Thermopower unveils the secret fractional quantum state in graphene

Imagine a super-cold, ultra-thin layer of electrons trapped in a strong magnetic field. Normally, electrons move independently, but under extreme conditions, they start behaving in a highly coordinated way — so what’s happening here?

Well, the electrons are forming a new quantum state where they move as if they were fractional particles rather than whole electrons. This rare and special state of matter is called the fractional quantum hall (FQH) effect.

It is of great importance as it could lead to the development of topological quantum computers, which are believed to be more stable against errors than current quantum computers. Plus, FQH may also facilitate the creation of new quantum materials and applications.

However, detecting and studying FQH in detail has been very challenging using the existing methods, which involves measuring a material’s ability to resist electric current flow. A team of researchers realized this problem and discovered an entirely different approach.

Catching FQH using thermopower

Instead of relying on electrical resistivity, the researchers tried a different method based on thermopower — a property where a material generates a small voltage when it is heated in a way that its one side is hot, and the other remains cool.

The heat causes the electrons to move from the hot region to the cooler region, creating a voltage. “It turns out that by measuring this voltage (i.e., thermopower), one can measure the entropy of the system, a thermodynamic quantity,” Fereshte Ghahari, one of the researchers and a professor of physics at George Mason Univesity (GMU), said.

Previous studies suggest that thermopower and entropy are directly proportional to each other, and they are closely linked to the unusual quantum behavior of FQH state. Therefore, thermopower measurements provide deeper insights into these exotic states than resistivity measurements alone. The study authors decided to test this phenomenon.

They picked Bernal-stacked bilayer graphene, a material in which graphene atoms in one layer align partially with the atoms in the second layer, forming a unique structure. This special stacking affects the way electrons move through the material, making it an excellent platform for studying quantum effects like the FQH states.

When the study authors heated the material and performed thermopower measurement, they could detect FQH states like never before. The thermopower signals revealed FQH states with exceptional sensitivity.

Such findings wouldn’t have been possible using the traditional resistivity approach. “We demonstrate that the magneto-thermopower detection of fractional quantum Hall states is more sensitive than resistivity measurements,” the researchers note.

“Overall, our findings reveal the unique capabilities of thermopower measurements, introducing a new platform for experimental and theoretical investigations of correlated and topological states in graphene systems, including moiré materials,” Ghahari concluded.

Hopefully, these findings will help us realize the true potential of the FQH effect. However, whether the same approach could be used to detect other exotic quantum states remains to be explored through further research.

The study is published in the journal Nature Physics.

This post was originally published on here