Electrons grow to be fractions of themselves in graphene

But in very special states of matter, electrons can splinter into fractions of their whole. This phenomenon, often known as “fractional charge,” is exceedingly rare, and if it will probably be corralled and controlled, the exotic electronic state could help to construct resilient, fault-tolerant quantum computers.

Up to now, this effect, known to physicists because the “fractional quantum Hall effect,” has been observed a handful of times, and mostly under very high, fastidiously maintained magnetic fields. Only recently have scientists seen the effect in a cloth that didn’t require such powerful magnetic manipulation.

Now, MIT physicists have observed the elusive fractional charge effect, this time in an easier material: five layers of graphene — an atom-thin layer of carbon that stems from graphite and customary pencil lead. They report their leads to Nature.

They found that when five sheets of graphene are stacked like steps on a staircase, the resulting structure inherently provides just the best conditions for electrons to go through as fractions of their total charge, without having for any external magnetic field.

The outcomes are the primary evidence of the “fractional quantum anomalous Hall effect” (the term “anomalous” refers back to the absence of a magnetic field) in crystalline graphene, a cloth that physicists didn’t expect to exhibit this effect.

“This five-layer graphene is a cloth system where many good surprises occur,” says study creator Long Ju, assistant professor of physics at MIT. “Fractional charge is just so exotic, and now we will realize this effect with a much simpler system and with out a magnetic field. That in itself is significant for fundamental physics. And it could enable the chance for a style of quantum computing that’s more robust against perturbation.”

Ju’s MIT co-authors are lead creator Zhengguang Lu, Tonghang Han, Yuxuan Yao, Aidan Reddy, Jixiang Yang, Junseok Search engine optimisation, and Liang Fu, together with Kenji Watanabe and Takashi Taniguchi on the National Institute for Materials Science in Japan.

A bizarre state

The fractional quantum Hall effect is an example of the weird phenomena that may arise when particles shift from behaving as individual units to acting together as a complete. This collective “correlated” behavior emerges in special states, as an example when electrons are slowed from their normally frenetic pace to a crawl that allows the particles to sense one another and interact. These interactions can produce rare electronic states, corresponding to the seemingly unorthodox splitting of an electron’s charge.

In 1982, scientists discovered the fractional quantum Hall effect in heterostructures of gallium arsenide, where a gas of electrons confined in a two-dimensional plane is placed under high magnetic fields. The invention later won the group a Nobel Prize in Physics.

“[The discovery] was a really big deal, because these unit charges interacting in a solution to give something like fractional charge was very, very bizarre,” Ju says. “On the time, there have been no theory predictions, and the experiments surprised everyone.”

Those researchers achieved their groundbreaking results using magnetic fields to decelerate the fabric’s electrons enough for them to interact. The fields they worked with were about 10 times stronger than what typically powers an MRI machine.

In August 2023, scientists on the University of Washington reported the primary evidence of fractional charge with out a magnetic field. They observed this “anomalous” version of the effect, in a twisted semiconductor called molybdenum ditelluride. The group prepared the fabric in a particular configuration, which theorists predicted would give the fabric an inherent magnetic field, enough to encourage electrons to fractionalize with none external magnetic control.

The “no magnets” result opened a promising path to topological quantum computing — a safer type of quantum computing, through which the added ingredient of topology (a property that continues to be unchanged within the face of weak deformation or disturbance) gives a qubit added protection when carrying out a computation. This computation scheme relies on a mixture of fractional quantum Hall effect and a superconductor. It was almost inconceivable to understand: One needs a powerful magnetic field to get fractional charge, while the identical magnetic field will normally kill the superconductor. On this case the fractional charges would function a qubit (the essential unit of a quantum computer).

Making steps

That very same month, Ju and his team happened to also observe signs of anomalous fractional charge in graphene — a cloth for which there had been no predictions for exhibiting such an effect.

Ju’s group has been exploring electronic behavior in graphene, which by itself has exhibited exceptional properties. Most recently, Ju’s group has looked into pentalayer graphene — a structure of 5 graphene sheets, each stacked barely off from the opposite, like steps on a staircase. Such pentalayer graphene structure is embedded in graphite and will be obtained by exfoliation using Scotch tape. When placed in a refrigerator at ultracold temperatures, the structure’s electrons slow to a crawl and interact in ways they normally would not when whizzing around at higher temperatures.

Of their recent work, the researchers did some calculations and located that electrons might interact with one another much more strongly if the pentalayer structure were aligned with hexagonal boron nitride (hBN) — a cloth that has the same atomic structure to that of graphene, but with barely different dimensions. Together, the 2 materials should produce a moiré superlattice — an intricate, scaffold-like atomic structure that would slow electrons down in ways in which mimic a magnetic field.

“We did these calculations, then thought, let’s go for it,” says Ju, who happened to put in a brand new dilution refrigerator in his MIT lab last summer, which the team planned to make use of to chill materials all the way down to ultralow temperatures, to review exotic electronic behavior.

The researchers fabricated two samples of the hybrid graphene structure by first exfoliating graphene layers from a block of graphite, then using optical tools to discover five-layered flakes within the steplike configuration. They then stamped the graphene flake onto an hBN flake and placed a second hBN flake over the graphene structure. Finally, they attached electrodes to the structure and placed it within the refrigerator, set to close absolute zero.

As they applied a current to the fabric and measured the voltage output, they began to see signatures of fractional charge, where the voltage equals the present multiplied by a fractional number and a few fundamental physics constants.

“The day we saw it, we didn’t recognize it at first,” says first creator Lu. “Then we began to shout as we realized, this was really big. It was a totally surprising moment.”

“This was probably the primary serious samples we put in the brand new fridge,” adds co-first creator Han. “Once we calmed down, we looked intimately to be sure that that what we were seeing was real.”

With further evaluation, the team confirmed that the graphene structure indeed exhibited the fractional quantum anomalous Hall effect. It’s the primary time the effect has been seen in graphene.

“Graphene may also be a superconductor,” Ju says. “So, you can have two totally different effects in the identical material, right next to one another. In case you use graphene to confer with graphene, it avoids a whole lot of negative effects when bridging graphene with other materials.”

For now, the group is constant to explore multilayer graphene for other rare electronic states.

“We’re diving in to explore many fundamental physics ideas and applications,” he says. “We all know there will probably be more to return.”

This research is supported partially by the Sloan Foundation, and the National Science Foundation.