Thermopower-based technique can detect fractional quantum Hall states

If one side of a conducting or semiconducting material is heated while the other remains cool, charge carriers move from the hot side to the cold side, generating an electrical voltage known as thermopower.

Past studies have shown that the thermopower produced in clean two-dimensional (2D) electron systems (i.e., materials with few impurities in which electrons can only move in 2D), is directly proportional to the entropy (i.e., the degree of randomness) per charge carrier.

The link between thermopower and entropy could be leveraged to probe exotic quantum phases of matter. One of these phases is the fractional quantum Hall (FQH) effect, which is known to arise when electrons in these materials are subject to a strong perpendicular magnetic field at very low temperatures.

Researchers at George Mason University, along with collaborators from Brown University and the National Institute of Standards and Technology (NIST), recently showed that FQH states could be better detected using thermopower measurements than with conventional electrical resistivity.

Their paper, published in Nature Physics, could open new possibilities for the precise study of strongly interacting quantum phases of matter, particularly FQH states in bilayer graphene.

"Some FQH states can support new particles that can serve as the building blocks for topological quantum computers," Fereshte Ghahari, senior author of the paper, told Phys.org.

Previous studies aimed at detecting and better understanding FQH states in bilayer graphene (i.e., a material made up of two layers of carbon atoms arranged in a specific stacked structure) only relied on measurements of a material's resistivity, or, in other words, the strength with which a material opposes the flow of electric current.

As part of their recent study, Ghahari and their colleagues set out to instead explore the potential of thermal measurements for studying these quantum states.

"These measurements ultimately allow us to access the entropy carried out by particles which can shed light on the properties of these new particles and if they can be used in future topological quantum computers," explained Ghahari.

"To measure entropy, we employed a thermopower-based technique. Thermopower is an effect where a material creates electricity from heat. When one side of a material becomes hot and the other remains cool, the heat makes the charge carriers move from the hot side to the cold side, creating a small voltage across the material.

"It turns out that by measuring this voltage (i.e., thermopower), one can measure the entropy of the system, a thermodynamic quantity."

By performing thermopower measurements in Bernal-stacked bilayer graphene, the researchers were able to detect fragile FQH states with greater sensitivity than that previously attained using resistivity measurements. Surprisingly, new FQH states appeared in the thermopower signal which had not been previously reported.

Their study thus demonstrates the potential of thermopower-based techniques applied to bilayer graphene for studying these strongly interacting quantum phases of matter, which could be ultimately leveraged to create new quantum technologies.

"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," added Ghahari.

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