The elementary charge is a fundamental constant of the universe. We call it simply e. Protons have a value of +e and electrons are -e. Depending on how familiar you are with physics, you might have heard that the quarks that make protons have a fractional charge, but we do not worry about that because they are never by themselves. And electrons don’t have components, so the elementary charge is truly elementary in all material interactions. Well, until it isn’t, as a new study has demonstrated. 

Overwhelmingly across materials and phenomena, the charge of an electron is -e, but some materials experience the fractional quantum Hall effect. In a handful of systems, under very high and carefully tuned magnetic fields, an exotic electronic state develops where its charge is no longer -e.

The new work uses graphene, which is considered a very interesting material. It is a single layer of carbon atoms, but it is incredibly strong and a good conductor. In this experiment, the team stuck five layers of graphene together like steps on a staircase, stamped them between two hexagonal boron nitride layers, and put the hybrid material at extremely low temperatures. The team saw something very weird as they sent electrons through this material.

Electrons passed through it as fractions of the total charge - but there was no external magnetic field. It is therefore the first evidence of the fractional quantum anomalous Hall effect in crystalline graphene, deemed anomalous because it does not have a magnetic field. Researchers were not expecting graphene to be able to do that.

"This five-layer graphene is a material system where many good surprises happen," study author Long Ju, assistant professor of physics at MIT, said in a statement. "Fractional charge is just so exotic, and now we can realize 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."

This is not the first time that the team witnessed something peculiar in a pentalayer of graphene. They reported last year that it also exhibited a "multiferroic" state. Twisted graphene is also superconductive at a very low temperature – just 1.7 Kelvins above absolute zero. The lab had a new fridge installed just last summer to make these investigations.

"The day we saw it, we didn't recognize it at first," said lead author Zhengguang Lu. "Then we started to shout as we realized, this was really big. It was a completely surprising moment."

"This was probably the first serious samples we put in the new fridge," added co-first author Tonghang Han. "Once we calmed down, we looked in detail to make sure that what we were seeing was real."

The group will continue to explore how multilayers of graphene might showcase different and rare electronic states.

"We are diving in to explore many fundamental physics ideas and applications," Ju added. "We know there will be more to come."

The study is published in the journal Nature.