A magnetic heterostructure of topological insulators as a candidate for an axion insulator

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Abstract
The axion insulator which may exhibit an exotic quantized magnetoelectric effect1,2,3,4,5,6 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 ZHP8,9,10. The manifestation of the axion insulator can lead to a new stage of research on novel magnetoelectric responses in topological matter.

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