In high school physics, we are taught that the electron is the basic unit of electricity, which carries a single negative charge. It is also the case in most materials in nature.
In very special states of matter, however, electrons can become fractions of themselves in an exceedingly rare phenomenon called "fractional charge." If the exotic electronic state can be corralled and controlled, it can be used in building resilient, fault-tolerant quantum computers.
Fractional Quantum Hall Effect
The fractional quantum Hall effect refers to a peculiar phenomenon when particles shift from acting as individual units to behaving collectively. The collective behavior emerges in particular states, like when electrons are slowed down to a crawl, allowing them to sense each other and interact. Such interactions can create rare electronic states, like the unusual splitting of an electron's charge.
It was in 1982 when the fractional quantum Hall effect was first discovered in gallium arsenide heterostructures. It involved an experiment where a gas of electrons confined in a 2D plane is positioned under high magnetic fields.
This discovery was such a big deal at the time since the unit charges interacting to give fractional charge was very, very bizarre. There were no theory predictions then, so the experiment surprised everyone.
The fractional quantum Hall effect has been observed often, mostly under very high, carefully maintained magnetic fields. In August 2023, the University of Washington experts demonstrated the first evidence of fractional charge without requiring a magnetic field.
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Discovery of Fractional Charge in Graphene
At the Massachusetts Institute of Technology, physicists have observed the fractional charge effect in five graphene layers. This simple material is an atom-thin carbon layer from graphite and common pencil lead. The research team reported their results in the paper "Fractional quantum anomalous Hall effect in multilayer graphene."
Led by Zhengguang Lu, the scientists fabricated two samples of hybrid graphene structure and placed them in a refrigerator near absolute zero. By applying a current to the material and measuring the voltage output, they began to witness signatures of fractional charge.
The researchers demonstrated that when five sheets of graphene are arranged like steps on a staircase, the resulting structure offers the right conditions to allow electrons to pass through as fractions of their total charge. This event can happen without requiring an external magnetic field.
This study is the first evidence of crystalline graphene's fractional quantum anomalous Hall effect. This type of graphene is a material that physicists did not expect to exhibit such an effect.
According to MIT physics assistant professor Long Ju, the fractional charge is so exotic, yet they realize this effect with a more straightforward system and without a magnetic field. That behavior alone is important for fundamental physics, and it can also allow the development of a type of quantum computing stronger against perturbation.
Since graphene can also be a superconductor, two completely different effects can occur in the same material right next to each other. This can potentially prevent many unwanted effects when graphene is connected to other materials.
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