Recent progress in the research of quantum anomalous <?A3B2 ACK?>Hall effect

Abstract

<p indent="0mm">The quantum anomalous Hall (QAH) effect is a quantum Hall effect that can occur without external magnetic field. The experimental realization of this effect in magnetically doped topological insulator thin films not only provides an ideal platform for research and application on chiral quantum Hall state, but also paves the way for constructing many other exotic topological quantum phases. But the realization temperature of QAH is quite low. The non-uniform magnetic disorder could be one of main reasons. To increase the uniformity of magnetic order, different element was doped, like V instead of Cr, or Cr-V co-doping, which enhance the magnetic anisotropy in the system. In Cr-V co-doping system, the realization temperature of the QAH was ten times enhanced. Also, a penta-layer structure with heavily doped sub-surface layer was formed to effectively increase the QAH temperature. In this structure, the modulation-doping originates from the suppression of the disorder in the surface state interacting with the rich magnetic moments. In the future, with combined element doping and modified structure, we may get much improved QAH temperature. To elevate the working temperature of the QAH effect, a TI sandwiched by two ferromagnetic insulator layers is also applied because of the more ordered structure and potentially high Curie temperature. Although several works on FMI/TI heterostructure reports the proximity ferromagnetic in TI, the QAH was observed at quite a low temperature in an all-telluride base heterostructure. A strong coupling between the surface state of TI and ferromagnetic layer could be required. In a magnetically doped TI system, it is natural to ask how a QAH system evolves in a magnetization reversal process. Two groups observed the zero plateau of Hall conductance around the coercive field independently, indicating transition between two QAH phases occurs via an ordinary insulator phase, which could be understood in a multi-magnetic domain structure. A similar zero plateau of Hall conductance have been observed in magnetic-doped TI (MTI) sandwich structure MTI/TI/MTI but with opposite out-of-plane magnetization direction on top and bottom surface. The system behaves like a normal insulator in a <italic>DC</italic> transport measurement but will have topological magnetoelectric effect in <italic>AC</italic> measurement, which is still a challenge. The QAH insulator is important not only as a rare topological quantum phase that has been unambiguously experimentally realized, but also as a solid and versatile building block to construct many other topological quantum states. The latter role requires a QAH system to be engineered or incorporated into various kinds of heterostructures, on which several exciting experimental progresses have been made in the past years: The QAH insulator-normal insulator heterostructures simulated high Chern number QAH insulator has been experimental reported. But it is still quite challenging because the multilayer structure acquires accurate growth parameter control; a QAH insulator acquiring proximity superconductivity from an adjacent <italic>s</italic> wave superconductor (SC) layer can show the properties of a chiral topological SC. Majorana bound states (MBSs) are expected to appear in the magnetic vortices of the topological SC and could be used to compose topological qubits which are presumably robust against decoherence with topological protection. A better-defined QAH-SC interface is crucial for further explorations in this direction to avoid disturbance of interface disorders; the first intrinsic anti-ferromagnetic TI material MnBi₂Te₄ with intercalation ferromagnetic layer MnTe was found. By alternating MBE growth of Bi₂Te₃ and MnTe, MnBi₂Te₄ thin film was formed and gapless Dirac surface state from ARPES in over 2 SL films indicate the “topology” of this material. Recently the quantized Hall resistance was reported in exfoliated thin flake of MnBi₂Te₄ with high magnetic field. The results establish that MnBi₂Te₄ could be an idea platform for further exploring topological phenomena. The recent progresses on QAH show us big opportunities to reach various novel topological states of matter experimentally. The explorations call for more exquisitely designed heterostructures and higher sample quality. With these efforts, the fantastic world of topological quantum matter is getting more and more realistic.