The combination of strong spin-orbit coupling and time-reversal symmetry is known to result in helical Dirac surface states in narrow band gap semiconductors such as Bi2Se3. When time-reversal symmetry in broken by introducing a magnetic order parameter, a gap opens up at the Dirac point. Electrical transport then occurs through dissipation free chiral edge states, accompanied by a precisely quantized Hall conductance, akin to the well-known quantum Hall effect, but observable even at zero magnetic field. This phenomenon is known as the “quantum anomalous Hall effect” (QAHE) and was predicted [1] and first observed [2] only recently. We provide an introductory overview of this fascinating and still incompletely understood phenomenon. We discuss recent experiments wherein we probe the unusual interplay between disorder, edge state transport and magnetism in the QAHE. In particular, we show how angle-dependent magneto-resistance measurements provide a quantitative measure of emerging edge state transport at relatively high temperature, before the full onset of the QAHE [3]. We also show how scanning SQUID microscopy reveals unexpected insights into the role of magnetic disorder underlying the QAHE [4]. Finally, we describe the surprising discovery of sharp magnetization reversal events at very low temperature, consistent with macroscopic tunneling of the magnetization in the presence of dissipation [5].
1. R. Yu et al., Science 329, 61 (2010).
2. C.-Z. Chang et al., Science 340, 167 (2013).
3. A. Kandala et al., Nature Comm. 6, 7434 (2015).
4. E. O. Lachman et al., Science Advances 1, e1500740 (2015).
5. Minhao Liu et al. arXiv:1603.02311.
