Insulators are not, by definition, good carriers of electrical current, but researchers in the US have discovered that a special type of insulator known as a quantum anomalous Hall insulator can nevertheless support electrical current within its interior. This surprising result – current was only thought to flow along the edges of such materials – could aid the development of next-generation quantum devices based on so-called topological insulators.

In the ordinary quantum Hall effect, which was discovered in 1980, the interior of a sample becomes an insulator when a strong magnetic field is applied to it. However, an electrical current still flows – in a single direction – along the edges of the sample. This leads to values of the resistance of the sample becoming quantized, or restricted to certain values determined by fundamental physical constants.

The quantum anomalous Hall effect (QAH) is slightly different. Here, the effect arises in a material that is already magnetized, and the material’s resistance is quantized even at even at weak (or indeed zero) magnetic fields. The result is that the current-carrying electrons travel at high speeds along the sample edge without dissipating energy – rather like what happens in a superconductor.

In the new work, which is described in Nature Materials, researchers led by physicist Katja Nowack of Cornell University studied a ferromagnetic topological insulator called chromium-doped bismuth antimony telluride. This the material in which the QAH was first observed in 2013. Using high-sensitivity magnetic imaging at very low temperatures, members of the team, which also included researchers at Pennsylvania State University, found that the previous “edge picture” may not be the only explanation for the material’s quantized resistance.

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