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Ferromagnetic topological insulator and quantum transport

In topological insulators (TI) the Dirac surface states are induced by the strong spin-orbit coupling (SOC) as well as protected by the time reversal symmetry (TRS). Breaking TRS in a TI with ferromagnetic perturbation can lead to many exotic quantum phenomena including the quantum anomalous Hall (QAH) effect. QAH state describes the dissipationless quantized Hall transport in ferromagnetic materials in the absence of external magnetic fields. The spontaneously broken TRS states can be introduced into TI by ferromagnetic ordering, achievable by two methods: i) conventional way, by doping with a magnetic element, ii) by ferromagnetic proximity coupling. The realization of the QAH effect in realistic materials required two conditions: ferromagnetic insulating materials and topologically non-trivial electronic band structures. We shall describe how in a hard ferromagnetic TI system a robust QAH state and dissipationless edge current flow is achieved. In another development, we have shown in the case of a TI interfaced with a ferromagnetic insulator (FMI), where we could drive the TI into magnetic state through interfacial exchange interaction to much higher temperatures than the Curie temperature of the FMI. The realization of QAH state and the occurrence of dissipationless conduction in hard ferromagnetic TIs could lead towards dissipationless electronic applications in the absence of external fields, making such devices more amenable for metrology and spintronics applications.


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  2. C-Z Chang et al., “High-precision realization of robust quantum anomalous Hall state in a hard ferromagnetic topological insulator” Nat. Matl 13, 473-477 (2015)
  3. C-Z Chang, et al, “Zero-Field Dissipationless Chiral Edge Transport and the Nature of Dissipation in the Quantum Anomalous Hall State”  Phys. Rev. Lett. 115, 057206 (2015)