Since it was first isolated only four years ago, graphene—a single layer of carbon atoms with honeycomb-like lattice—has been found to exhibit a series of extraordinary electronic properties, due to its unusual electronic structure, which resembles that of relativistic particles.

Graphene is a very high quality material, which is seen by the ballistic transport of electrons over long distances. But disorder originating from missing atoms, adsorbed molecules, lattice distortions, or simply from the influence of a substrate, can dramatically affect electron motion.

Now researchers in China1 have carefully examined the effect of disorder on the transport properties of graphene from a theoretical perspective, explaining several recent unexpected experimental observations.

Fig. 1: Scheme of a grapene p-n junction. The different colours represent the different doping of the layer (red for n-type and blue for p-type).

The researchers simulated the effect of disorder on electron transport in a strip of graphene with zigzag edges (Fig. 1), with a magnetic field perpendicular to the plane. Under these conditions, the conductance exhibits a series of plateaus due to the quantum Hall effect. Recent experiments on a graphene p-n junction2—consisting of a positively charged region adjacent to a negatively charged one—revealed the emergence of unexpected plateaus. “We felt that there are still many features in the experiments that need to be explained. By numerical simulations we can also explore new physics that may be inaccessible experimentally,” says Qing-feng Sun.

In the simulations the degree of disorder was controlled very carefully. “We found that the conductance was quite small for the clean p-n junction, but that disorder drastically enhanced it.” The enhancement of the conductance allows the emergence of plateaus that cannot otherwise be observed in clean graphene. And these plateaus persist up to a very high degree of disorder, at which point the quality of the lattice is too badly damaged and the conductance deteriorates.

“We numerically reproduced all the results in the experiment,”says Sun. Most importantly, “we have shown that the new conductance plateaus are a generic feature of p-n graphene junctions regardless of the chirality or the geometry.”