Rare fractions of electron charge seen in graphene

In rare cases, electrons can show a fraction of their usual charge. It’s an effect that’s only been seen a handful of times – now physicists at the Massachusetts Institute of Technology (MIT) have seen it in graphene.

And it could prove useful in technologies such as quantum computing.

Electrons have a single negative charge, equal and opposite to the positive charge of the proton.

But it’s not that simple, as it turns out. Electrons can also have a “fractional charge.”

Physicists call this the “fractional quantum Hall effect.” It is usually seen in very high, carefully maintained magnetic fields. But recently, it’s been shown that such strong magnetic fields are not necessary to see fractional electron charges.

Four scientists standing in front of fridge in lab quantum
A photo of the team. Credit: Jixiang Yang.

The MIT condensed matter physicists found the effect in a much simpler material: 5 layers of graphene. Their results are described in a paper published in Nature.

Graphene, which can be extracted from graphite, like that in a pencil, is made up of layers of carbon atoms arranged in a hexagonal pattern.

When stacked like steps on a staircase, the structure produces some unique conditions which allows electrons to pass through the layers as fractions of their total charge. No external magnetic field needed.

“This five-layer graphene is a material system where many good surprises happen,” says study author Long Ju, assistant professor of physics at MIT.

“Fractional charge is just so exotic, and now we can realise this effect with a much simpler system and without a magnetic field. That in itself is important for fundamental physics. And it could enable the possibility for a type of quantum computing that is more robust against perturbation.”

Quantum computers usually suffer from deformation and disturbances due to their quantum nature.

Topological quantum computers could be more secure because their topology (arrangement) makes them unchanging in the face of such disturbances. They could be built based on a combination of the fractional quantum Hall effect and superconductors. But the powerful magnetic fields needed for the fractional quantum Hall effect in most materials would kill the superconductor.

“Graphene can also be a superconductor,” Ju says. “So, you could have two totally different effects in the same material, right next to each other. If you use graphene to talk to graphene, it avoids a lot of unwanted effects when bridging graphene with other materials.” 

The team is looking for other rare electron modes in multilayer graphene. 

“We are diving in to explore many fundamental physics ideas and applications. We know there will be more to come.” 

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