The quantum anomalous Hall effect (QAHE) is a fascinating phenomenon in condensed matter physics that has attracted diverse attention and great interest in recent years. It was first predicted in the 1980s as a consequence of the quantum Hall effect, where an external magnetic field induces a quantized transverse conductance in a two-dimensional electron gas. However, unlike the quantum Hall effect, which requires a strong magnetic field to manifest, the QAHE occurs in a zero magnetic field due to intrinsic magnetization and topological properties of the material.

In a QAHE system, the electron spins align ferromagnetically, breaking time-reversal symmetry and creating a gap in the electronic energy spectrum. At the same time, a nontrivial topology of the electronic structure generates chiral edge states that run around the edges of the sample. These edge states are responsible for the quantized transverse conductance observed in experiments, while the vanishing longitudinal conductance ensures dissipation-less transport.

The unusual properties of QAHE make it an exciting area of research with potential applications in low-energy electronics, spintronics, and quantum computing. One such application is the use of QAH materials as a resistance standard operating at zero magnetic field. Additionally, QAH materials could be utilized to develop dissipationless charge/spin currents for low-power-consumption devices. Another application lies in the use of QAHE materials as quantum bits (qubits) in quantum computing, where the robustness against local perturbations like noise and disorder is crucial for reliable operation. Overall, the study of QAHE presents exciting opportunities for both fundamental research and technological application.

Despite the great triumphs achieved in the last decade, there exist several challenges associated with the study and implementation of QAHE. For example, the QAHE effect is only realized at extremely low temperatures, the mesoscopic properties of QAH materials remain largely unexplored, and finally, the long-anticipated QAHE-based Majorana zero modes, which play a pivotal role in realizing quantum computing, are still missing.

In our lab, we use state-of-the-art molecular beam epitaxy to grow QAH thin films and heterostructures, and use nanofabrication to make these films into QAH devices. We conduct cryogenic transport and spectroscopy measurements to access its electrical and magnetic properties. The ongoing project includes searching for new states of matter with high QAH temperatures, fabricating QAH devices with novel functions, exploring the diverse phases and phase transition critical phenomena in QAH materials, as well as realizing QAH-based Majorana physics.