Electrons become fractions of themselves in graphene, study finds

The electron is the basic unit of electricity because it carries a single negative charge. This is what we learn in physics in high school, and it is mostly the case for most natural materials.

But in very special states of matter, electrons can split into fractions. This phenomenon, known as “fractional charging,” is extremely rare, and if it can be pinned down and controlled, the exotic electronic state could help build resilient, fault-tolerant quantum computers.

To date, this effect, known to physicists as the “fractional quantum Hall effect,” has been observed repeatedly, and mostly under very high, carefully maintained magnetic fields. Only recently have scientists observed this effect in a material that did not require such powerful magnetic manipulation.

Now MIT physicists have observed the elusive fractional charge effect, this time in a simpler material: five layers of graphene, an atom-sized layer of carbon that comes from graphite and mine of common pencil. They report their results in Nature.

They discovered that when five sheets of graphene are stacked like steps on a staircase, the resulting structure inherently provides the ideal conditions for electrons to pass through as fractions of their total charge, without the need for a field. external magnetic.

The results are the first evidence of the “fractional quantum anomalous Hall effect” (the term “anomalous” refers to the absence of a magnetic field) in crystalline graphene, a material that physicists did not expect to exhibit this effect.

“This five-layer graphene is a material system in which many good surprises occur,” says study author Long Ju, an assistant professor of physics at MIT. “Fractional charge is so exotic, and we can now achieve this effect with a much simpler system and without a magnetic field. That in itself is important for fundamental physics. And it could pave the way for a more simple type of quantum computing. complex.” robust against disturbances.

Ju’s co-authors at MIT are lead authors Zhengguang Lu, Tonghang Han, Yuxuan Yao, Aidan Reddy, Jixiang Yang, Junseok Seo and Liang Fu, as well as Kenji Watanabe and Takashi Taniguchi of the National Institute of Materials Science at Japan.

A weird state

The fractional quantum Hall effect is an example of the strange phenomena that can occur when particles move from behaving as individual units to behaving as a whole. This “correlated” collective behavior appears in particular states, for example when electrons are slowed from their normally frenetic pace to a rate that allows particles to sense each other and interact. These interactions can produce rare electronic states, such as the seemingly unorthodox splitting of an electron’s charge.

In 1982, scientists discovered the fractional quantum Hall effect in gallium arsenide heterostructures, where an electron gas confined in a two-dimensional plane is placed under high magnetic fields. This discovery later earned the group a Nobel Prize in physics.

“(The discovery) was very important, because these unit charges interacting in a way to give something like a fractional charge was very, very bizarre,” says Ju. “At the time, there were no theoretical predictions and the experiments surprised everyone.”

These researchers achieved groundbreaking results using magnetic fields to slow down the material’s electrons enough so that they could interact. The fields they worked with were about 10 times more powerful than those that typically power an MRI machine.

In August 2023, scientists at the University of Washington reported the first evidence of a fractional charge without a magnetic field. They observed this “anomalous” version of the effect, in a twisted semiconductor called molybdenum ditelluride. The group prepared the material in a specific configuration, which theorists believe would give the material an inherent magnetic field, enough to encourage electrons to split apart without any external magnetic control.

The “magnet-free” result opened a promising path toward topological quantum computing: a more secure form of quantum computing, in which the additional ingredient of topology (a property that remains unchanged in the face of small deformation or disturbance) provides additional protection to the qubit. when performing a calculation.

This calculation scheme is based on a combination of fractional quantum Hall effect and a superconductor. In the past, this was almost impossible to achieve: you need a strong magnetic field to get a fractional charge, while the same magnetic field usually kills the superconductor. In this case, the fractional charges would serve as a qubit (the basic unit of a quantum computer).

take steps

The same month, Ju and his team also observed signs of anomalous fractional charge in graphene, a material for which there was no prediction of such an effect.

Ju’s group explored the electronic behavior of graphene, which exhibits exceptional properties on its own. More recently, Ju’s group has looked at five-layer graphene, a structure of five graphene sheets, each lightly stacked on top of each other, like steps on a staircase.

Such a five-layer graphene structure is embedded in graphite and can be obtained by exfoliation using adhesive tape. When placed in a refrigerator at ultracold temperatures, the structure’s electrons slow down to the point and interact in ways they normally wouldn’t when swirling at higher temperatures.

In their new work, the researchers performed some calculations and found that electrons could interact even more strongly with each other if the five-layer structure was aligned with hexagonal boron nitride (hBN), a material that has an atomic structure similar to that of graphene, but with slightly different dimensions.

In combination, the two materials are expected to produce a moiré superlattice, a complex scaffold-like atomic structure that could slow down electrons in a way that mimics a magnetic field.

“We did these calculations, and then we thought, let’s do this,” says Ju, who last summer installed a new dilution refrigerator in his MIT lab, which the team planned to use to cool the materials down to ultra-low temperatures, to study exotic electronic behavior.

The researchers made two samples of graphene’s hybrid structure by first exfoliating graphene layers from a block of graphite, then using optical tools to identify five-layer flakes in a staircase configuration. They then printed the graphene flake onto an hBN flake and placed a second hBN flake onto the graphene structure. Finally, they attached electrodes to the structure and placed it in the refrigerator, set at a level close to absolute zero.

By applying a current to the material and measuring the output voltage, they began to see signatures of fractional charging, where voltage equals current times a fractional number and some fundamental physical constants.

“The day we saw it, we didn’t recognize it at first,” says first author Lu. “Then we started screaming when we realized it was really huge. It was a completely surprising.”

“Those were probably the first serious samples we put in the new refrigerator,” adds Han, co-first author. “Once we calmed down, we looked in detail to make sure what we were seeing was real.”

With further analysis, the team confirmed that the graphene structure indeed exhibited the fractional quantum anomalous Hall effect. This is the first time that this effect has been observed in graphene.

“Graphene can also be a superconductor,” says Ju. “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.”

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

“We are exploring many fundamental ideas and applications in physics,” he says. “We know there will be more to come.”

This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and education.