MIT Physicists Discover Fractional Charge in Electrons of Pentalayer Graphene
Physicists at MIT have made an unexpected discovery: electrons in pentalayer graphene can exhibit fractional charge. New theoretical research from the team provides an explanation, suggesting that electron interactions in confined two-dimensional spaces give rise to unique quantum states, independent of magnetic fields.
MIT physicists have made a pioneering advancement in understanding how electrons can divide into fractional charges, shedding light on the creation of unique electronic states in graphene and other two-dimensional materials.
This new discovery builds on previous work by another MIT team led by Assistant Professor Long Ju. Ju’s group observed that electrons appear to carry "fractional charges" within pentalayer graphene, a material consisting of five stacked graphene layers placed on a boron nitride sheet.
Revealing Fractional Charges
Ju found that when an electric current was passed through the pentalayer graphene structure, the electrons seemed to travel as fractional parts of their total charge, even without the presence of a magnetic field. While scientists have previously shown that electrons can split into fractions under strong magnetic fields in the fractional quantum Hall effect, Ju’s research was the first to demonstrate this phenomenon occurring in graphene without a magnetic field—an unexpected outcome.
This discovery led to the identification of a new effect, called the fractional quantum anomalous Hall effect, and researchers are now focused on understanding how fractional charges arise in pentalayer graphene.
Theoretical Advances and Collaborative Breakthroughs
A new study led by MIT physics professor Senthil Todadri provides a key piece of the puzzle. By conducting quantum mechanical calculations, Todadri and his team discovered that the electrons in pentalayer graphene arrange themselves into a crystal-like structure, which is perfectly suited for fractional electron behavior to emerge.
"This is an entirely new mechanism," says Todadri. "In the history of the field, no one has seen a system behave in this way to produce fractional electron phenomena. It's incredibly exciting because it opens up the possibility for experiments that were previously only theoretical."
The team's findings were recently published in Physical Review Letters, and two other research teams—one from Johns Hopkins University and another from Harvard University, University of California at Berkeley, and Lawrence Berkeley National Laboratory—have published similar findings in the same issue. The MIT team includes Zhihuan Dong (PhD ’24) and former postdoc Adarsh Patri.
The Birth of "Fractional Phenomena"
In 2018, MIT physicist Pablo Jarillo-Herrero and his colleagues discovered that stacking and twisting two sheets of graphene could give rise to new electronic behaviors. These graphene sheets are each only one atom thick and arranged in a hexagonal lattice. When stacked at a specific angle, the interference between the layers, called a moiré pattern, produced unexpected phenomena, such as both superconductivity and insulating behavior in the same material. This breakthrough led to the creation of magic-angle graphene, sparking the field of twistronics, which studies the electronic behavior of twisted 2D materials.
"After his experiments, we realized that these moiré systems would be ideal for discovering conditions that could produce fractional electron phases," says Todadri, who worked with Jarillo-Herrero to show that such twisted systems could theoretically exhibit fractional charge without a magnetic field. "We strongly advocated these systems as the best candidates for exploring these fractional phenomena."
Unexpected Experimental Findings
In September 2023, Senthil Todadri received an unexpected call from Long Ju, a physicist who had been working on similar experimental studies. Ju shared new data showing that electrons in pentalayer graphene seemed to split into fractions, defying Todadri's original predictions.
“I was surprised when he called me on a Saturday to show me his results,” Todadri recalls. “It didn’t go as we thought.”
In his 2018 paper, Todadri theorized that fractional charge would emerge from a precursor phase, where the electron wavefunction undergoes a specific twist. He predicted that the degree of this twisting should increase as more graphene layers were added. Specifically, for pentalayer graphene, Todadri expected the wavefunction to wind around five times, setting the stage for fractional charge. However, Ju’s experiments showed that the wavefunction only twisted once, sparking new questions about what was happening.
Reevaluating Electron Interactions
Following this surprising result, Todadri and his team reassessed their theory. They realized that they had likely overlooked a critical aspect of the system.
“In most materials, we treat electrons as independent particles, but in two-dimensional systems like pentalayer graphene, the electrons are much more confined,” Todadri explains. “This confinement means the electrons interact with each other, and their quantum correlations affect their behavior.”
Once they added electron-electron interactions into their model, the team’s predictions matched Ju’s experimental observations. They discovered that pentalayer graphene’s structure—where each carbon layer sits on top of the other, with boron nitride below—creates a weak electrical potential. This potential confines the electrons and forces them to interact via quantum correlations. The interactions lead to a wavefunction pattern, where the electrons “wind” in a way that supports the fractional charge phenomenon.
“This new crystal structure behaves in ways that ordinary crystals do not, leading to many intriguing questions for future research,” Todadri says. “In the short term, our model provides a theoretical foundation for understanding the fractional charges in pentalayer graphene and predicting similar behaviors in other systems.”
