena. The authors describe the compound's AFM ground state, characterized by a substantial topological energy gap (~0.2 eV), and identify the presence of a topological axion state. They suggest that the AFM phase of MnBi$_2$Te$_4$ exhibits the quantized topological magnetoelectric (TME) effect, which is intrinsically linked to the material's axion dynamics. This state is notable for having gapped bulk and surface states.

Additionally, the paper discusses the effects of an external magnetic field that could transition MnBi$_2$Te$_4$ into a ferromagnetic (FM) phase, potentially resulting in a minimal ideal Weyl semimetal. This transition is associated with an ideal realization of Weyl points at the Fermi level—which is a highly sought-after phenomenon due to its stability and isolation from trivial bands.

Theoretical and Practical Implications

Theoretical Insights

The paper extends the understanding of topological quantum matter to include exotic magnetic phases that were previously underexplored. The demonstration of a substantial topological energy gap in MnBi$_2$Te$_4$ is particularly significant, as it affirms the feasibility of realizing large-gap topological states in intrinsic antiferromagnets without relying on external doping or fields. The discussion of the effective Hamiltonian models deepens the theoretical comprehension of symmetry-protected topological phases, emphasizing the role of combined symmetries like $\mathcal{S}$ (an antiunitary symmetry) in determining topological classifications.

Practical Applications

On the practical side, MnBi$_2$Te$_4$'s substantial surface gap and inherent magnetism make it a prime candidate for applications that utilize the quantized TME, including topological quantum computing and robust spintronic devices. The material's capability to host a variety of topological phases suggests it can be engineered for specific applications by modulating its magnetic and structural properties.

The work also opens avenues for experimental verification and potentially for the synthesis of related tetradymite-type compounds, such as EuBi$_2$Te$_4$ or MnSb$_2$Te$_4$, that might exhibit similar or novel topological properties.

Future Research Directions

The authors hint at the possibility of exploring the dynamical axion field emerging from the Neél order fluctuations in such materials, which could lead to new topological phenomena beyond static axion states. The rich interplay between magnetism and topological order indicated in MnBi$_2$Te$_4$ also motivates further investigation into its thin-film phases, particularly with regards to the quantum anomalous Hall (QAH) effect in odd-layer films.

Furthermore, the insights drawn from MnBi$_2$Te$_4$ spur interest in designing materials with negotiated structural and electronic parameters to stabilize or transition between desirable topological phases under experimental conditions.

In summary, this paper establishes MnBi$_2$Te$_4$ as a crucial material for future studies in topological matter, with theoretical predictions that could foster experimental breakthroughs in the realization and manipulation of magnetic topological insulators and related quantum technologies.