Gapless surface Dirac cone in antiferromagnetic topological insulator MnBi$_2$Te$_4$ (1907.03722v2)

Abstract: The recent discovered antiferromagnetic topological insulators in Mn-Bi-Te family with intrinsic magnetic ordering have rapidly drawn broad interest since its cleaved surface state is believed to be gapped, hosting the unprecedented axion states with half-integer quantum Hall effect. Here, however, we show unambiguously by using high-resolution angle-resolved photoemission spectroscopy that a gapless Dirac cone at the (0001) surface of MnBi$_2$Te$_4$ exists between the bulk band gap. Such unexpected surface state remains unchanged across the bulk N\'eel temperature, and is even robust against severe surface degradation, indicating additional topological protection. Through symmetry analysis and $\textit{ab}$-$\textit{initio}$ calculations we consider different types of surface reconstruction of the magnetic moments as possible origins giving rise to such linear dispersion. Our results reveal that the intrinsic magnetic topological insulator hosts a rich platform to realize various topological phases such as topological crystalline insulator and time-reversal-preserved topological insulator, by tuning the magnetic configurations.

• The paper demonstrates the unexpected discovery of a gapless Dirac cone on MnBi₂Te₄’s (0001) surface using high-resolution ARPES techniques.
• It challenges prevailing theoretical models by showing that surface magnetic reconstructions can preserve gapless states despite time-reversal symmetry breaking.
• These findings pave the way for improved control of magnetic topological insulators in applications such as spintronics and quantum computing.

Gapless Surface Dirac Cone in Antiferromagnetic Topological Insulator MnBi₂Te₄

The paper investigates the electronic properties of the antiferromagnetic topological insulator MnBi₂Te₄, deploying high-resolution angle-resolved photoemission spectroscopy (ARPES) to reveal an unexpected surface state—a gapless Dirac cone—at the (0001) surface. This research contradicts prior theoretical predictions and experimental observations which suggested a sizeable magnetic gap at the surface state due to time-reversal symmetry breaking, highlighting critical insights about the topological structure of such antiferromagnetic materials.

The exploration into MnBi₂Te₄ arises from the pursuit to comprehend the interplay of magnetism and non-trivial topological phases like Quantum Anomalous Hall (QAH) and axion states within Magnetic Topological Insulators (MTIs). These materials promise considerable advancements in fields such as spintronics and quantum computing, especially when configured correctly to manifest QAH or axion states. Traditionally, such phases require precise magnetic configurations, often challenging to maintain due to natural material imperfections and external conditions.

This paper showcases compelling ARPES data, demonstrating the presence of a gapless, X-shaped Dirac cone traversing the bulk band gap of MnBi₂Te₄. This surface Dirac cone is notably robust, maintaining its gapless character across the bulk Neel temperature and proving resilient even under significant surface degradation. The research highlighted discrepancies with previous works that identified gapped bands as primarily bulk in nature—these bands exhibit clear k-dispersion, unlike the surface state.

A series of symmetry analyses and density functional theory (DFT) calculations were performed to delve into the possible origins of the observed gapless surface state. Through these analyses, the paper inferred that the anomalous surface state could emerge from surface-mediated reconstruction of the magnetic moments which diverge from the bulk configuration. Several potential magnetic reconstructions, including spin disorder or intralayer antiferromagnetic (AFM) alignments, were posited as feasible mechanisms for the surface state.

Subsequent theoretical analysis suggested that gapless Dirac states are possible via multiple symmetry protections, i.e., through time-reversal, mirror symmetry, or translation symmetries. Critically, the findings imply that surface states in MnBi₂Te₄ are likely to be highly sensitive to the surface magnetic and structural environment—variations within these factors play a decisive role in determining surface electronic properties.

The discovery underscores a requisite consideration of possible surface reconstructions in antiferromagnetic topological insulators, highlighting the nuanced behavior that can arise due to deviations from idealized magnetic configurations. It challenges previous assumptions about the pristine nature of surface states in magnetic topological insulators and sets the stage for studying similar topological material systems where surface effects profoundly influence bulk properties.

The implications of this paper extend towards advancing theoretical models and experimental methods for synthesizing magnetic topological insulators with desired topological characteristics. Moreover, it emphasizes the necessity to comprehend and control surface phenomena if one aims to harness the unusual spin-dependent transport properties promised by axion insulators and related quantum states in practical applications. Future works could explore strategies to either mitigate surface reconstruction effects for ideal state realization or exploit these reconstructions to engineer novel quantum devices.