Direct observation of chiral edge current at zero magnetic field in odd-layer MnBi$_2$Te$_4$

TL;DR

This study directly visualizes chiral edge currents in odd-layer MnBi₂Te₄ at zero magnetic field, demonstrating their robustness and establishing a new method to identify quantum anomalous Hall states in topological antiferromagnets.

Contribution

It provides the first direct imaging of chiral edge currents in MnBi₂Te₄, confirming their existence without the need for quantized Hall conductance.

Findings

Edge currents flow unidirectionally on odd-layer boundaries.

Chiral edge current reverses with bulk magnetization.

Edge channels coexist with finite bulk conduction.

Abstract

The chiral edge current is the boundary manifestation of the Chern number of a quantum anomalous Hall (QAH) insulator. Its direct observation is assumed to require well-quantized Hall conductance, and is so far lacking. The recently discovered van der Waals antiferromagnet MnBi$_2$Te$_4$ is theorized as a QAH in odd-layers but has shown Hall resistivity below the quantization value at zero magnetic field. Here, we perform scanning superconducting quantum interference device (sSQUID) microscopy on these seemingly failed QAH insulators to image their current distribution. When gated to the charge neutral point, our device exhibits edge current, which flows unidirectionally on the odd-layer boundary both with vacuum and with the even-layer. The chirality of such edge current reverses with the magnetization of the bulk. Surprisingly, we find the edge channels coexist with finite bulk conduction even though the bulk chemical potential is in the band gap, suggesting their robustness under significant edge-bulk scattering. Our result establishes the existence of chiral edge currents in a topological antiferromagnet and offers an alternative for identifying QAH states.