Physicists Observe Exotic Quantum States in Double-Layer Graphene

An international team of physicists from the United States and Japan has demonstrated a series of fractional quantum Hall effect states that arise in double-layer stacks of graphene. The team’s paper was published in the journal Nature Physics.

A new type quasiparticle discovered in graphene double-layer structure. This so-called composite fermion consists of one electron and two different types of magnetic flux, illustrated as blue and gold colored arrows in the figure. Composite fermions are capable of forming pairs, such unique interaction lead to experimental discovery of unexpected new quantum Hall phenomena. Image credit: Michelle Miller Jia Li, Brown University.

A new type quasiparticle discovered in graphene double-layer structure. This so-called composite fermion consists of one electron and two different types of magnetic flux, illustrated as blue and gold colored arrows in the figure. Composite fermions are capable of forming pairs, such unique interaction lead to experimental discovery of unexpected new quantum Hall phenomena. Image credit: Michelle Miller Jia Li, Brown University.

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.

A few years later, experimental physicists 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.

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.

The new research, carried out by physicists from Brown University, Columbia University and Japan’s National Institute for Materials Science, 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.

For the experiments, the team created ultra-clean devices entirely from atomically flat 2D materials.

The core of the structure consists of two graphene layer separated by a thin layer of hexagonal boron nitride as an insulating barrier. The double-layer structure is encapsulated by hexagonal boron nitride as a protective insulator, and graphite as a conductive gate to change the charge carrier density in the channel.

The graphene structures were then exposed to strong magnetic fields — millions of times stronger than Earth’s magnetic field.

The scientists produced a range of fractional quantum Hall states, some of which demonstrate excellent agreement with the composite fermion model, and some that had never been predicted or seen.

“Apart from the interlayer composite fermions, we observed other features that cannot be explained within the composite fermion model,” said Dr. Qianhui Shi, a postdoctoral researcher at Columbia University.

“A more careful study revealed that, to our surprise, these new states result from pairing between composite fermions. Pairing interaction between adjacent layers and within the same layer give rise to a variety of new quantum phenomena, making double-layer graphene an exciting platform to study.”

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J.I.A. Li et al. Pairing states of composite fermions in double-layer graphene. Nature Physics, published online June 24, 2019; doi: 10.1038/s41567-019-0547-z

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