Quantum anomalous Hall effect in magnetic topological insulators

Abstract

The quantum Hall effect, a quantum phenomenon that appears in macroscopic length scale, is one of the most important topics in condensed matter physics, is. It has long been expected that the quantum Hall effect may occur without Landau levels so that no external magnetic field nor high sample mobility is required for its studies and applications. Such a quantum Hall effect free of Landau levels can be realized in a topological insulator with its time-reversal symmetry broken by ferromagnetism as the quantized version of the anomalous Hall effect, i.e. the quantum anomalous Hall effect (see Figure 1 for the schematics of the quantum Hall effect and quantum anomalous Hall effect). Combing molecular beam epitaxy, scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we prepared high quality thin films of magnetically doped (Bi, Sb)2Te3 topological insulator with well-controlled composition, thickness and chemical potential and systematically studied their transport properties. In 5 quintuple layer thick Cr-doped (Bi, Sb)2Te3 films we experimentally observed the quantization of the Hall resistance at h/e2 at zero field, accompanied by a considerable reduction in the dissipation of electron transport. Under a strong magnetic field, the longitudinal resistance vanishes, whereas the Hall resistance remains at the quantized value (see Figure 2). The observations unambiguously demonstrate the occurrence of the quantum anomalous Hall effect. The temperature, thickness and magnetic-doping-concentration dependences of the quantum anomalous Hall effect were systematically studied, which clarifies the roles of the band structure, electron localization and magnetic order in the effect and provides clues for obtaining the effect at a higher temperature. The experimental progresses in the quantum anomalous Hall effect pave the ways for applications of dissipationless quantum Hall edge states in low-energy-consuming devices and for realizations of other novel quantum phenomena such as chiral topological superconductivity and axion electrodynamics.