Layer Hall effect in a 2D topological Axion antiferromagnet (2107.10233v1)

Abstract: While ferromagnets have been known and exploited for millennia, antiferromagnets (AFMs) were only discovered in the 1930s. The elusive nature indicates AFMs' unique properties: At large scale, due to the absence of global magnetization, AFMs may appear to behave like any non-magnetic material; However, such a seemingly mundane macroscopic magnetic property is highly nontrivial at microscopic level, where opposite spin alignment within the AFM unit cell forms a rich internal structure. In topological AFMs, such an internal structure leads to a new possibility, where topology and Berry phase can acquire distinct spatial textures. Here, we study this exciting possibility in an AFM Axion insulator, even-layered MnBi$_2$Te$_4$ flakes, where spatial degrees of freedom correspond to different layers. Remarkably, we report the observation of a new type of Hall effect, the layer Hall effect, where electrons from the top and bottom layers spontaneously deflect in opposite directions. Specifically, under no net electric field, even-layered MnBi$_2$Te$_4$ shows no anomalous Hall effect (AHE); However, applying an electric field isolates the response from one layer and leads to the surprising emergence of a large layer-polarized AHE (~50%$\frac{e^2}{h}$). Such a layer Hall effect uncovers a highly rare layer-locked Berry curvature, which serves as a unique character of the space-time $\mathcal{PT}$-symmetric AFM topological insulator state. Moreover, we found that the layer-locked Berry curvature can be manipulated by the Axion field, E$\cdot$B, which drives the system between the opposite AFM states. Our results achieve previously unavailable pathways to detect and manipulate the rich internal spatial structure of fully-compensated topological AFMs. The layer-locked Berry curvature represents a first step towards spatial engineering of Berry phase, such as through layer-specific moir\'e potential.

• The paper reveals a novel layer Hall effect driven by a layer-locked Berry curvature in even-layered MnBi2Te4.
• It employs dual-gated devices to precisely modulate charge density and electric fields, showing sign reversal of the Hall effect with field direction change.
• The results highlight significant potential for low-energy topological spintronics and advanced quantum devices.

The Layer Hall Effect in a 2D Topological Axion Antiferromagnet

The investigation of topological antiferromagnets (AFMs) has garnered substantial research interest in recent years, largely due to their potential to host unique quantum phenomena, such as the layer Hall effect, which arise from the interplay of antiferromagnetism and topological states. This paper presents an in-depth exploration of the layer Hall effect in even-layered MnBi$_2$Te$_4$ flakes, a material system poised at the intersection of magnetism, topology, and two-dimensional van der Waals structures.

The paper is grounded on the premise of a novel Hall effect observed in an Axon insulator state of MnBi$_2$Te$_4$, an AFM material. The system under consideration demonstrates a unique layer-dependent Hall effect where electrons in opposing layers deflect in opposite directions. This phenomenon is captured in the absence of a net electric field, distinguishing itself from anomalous Hall effects observed in ferromagnetic materials, which rely on a net magnetization for their detection.

Key Observations and Methodology

The researchers employed dual-gated MnBi$_2$Te$_4$ devices for their experiments, a pivotal choice that allowed precise control over charge density and the out-of-plane electric field independently. This experimental setup facilitated the detailed paper of how electric fields influence the material’s internal Berry curvature and consequently, its Hall response.

Key findings include:

• Layer-locked Berry Curvature: The application of an electric field induced a significant layer-polarized anomalous Hall effect $(\sim50\% \frac{e^2}{h})$, attributed to a layer-locked Berry curvature. This effect is driven by an electric field breaking the system's space-time $\mathcal{PT}$-symmetry.
• E-field Sign Reversal: The induced AHE flips its sign with the direction reversal of the electric field, substantiating the effect's dependency on the layer-specific Berry curvature contribution.
• Charge Density Dependence: The layer Hall effect was observed to have differing signs under electron-doped and hole-doped conditions, further evidencing the unique spatial structuring of Berry curvature in even-layered MnBi$_2$Te$_4$.
• Temperature Dependence: The AHE vanished at temperatures exceeding the Néel temperature of MnBi$_2$Te$_4$, affirming its origination from the magnetically ordered state.

Through symmetry analysis and first-principles calculations, the work highlights the intrinsic topological nature of the observed effects, which stem from macroscopically $\mathcal{PT}$-symmetric AFM states that break $\mathcal{T}$ symmetry at the microscopic level.

Theoretical and Practical Implications

Theoretical implications suggest a new pathway in the control and manipulation of quantum materials through electrical means, particularly emphasizing the potential for $\mathbf{E}\cdot\mathbf{B}$ field coupling to manipulate AFM states. This manipulation leads to controlling the layer-locked Berry curvature, which could pave the way for future developments in topological AFM spintronics.

Practically, the experimental methodology demonstrates an achievable engagement with the intrinsic properties of MnBi$_2$Te$_4$, which suggests promising applications in low-energy-dissipation quantum devices. The utilization of dual-gated devices to investigate layer-specific phenomena could inspire similar approaches across other low-dimensional topological materials.

Future Directions

The paper identifies several potential avenues for ongoing research, including:

• Layer-specific Berry Phase Engineering: Through further exploration of layer-specific Berry phases, particularly within certain heterostructures, novel magnetic and electronic properties could be developed.
• Axion Field Manipulation: The $\mathbf{E}\cdot \mathbf{B}$ field offers a novel means of tuning topological and magnetic properties, pointing to the possibility of designing advanced quantum devices that leverage these unique field effects.
• Exploiting Topological Domain Walls: Understanding the control and capabilities of topological domain walls within this context could lead to substantial breakthroughs in information storage and quantum computing.

In summary, this paper underscores an emerging frontier in condensed matter physics, where the intricate balance of AFM order, topology, and electrical control harnesses a potentially transformative capacity for novel device functionalities and fundamental scientific explorations.