Topological insulators are a new class of materials that act like insulators in their bulk but allow charge transport at their surface. Their complex character has made topological insulators one of the hottest research topics in condensed matter physics, with potential uses in ‘spintronics’ — electronics that also exploits an electron’s spin. Now, researchers from the Chinese Academy of Sciences in Beijing and Stanford University in the USA1 have predicted that topological insulators are also good candidates for observation of the ‘quantum anomalous Hall effect’ (QAHE).

Fig. 1: The classical (left) and anomalous (right) Hall effects. In the classical Hall effect, an electrical potential is generated perpendicular to an external magnetic field and an electrical current flowing through a conductor. In the anomalous Hall effect, the same potential is produced without an external magnetic field by the intrinsic magnetization of the magnetic conductor.

The classical Hall effect describes how a magnetic field applied perpendicular to an electric current generates an electric potential that is orthogonal to both the current and magnetic field. When the generated voltage appears only in discrete steps, or quanta, it is referred to as the quantum Hall effect. Some magnetic materials display an anomalous Hall effect, by which a voltage appears even in the absence of an external magnetic field, originating from strong coupling between the conducting electrons and the magnetic moments of the material’s atoms. Whether the anomalous form of the quantum Hall effect can be realized experimentally, however, has yet to be determined. “Observing the QAHE is a very big issue in condensed matter physics,” says Xi Dai from the research team. “Through the QAHE we can realize dissipation-free electrical transport at temperatures close to room temperature.” The absence of any loss in electrical current would be of significant benefit for spintronics applications and could be an important step towards applications in quantum computing.

The newly discovered topological insulators have now been shown to be natural candidates for the observation of the QAHE, as their electrons display strong coupling with the magnetic moments of the material’s atoms. In their study, the researchers demonstrated that to make the QAHE work, existing topological insulators such as Sb2Te3 need to be 'doped' with magnetic elements. This has been investigated before, but the insufficient quality of samples fabricated so far means that impurities had made the materials electrically conductive, when topological insulators need to be electrically insulating. Dai is optimistic that once samples with sufficient quality can be fabricated, “we have the chance to discover the QAHE experimentally.”