By Emily Hubbell

An Ohio University research team recently published a model illustrating the effects of confinement, spin-orbit interaction and the Coulomb Interaction on an electron’s movement along a zigzag graphene ribbon—research that could have potential applications for future electronic devices.

The team—comprised of assistant professor of physics and astronomy Nancy Sandler, postdoctoral research assistants Mahdi Zarea and Carlos Busser, currently at Oakland University—recently published its findings in the paper, “Unscreened Coulomb Interactions and the Quantum Spin Hall Phase in Neutral Zigzag Graphene Ribbons.” The paper appeared in Physical Review Letters in October and was featured in the Virtual Journal of Nanoscale Science and Technology.

Graphene—a single sheet of carbon that comprises graphite when layered—is considered to be the natural successor for silicon—the semiconductor used in about 99 percent of all electronics today, Zarea said.

Because the material is rigid and has high electron mobility, graphene can be used to create transistors in which electrons move without significant scattering. This high electron mobility also makes graphene a well-suited material for spintronics—a field in which researchers focus on carrying information using an electron’s spin instead of its change, Zarea said. The tiny magnet attached to electrons is called spin. 

Graphene can be cut in two ways—along the zigzag edge or along the armchair edge. The type of cut alters the movement of electrons along the graphene ribbon, Zarea said.

After cutting graphene into a zigzag ribbon, the team learned that some of the electrons moving inside the ribbon moved to the outer edges. This change to the energy of electrons is produced by transversal confinement, Zarea said, adding that this effect is unique to zigzag graphene ribbons.

Once the electrons are confined to the edges of the ribbon, their path is altered again by a new type of interaction—the spin-orbit interaction.

“Take two electrons, with one jumping from atom to atom around the other one. For the electron ‘sitting still,’ the moving electron is really a current,” Zarea said. “Any current produces a magnetic field and because all electrons are like tiny magnets (they carry spin), then the ‘sitting still’ electron spin is affected by the magnetic field produced by the moving electron.”

Including this interaction in the model illustrates why electrons redistribute themselves based on their spin. This redistribution means that the electrons along the upper edge of the ribbon have spin up and the electrons along the bottom edge have spin down.

These electrons are separated using only a voltage, not a magnetic field, Zarea said.

By taking into account the negative charge within an electron, the researchers learned that the Coulumb Interraction threatens to destroy the separation created during the spin Hall phase. The strong repulsion between electrons prevents the spin separation that they observed during the spin Hall phase.

The more narrow the graphene ribbon, the stronger the Coulomb Interaction between electrons, Zarea said. He added that for this reason wires made of graphene ribbons must be above certain limiting widths in order to carry spin-polarized currents.