Controlling electron ‘colour’ step forward in valleytronics

Article By : Walt Mills

The experimental device of bilayer graphene proves the ability to control the momentum of electrons, offering a path to electronics that could require less energy and give off less heat.

Researchers at Penn State have developed an experimental proof of the ability to control the momentum of electrons, offering a path to electronics that could require less energy and give off less heat than standard silicon-based transistors.

The device is made of bilayer graphene, an atomically thin hexagonal arrangement of carbon atoms. Researchers considered it a step forward in a new field of physics called valleytronics.

"Current silicon-based transistor devices rely on the charge of electrons to turn the device on or off, but many labs are looking at new ways to manipulate electrons based on other variables, called degrees of freedom," said Jun Zhu, associate professor of physics, Penn State, who directed the research. "Charge is one degree of freedom. Electron spin is another, and the ability to build transistors based on spin, called spintronics, is still in the development stage. A third electronic degree of freedom is the valley state of electrons, which is based on their energy in relation to their momentum."

[EETI graphene 1]
__Figure 1:__ *One-dimensional wires created in bilayer graphene gated by two pairs of split gates above and below the sheet. Wires traveling in opposite directions carry electrons of different valley states labelled as K and K’ in the figure. (Source: Jun Zhu/Penn State)*

Think of electrons as cars and the valley states as blue and red colours, Zhu suggested, just as a way to differentiate them. Inside a sheet of bilayer graphene, electrons will normally occupy both red and blue valley states and travel in all directions. The device her Ph.D. student, Jing Li, has been working on can make the red cars go in one direction and the blue cars in the opposite direction.

"The system that Jing created puts a pair of gates above and below a bilayer graphene sheet. Then he adds an electric field perpendicular to the plane," Zhu said.

"By applying a positive voltage on one side and a negative voltage on the other, a bandgap opens in bilayer graphene, which it doesn't normally have," Li explained. "In the middle, between the two sides, we leave a physical gap of about 70 nanometres."

Inside this gap are live one-dimensional metallic states, or wires, that are colour-coded freeways for electrons. The red cars travel in one direction and the blue cars travel in the opposite direction.

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