Prof. X. C. Xie and his collaborators publish a paper on Physical Review Letters reporting the phase transition of a disordered MnBi2Te4 under an external magnetic field

Recently, Prof. Xin-Cheng Xie from Peking University and his collaborators investigate the phase transition of a disordered MnBi₂Te₄ under an external magnetic field. The authors elucidate a physical picture based on the Anderson localization to understand the coexistence states of quantum Hall (QH) and quantum anomalous Hall (QAH) effects. They also explain the observed Hall plateaus in experiments [Deng et al., Science 367, 895 (2020)] and propose more coexistence states of QAH and QH to be verified by further experiments. This work has been published in Physical Review Letters [Phys. Rev. Lett. 127, 236402. https://link.aps.org/doi/10.1103/PhysRevLett.127.236402].

Since successfully synthesized in experiments, MnBi₂Te₄ as an intrinsic magnetic topological insulator has attracted great attention. However, to observe quantized Hall plateau, an external magnetic field is applied in most cases. On one hand, the magnetic field can induce an exchange gap to make it easier to observe the QAH effect, which gives rise to higher Chern numbers under stronger magnetic fields. Thus, whether the nonzero Chern number (C≠0) phase is a QAH state, or a QH state, or a coexistence state of both is still a puzzle. Usually, the fabricated MnBi₂Te₄in experiments owns extremely low mobility from 74 cm²·V^-1·s^-1～1500 cm²·V^-1·s^-1, which suggests that the Anderson localization may play a key role in these systems. Thus, the authors elucidate a physical picture based on the Anderson localization and construct coexistence states of QH and QAH effects.

Specifically, the bulk states are localized without a magnetic field, and a C=1 QAH phase emerges. However, under a strong magnetic field, localized bulk states can still form Landau bands, and the QH edge state coexists with the QAH edge state, leading to a C=2 phase. Thus, the authors attribute the observed Hall plateaus to a phase transition from a pure QAH phase to a phase composing of QAH and QH edge states in disordered MnBi₂Te₄. They also propose more coexistence states of QAH and QH effects to be verified by further experiments.

Hailong Li (a Ph. D. candidate in Prof. X. C. Xie’s group) and Prof. Chui-Zhen Chen from Soochow University are co-first authors, and both Prof. X. C. Xie from Peking University and Prof. Hua Jiang from Soochow University are the corresponding authors of this Letter. 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) A schematic plot of the coexisting phase of QH and QAH states in a disordered MnBi₂Te₄  device under a strong magnetic field; (b) Hall resistance under different Fermi energies.; (c) Phase diagram.