Raise quantum anomalous Hall states up

Abstract

Experimental progress on the quantum anomalous Hall (QAH) effect has been significantly accelerated recently by the discovery of an intrinsic magnetic topological insulator MnBi2Te4 [1]. The material is natively antiferromagnetic, but an external magnetic field of several tesla can overcome its weak interlayer antiferromagnetic coupling, making it ferromagnetic. Interestingly, ferromagnetic MnBi2Te4 is predicted to be a magnetic Weyl semimetal, a topological phase hunted for almost a decade but with few cases confirmed experimentally [2]. A characteristic property of a magnetic Weyl semimetal is that its thin films can show the QAH effect with the Chern number (C), i.e. the number of the dissipationless edge channels, increasing with their thicknesses [3]. It provides an elegant way to engineer the QAH edge states for various studies and applications, but has never been experimentally demonstrated. In a recent work published in National Science Review, Prof. Jian Wang from Peking University and his collaborators observed the Hall resistance plateaus of both one quantum resistance (∼25.8 k ) and half quantum resistance (∼12.9 k ), corresponding to the C = 1 and C = 2 QAH states, respectively, in MnBi2Te4 flakes of different thicknesses under a moderate magnetic field of about 5 tesla (Fig. 1) [4]. This unambiguously confirms the magnetic Weyl semimetal phase in ferromagnetic MnBi2Te4, and, for the first time, showed us the unique aspect of magnetic Weyl semimetals. An astonishing observation is that the QAH states can survive rather a high temperature in MnBi2Te4 flakes. C = 2 QAH state is observed at T >13 K. In some C = 1 samples, almost quantized anomalous Hall resistance is observed at a temperature even higher than the magnetic ordering temperature (90.4% at 45 K in a sevenseptuple-layer device, 96.7% at 30 K in an eight-septuple-layer device). This appears counter-intuitive, but is actually a natural result of the two-dimensional magnetism of MnBi2Te4. According to theMermin-Wagner theorem, the ordering temperature of such a 2D magnetic system is not limited by the exchange energy but the magnetic anisotropic energy, which suppresses themagnetic fluctuation resulting from low dimension. A perpendicular magnetic field increases the effective anisotropic energy and thus elevates the effective magnetic ordering temperature. The topological electronic states of MnBi2Te4 are predicted to have a large magnetically induced gap Figure 1. Schematics of the C = 1 QAH state (a) and the C = 2 QAH state (b) in thinner (7 septuple-layer) and thicker (10 septuplelayer) MnBi2Te4 films, respectively. The black arrows indicate the magnetization vectors. The blue lines with arrows indicate the chiral edge states.