Electrons in graphene exhibit very strange fractional behavior

In the realm of physics, where the boundaries of our understanding are constantly being pushed, a team of MIT physicists has made an intriguing discovery involving electrons and graphene, a material no thicker than a single atom and derived from common graphite.

Their research, recently published in Nature, reveals the observation of the elusive fractional charge effect in a relatively simple setup: five layers of graphene, stacked in a stair-like formation.

This finding simplifies previous methods that required intense magnetic fields explores new areas for the development of fault-tolerant quantum computers.

Quantum leap: Fractional electrons and graphene

The concept of the electron as the fundamental unit of electricity, carrying a single negative charge, is a basic principle taught in high school physics.

However, under certain rare conditions, electrons can exhibit behavior that seems to defy this understanding, fragmenting into fractional charges.

This phenomenon, known as the “fractional quantum Hall effect,” has been observed under specific, high-magnetic field conditions.

Yet, the MIT team’s discovery showcases the “fractional quantum anomalous Hall effect” in graphene without the need for such external magnetic influences.

“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 strange, 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.”

Splintering electrons: Mystery of fractional charge

The research team, including lead author Zhengguang Lu, Tonghang Han, Yuxuan Yao, Aidan Reddy, Jixiang Yang, Junseok Seo, Liang Fu, and collaborators Kenji Watanabe and Takashi Taniguchi from the National Institute for Materials Science in Japan, embarked on this exploration into the peculiar properties of graphene.

Their findings mark the first evidence of fractional electron charge in graphene, a material previously not expected to exhibit such effects.

The fractional quantum Hall effect has been a topic of intrigue since its discovery in 1982 in gallium arsenide heterostructures, earning a Nobel Prize in Physics.

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

It exemplifies the strange phenomena that can occur when particles, such as electrons, transition from individual to collective behavior under certain conditions.

Graphene’s game-changing role

The MIT team’s work builds on this legacy by demonstrating the effect in graphene, a material known for its exceptional properties, without the need for the strong magnetic fields that were previously considered essential.

In August 2023, parallel research at the University of Washington reported the first evidence of fractional charge without a magnetic field in a twisted semiconductor.

This “no magnets” discovery paved the way for advancements in topological quantum computing, offering a more secure form of quantum computation.

Ju’s team’s observation in graphene further expands the potential for utilizing fractional charges as qubits in quantum computing, combining the benefits of fractional quantum Hall effects and superconductivity in a single material.

Graphene, electrons and the quantum code

The experimental journey to this discovery involved meticulous preparation and analysis. The team fabricated samples by exfoliating graphene layers and stacking them with hexagonal boron nitride (hBN) to create a moiré superlattice structure.

This setup mimics the effects of a magnetic field, allowing for the observation of fractional charges without external magnetic control. The significance of their findings was initially overlooked but quickly recognized as a major breakthrough.

“The day we saw it, we didn’t recognize it at first,” says first author 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,” adds co-first author Han. “Once we calmed down, we looked in detail to make sure that what we were seeing was real.” 

According to Ju, graphene’s potential as a superconductor, coupled with its ability to exhibit fractional charge, suggests the possibility of harnessing two distinct phenomena within the same material.

“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,” Ju explained.

This dual capability could simplify the design of quantum computing systems by avoiding the complications that arise when integrating different materials.

Graphene’s place in future technologies

In summary, the MIT team’s discovery of the fractional quantum anomalous Hall effect in pentalayer graphene represents a significant leap forward in the study of strange electronic states and the development of quantum computing.

By observing fractional electron charges in a material system as simple as graphene, without the need for strong magnetic fields, they have opened new possibilities for creating more robust, fault-tolerant quantum computers.

This achievement highlights the unexpected and fascinating properties of graphene while setting the stage for future explorations into rare electronic states.

As the researchers continue to delve into the potential of multilayer graphene, their work promises to unlock many more fundamental physics insights and technological applications, paving the way for innovative quantum computing solutions.

More about graphene and electrons

As discussed above, graphene, a single layer of carbon atoms tightly bound in a hexagonal honeycomb lattice, stands as the thinnest, strongest, and most conductive material ever discovered.

Isolated for the first time in 2004 by scientists Andre Geim and Konstantin Novoselov at the University of Manchester, graphene has since captivated the scientific community and industry leaders alike.

This breakthrough earned them the Nobel Prize in Physics in 2010, highlighting graphene’s potential to revolutionize numerous fields.

Exceptional properties of graphene

Graphene’s extraordinary properties stem from its unique structure. Despite being only one atom thick, it is about 200 times stronger than steel by weight, making it the strongest material known.

Moreover, graphene’s ability to conduct electricity better than copper, combined with its exceptional thermal conductivity and transparency, opens up a wide array of applications, from electronics to energy storage.

Electronics, computing and energy

In the realm of electronics, graphene promises to usher in a new era of devices. Its high electrical conductivity and thinness make it ideal for creating faster, more efficient transistors, potentially leading to computers that are quicker and consume less power than today’s silicon-based systems.

Graphene’s role in energy technologies is equally transformative. It can significantly improve the efficiency of solar panels and batteries.

Graphene-based batteries not only charge much faster but also hold more power and last longer than conventional ones, making electric vehicles more practical and accessible.

Graphene’s unmatched strength and flexibility are set to revolutionize material science and engineering. By incorporating graphene into composite materials, manufacturers can create lighter, stronger, and more durable products, ranging from aircraft and vehicles to construction materials and protective gear.

Graphene, electrons and the future

The potential applications of graphene are vast and varied. Ongoing research aims to harness its properties for use in medical technologies, such as drug delivery systems and new types of sensors, as well as in water purification systems, offering a more efficient means of removing contaminants.

Despite its promise, the widespread adoption of graphene faces challenges, primarily related to cost and production scalability. However, as research progresses and new methods of producing graphene more efficiently are developed, it is expected to become a cornerstone of modern technology.

In summary, graphene, with its remarkable properties, has the potential to transform industries and improve daily life in unprecedented ways.

As scientists and engineers continue to explore and unlock its possibilities, the future of graphene appears bright, promising a new era of technological advancements that were once thought impossible.

The full study was published in the journal Nature.

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