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Prof. X. C. Xie and his collaborators publish a paper on Physical Review Letters reporting the disorder-induced phase transitions of an axion insulator and its critical behaviors

Recently, Prof. Xin-Cheng Xie from Peking University and his collaborators investigate the disorder-induced phase transition of an axion insulator state and elucidate a global phase diagram. The authors also find that there exists a 2D phase transition which shares the same universality class with the quantum Hall plateau to plateau transition. Therefore, they propose to probe the axion insulator state by investigating the universal signature of 2D quantum-Hall-type critical behaviors in the 3D magnetic TI. This work has been published in Physical Review Letters [Phys. Rev. Lett. 126, 156601. https://link.aps.org/doi/10.1103/PhysRevLett.126.156601].

Topology and symmetry breaking play a key role in describing phases of matter. The axion insulator state shows up when the time-reversal symmetry of surface states of a 3D TI is broken. In comparison with a trivial insulator state, the axion insulator state possesses a unique electromagnetic response from the massive Dirac surface states, giving rise to novel phenomena such as a quantized topological magnetoelectric effect and half-quantized surface Hall conductance. Thus, the search for an axion insulating state in the ferromagnetic-3D topological insulator (TI) heterostructure and MnBi2Te4 has attracted extensive attention in recent years. Although three groups recently claimed the discovery of axion insulators in ferromagnet-TI heterostructures and antiferromagnetic TI MnBi2Te4, respectively, the evidence that the axion insulator shows huge longitudinal resistance and zero Hall conductance coincides with the behavior of a trivial insulator. Therefore, a definitive experimental evidence for the axion insulator state is still missing.

Typical properties of Anderson phase transitions in disordered systems, especially the critical exponents, depend only on general properties of the model, such as spatial dimensionality, symmetry, etc. The experimental studies of the quantum phase transition have already been extensively performed in magnetic TIs and revealed unique properties of topological states. Thus, they propose probing the axion insulator state by shedding light on the disorder-induced metal-insulator transition in 3D magnetic TIs.

Specifically, with the weak disorder and increase in the Fermi energy, the axion insulator will undergo a 2D delocalized transition on the surfaces of the 3D TI, become an Anderson insulator, and then transform into a diffusive metal after a 3D insulator metal transition. They also provide a phenomenological theory that relates the disordered axion insulator to the Chalker-Coddington network model. Furthermore, the 2D phase transition remains in the presence of bulk antiferromagnetism for MnBi2Te4, and thereby, it is model independent. The universal phase transition behavior of the axion insulator we predicted can be detected in the ferromagnetic-3D TI heterostructure and the antiferromagnetic TI MnBi2Te4.

Hailong Li (a Ph. D. candidate in Prof. X. C. Xie’s group) is the first author, and both Prof. X. C. Xie from Peking University and Prof. Chui-Zhen Chen from Soochow University are the corresponding authors of this Letter. Other collaborators include Prof. Hua Jiang from Soochow University. This work is financially supported by the National Basic Research Program of China, the National Natural Science Foundation of China, the Strategic Priority Research Program of the Chinese Academy of Sciences, the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions, and the Natural Science Foundation of Jiangsu Province.

Figure: (a) Schematic plot of an axion insulator; (b) Chiral edge states along domain walls scatter with each other and form conducting channels; (c) Renormalized localization length as a function of the Fermi energy; (d) A numerical fit of the one-parameter scaling; (e) A phase diagram of a disordered axion insulator.