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Graphene Enables Next Step Towards the Age of "Valleytronics"

Graphene opens up possibilities beyond electronics and spintronics to a new era of valleytronics

3 min read

The pair of gates sandwiching a sheet of bilayer graphene create separate electron roadways (red and blue arrows) that dramatically reduce a circuit's power consumption and decrease the amount of heat it generates.
The pair of gates sandwiching a sheet of bilayer graphene create separate electron roadways (red and blue arrows) that dramatically reduce a circuit's power consumption and decrease the amount of heat it generates.
Illustration: Zhu Lab

Researchers at the Penn State Materials Research Institute have fabricated a device made from bilayer graphene that is able to control the momentum of electrons. The result is a less energy intensive approach to computing and a new path for digital logic that leverages a new field of electron physics known as “valleytronics.”

Valleytronics is a portmanteau in which the word “valley” refers to an additional degree of freedom for electrons. Electrons are commonly manipulated based on the two degrees of freedom (or properties) that have made possible our computer age: charge and spin. Electronics have long exploited the charge of electrons to make devices that can turn on or off. And more recently, we have also seen the spin of electrons leveraged; scientists have dubbed circuits based on this property spintronics.

Both electronics and spintronics have their strengths and weaknesses when it comes to establishing the on-off states that are so critical for digital logic. So researchers have been in search of another degree of freedom in electrons that avoids those weakness, and maximizes the strengths. Instead of relying on the electrons’ spin or their charge, valleytronics exploits their energy level in relation to their momentum.

Pioneering exploration of the valley began in earnest in 2007, when researchers first discovered that the recently developed 2-D material graphene could sort electrons and holes according to which valley they occupied. Prior to this discovery, electrons and holes had only been observed occupying different valleys at random.

Now Jun Zhu, associate professor of physics at Penn State, who directed this most recent research, believes these experimental results offer a realistic approach to controlling the momentum of the electrons and determining which valley they end up occupying. To illustrate how this works, Zhu provides a useful metaphor for visualizing valleytronics. He proposes thinking of electrons as cars and the valleys either being blue or red. Typically in bilayer graphene, the electrons (or cars) could travel freely between the blue and red valleys. What Zhu and his team at Penn State have done is to mark the electrons as either blue or red, causing them to travel only to their respective valleys.

In research described in the journal Nature Nanotechnology, the Penn State team was able to direct the electrons by placing a pair of gates above and below the bilayer graphene sheet and then applying an electrical field perpendicular to the plane.

“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,” explained Jing Li, a doctoral student who worked on the research, in a press release. “In the middle, between the two sides, we leave a physical gap of about 70 nanometers.”

In this small gap are one-dimensional metallic states that can be thought of as virtual metal wires. These metal wires act as color-coded highways (to reference Zhu’s metaphor) where the electrons travel. This means that the “red” and “blue” electrons can be sorted into different valleys and sent into separate locations with very little resistance, translating into very little power consumption and a dramatic decrease in heat.

“It’s quite remarkable that such states can be created in the interior of an insulating bilayer graphene sheet, using just a few gates,” said Zhu. “They are not yet resistance-free, and we are doing more experiments to understand where resistance might come from. We are also trying to build valves that control the electron flow based on the color of the electrons.”

While the researchers concede that this research is still a long way from yielding real-world valleytronic devices, it does offer an experimental framework onto which the further development of the technology will hang.

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