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

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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Figure 1: ZHP state in a MTI heterostructure towards an axion insulator.
Figure 2: Comparison between ZHP and QAH states in gate voltage dependence.
Figure 3: Zero longitudinal conductivity and the persistence at zero magnetic field in the ZHP state.
Figure 4: Conductivity under various magnetization alignments.

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Acknowledgements

We thank J. Wang, T. Morimoto and N. Nagaosa for fruitful discussions. We thank T. Yokouchi for experimental support. This research was supported by the Japan Society for the Promotion of Science through the Funding Program for World-Leading Innovative R & D on Science and Technology (FIRST Program) on ‘Quantum Science on Strong Correlation’ initiated by the Council for Science and Technology Policy, JSPS/MEXT Grant-in-Aid for Scientific Research (No. 24224009, 24226002, 15H05867), and CREST, JST.

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Authors

Contributions

M.M. and R.Y. grew and characterized the samples. M.M. and M.Kawamura conducted the device fabrication. M.Kawamura, M.M., R.Y. and Y.K. performed transport measurements. N.S. and M.M. conducted magnetization measurements. M.M. and M.Kawamura analysed the data. A.T., K.S.T., M.Kawasaki and Y.T. contributed to discussion of the results and guided the project. Y.T. conceived and coordinated the project.

Corresponding authors

Correspondence to M. Mogi or Y. Tokura.

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The authors declare no competing financial interests.

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Mogi, M., Kawamura, M., Yoshimi, R. et al. A magnetic heterostructure of topological insulators as a candidate for an axion insulator. Nature Mater 16, 516–521 (2017). https://doi.org/10.1038/nmat4855

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