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Theorists at The University of Texas at Dallas, along with colleagues in Germany, have for the first time observed a rare phenomenon called the quantum anomalous Hall effect in a very simple material. Previous experiments have detected it only in complex or delicate materials.

Dr. Fan Zhang, associate professor of physics in the School of Natural Sciences and Mathematics, is an author of a study published on Oct. 6 in the journal Nature that demonstrates the exotic behavior in bilayer graphene, which is a naturally occurring, two-atom thin layer of carbon atoms arranged in two honeycomb lattices stacked together.

The quantum Hall effect is a macroscopic phenomenon in which the transverse resistance in a material changes by quantized values in a stepwise fashion. It occurs in two-dimensional electron systems at low temperatures and under strong magnetic fields. In the absence of an external magnetic field, however, a 2D system may spontaneously generate its own magnetic field, for example, through an orbital ferromagnetism that is produced by interactions among electrons. This behavior is called the quantum anomalous Hall effect.

"When the rare quantum anomalous Hall effect was investigated previously, the materials studied were complex," Zhang said. "By contrast, our material is comparably simple, since it just consists of two layers of graphene and occurs naturally."

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