Graphene consists of single layers of carbon atoms that one would normally find stacked in graphite. Steven Louie from the University of California in Berkeley has now looked at long "nanoribbons" of graphene, which like a semiconductor contain both occupied and unoccupied electron states separated by an energy gap. But because of the peculiar zigzag geometry left by broken hexagonal bonds, the electrons at opposite edges of a graphene ribbon are in different states. On one edge, the occupied states are spin-up and the unoccupied states are spin-down, whereas on the other edge the reverse is true. (See image: "Moving state".)

Louie discovered that applying an electric field across the zigzag edges shifts the energies of the states, thus eliminating the gap for one spin orientation between the occupied states on one edge and the unoccupied states on the other. In other words, electrons within those spin states can conduct freely, but electrons within the opposite spin state cannot. Louie said that this "half-metallic" behaviour could find applications in the emerging area spintronics, where electronic spin as well as charge could be used to govern the movement of electron current.

According to Louie, the effect could be tailored by altering the width of the graphene ribbon. In a wider ribbon, for example, the interaction between the states at opposite edges would be weak because they would not overlap much, meaning that the gap would be closed less than in a narrower ribbon. Louie found that if the ribbon is more than 32 chains of atoms wide, the interaction would become negligible and the states would be unable to maintain their opposing spins. On the other hand, he said, a thin ribbon would need a much larger electric field to make the half-metal transformation.