George Mason Materials Science Collaboration Reports First Thermopower Detection of Fractional quantum Hall Effect in Bilayer Graphene
Novel thermopower measurement capabilities open new avenues for probing the topological properties of exotic quasiparticles, with potential implications in future quantum computers.
George Mason University physics and astronomy assistant professor Fereshte Ghahari and her team at George Mason University, along with collaborators including colleagues from Brown University and National Institute of Standards and Technology (NIST), report the first thermopower detection of Fractional quantum Hall (FQH) effect in bilayer graphene, a material consisting of two atom-thin layers of graphene. Their findings, published in Nature Physics, demonstrate that thermopower is a more sensitive probe of FQH effect compared to resistivity.
FQH effect is a quantum phenomenon that emerges in certain ultra-thin materials under high magnetic fields and extremely low temperatures. It is a particular state of matter where electrons interact strongly and in unexpected ways, giving rise to new particles that can potentially serve as the building blocks for future quantum computers.
“For many years, electrical resistance measurements have been the primary tool to probe FQH effect in graphene systems, leaving alternate approaches underexplored,” said Ghahari.
In essence, thermopower is the heating of one side of the material while keeping the other side cool. This causes the charged particles inside to shift toward the cooler side, creating a small electrical voltage. By looking at how the charges flow and the heat they carry in response to the temperature gradient, one can measure entropy, the degree of randomness. This entropic connection makes thermopower a powerful alternative tool to probe the topological properties of new particles and assess if they can be used in future quantum computers.
George Mason physics PhD student Nishat Sultana played an important role in this research by making specialized thermopower devices consisting of an isolated heater and local thermometers for producing and measuring the temperature gradient, respectively. She created a tiny sample made of bilayer graphene and then made the thermopower device using the nanofabrication facility at NIST's Center for Nanoscale Science and Technology. She then measured it using a low temperature cryostat located at Ghahari’s lab at George Mason.
“We performed thermopower and resistivity measurements for a variety of devices at various temperatures and magnetic fields,” said Sultana.
What the team observed was that FQH effect appears more strongly in thermopower compared to electrical resistance measurements at lower magnetic fields and higher temperatures, where resistivity measurements were inconclusive. Strikingly, new FQH states emerged in the thermal signal which had not been previously reported using resistivity measurements. Moreover, these studies raise the possibility that one of the newly observed FQH states may host topological entropy, making it a potential candidate for future topological quantum computing applications.
"This paper reports on the implementation of a unique tool for unlocking correlated phases in quantum materials,” shared Joel Schnur, a biomolecular science professor who has supported Ghahari’s efforts since she joined George Mason. “For example, this advance could play an important part in the development of quantum devices and lead to implementation of topological quantum computing,” Schnur explained.
These new measurements promise to provide additional insights about the FQH effect which cannot be accessed by other techniques. Overall, the findings by Ghahari and collaborators highlights the novel potential of thermopower measurements, opening new avenues for experimental and theoretical investigations of correlated and topological states in graphene systems, including moiré materials.
This Mason Science discovery was also highlighted in phys.org.
