Via Sci-News.com: 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.
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...
