New York, NY—June 24, 2019—Researchers from Brown and Columbia Universities have demonstrated previously unknown states of matter that arise in double-layer stacks of graphene, a two-dimensional nanomaterial. These new states, known as the fractional quantum Hall effect, arise from the complex interactions of electrons both within and across graphene layers.

“The findings show that stacking 2D materials together in close proximity generates entirely new physics,” said Jia Li, assistant professor of physics at Brown, who initiated this work while a post-doc at Columbia working with Cory Dean, professor of physics, and Jim Hone, professor of mechanical engineering. “In terms of materials engineering, this work shows that these layered systems could be viable in creating new types of electronic devices that take advantage of these new quantum Hall states.”

The research is published in the journal Nature Physics.

Importantly, says Hone, Wang Fong-Jen Professor of Mechanical Engineering at Columbia Engineering, several of these new quantum Hall states “may be useful in making fault-tolerant quantum computers.”

The Hall effect emerges when a magnetic field is applied to a conducting material in a perpendicular direction to a current flow. The magnetic field causes the current to deflect, creating a voltage in the transverse direction, called the Hall voltage. The strength of the Hall voltage increases with the strength of the magnetic field. The quantum version of the Hall effect was first discovered in experiments performed in 1980 at low temperatures and strong magnetic fields. The experiments showed that rather than increasing smoothly with magnetic field strength, the Hall voltage increases in step-wise (or quantized) fashion. These steps are integer multiples of fundamental constants of nature and are entirely independent of the physical makeup of the material used in the experiments. The discovery was awarded the 1985 Nobel Prize in Physics.

A few years later, researchers working at temperatures near absolute zero and with very strong magnetic fields found new types of quantum Hall states in which the quantum steps in Hall voltage correspond to fractional numbers, hence the name fractional quantum Hall effect. The discovery of the fractional quantum Hall effect won another Nobel Prize, in 1998. Theorists later posited that the fractional quantum Hall effect is related to the formation of quasi-particles called composite fermions. In this state, each electron combines with a quantum of magnetic flux to form a composite fermion carrying a fraction of an electron charge giving rise to the fractional values in Hall voltage.

The composite fermion theory has been successful in explaining a myriad of phenomena observed in single quantum well systems. This new research used double-layer graphene to investigate what happens when two quantum wells are brought close together. Theory had suggested that the interaction between two layers would lead to a new type of composite fermion, but this had never been observed in experiment.