MIT physicists have observed an exotic electronic state, “fractional charge,” in a simple material: five graphene layers. This phenomenon, where electrons splinter into fractions of their whole, could help build resilient, fault-tolerant quantum computers. The team, led by Long Ju, found that the stacked graphene structure allows electrons to pass through as fractions of their total charge without the need for an external magnetic field. This is the first evidence of the “fractional quantum anomalous Hall effect” in graphene, a material not expected to exhibit this effect. The research could enable a more robust form of quantum computing.
Discovery of Fractional Charge in Graphene
In a groundbreaking study, physicists at MIT have observed an unusual electronic state in graphene, where electrons behave as fractions of their whole. This rare phenomenon, known as “fractional charge,” has been observed only a few times before, typically under high, carefully controlled magnetic fields. The discovery of this effect in graphene, a simpler material, could potentially pave the way for more resilient forms of quantum computing.
The researchers found that when five sheets of graphene are stacked in a specific manner, the resulting structure provides the right conditions for electrons to pass through as fractions of their total charge, without the need for any external magnetic field. This is the first evidence of the “fractional quantum anomalous Hall effect” in crystalline graphene, a material that was not expected to exhibit this effect.
The Fractional Quantum Hall Effect
The fractional quantum Hall effect is a peculiar phenomenon that occurs when particles shift from behaving as individual units to acting collectively. This collective behavior emerges in special states, for instance when electrons are slowed down to a crawl, enabling them to sense each other and interact. These interactions can produce rare electronic states, such as the seemingly unorthodox splitting of an electron’s charge.
The fractional quantum Hall effect was first discovered in 1982 in heterostructures of gallium arsenide, where a gas of electrons confined in a two-dimensional plane is placed under high magnetic fields. In 2023, scientists at the University of Washington reported the first evidence of fractional charge without a magnetic field, in a twisted semiconductor called molybdenum ditelluride.
Graphene: A Material Full of Surprises
Graphene, an atom-thin layer of carbon that stems from graphite and common pencil lead, has exhibited exceptional properties. The MIT team has been exploring electronic behavior in pentalayer graphene — a structure of five graphene sheets, each stacked slightly off from the other, like steps on a staircase. When placed in a refrigerator at ultracold temperatures, the structure’s electrons slow down and interact in ways they normally wouldn’t when moving at higher temperatures.
The researchers found that electrons might interact with each other even more strongly if the pentalayer structure were aligned with hexagonal boron nitride (hBN), a material with a similar atomic structure to that of graphene, but with slightly different dimensions. The combination of the two materials produces a moiré superlattice, an intricate atomic structure that slows down electrons in ways that mimic a magnetic field.
Observing the Fractional Charge in Graphene
The researchers fabricated two samples of the hybrid graphene structure and placed them in a refrigerator set to near absolute zero. As they applied a current to the material and measured the voltage output, they started to see signatures of fractional charge. After further analysis, the team confirmed that the graphene structure indeed exhibited the fractional quantum anomalous Hall effect, marking the first time the effect has been seen in graphene.
The discovery of this effect in graphene opens up new possibilities for quantum computing. Graphene can also be a superconductor, meaning two completely different effects can occur in the same material, right next to each other. This could potentially avoid many unwanted effects when bridging graphene with other materials.
Future Research and Applications
The MIT team is continuing to explore multilayer graphene for other rare electronic states. The discovery of the fractional quantum anomalous Hall effect in graphene not only contributes to fundamental physics but also opens up potential applications in quantum computing. The team is optimistic about uncovering more surprises in the future as they delve deeper into the exploration of graphene’s unique properties.