The axion insulator which may exhibit an exotic quantized magnetoelectric effect^1,2,3,4,5,6 is one of the most interesting quantum phases predicted for the three-dimensional topological insulator (TI). The axion insulator state is expected to show up in magnetically doped TIs with magnetizations pointing inwards and outwards from the respective surfaces. Towards the realization of the axion insulator, we here engineered a TI heterostructure in which magnetic ions (Cr) are modulation-doped only in the vicinity of the top and bottom surfaces of the TI ((Bi,Sb)₂Te₃) film⁷. A separation layer between the two magnetic layers weakens interlayer coupling between them, enabling the magnetization reversal of individual layers. We demonstrate the realization of the axion insulator by observing a zero Hall plateau (ZHP) (where both the Hall and longitudinal conductivity become zero) in the electric transport properties, excluding the other possible origins for the ZHP^8,9,10. The manifestation of the axion insulator can lead to a new stage of research on novel magnetoelectric responses in topological matter.

可能表现出奇特的量化磁电效应^1,2,3,4,5,6的轴突绝缘体是为三维拓扑绝缘体（TI）预测的最有趣的量子相之一。预计轴突绝缘体状态会出现在磁性掺杂的TI中，磁化强度从相应表面向内和向外。为了实现轴突绝缘体，我们在这里设计了一种TI异质结构，其中仅在TI（（Bi，Sb）₂ Te ₃）膜⁷的顶面和底面附近调制掺杂了磁离子（Cr）。。两个磁性层之间的隔离层会削弱它们之间的层间耦合，从而使各个层的磁化反转。我们通过观察零电霍尔特性（ZHP）（霍尔和纵向电导率均变为零）证明了轴绝缘子的实现，其中不包括ZHP ^8,9,10的其他可能来源。轴绝缘子的出现可导致对拓扑物质中新型磁电响应的研究进入一个新的阶段。