Researchers have found fractional quantum anomalous Hall effect in graphene molecules which could pave the way for making certain type of quantum computers.
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Electrons in stacked graphene layers have shown anomalous
behavior which has physicists excited. |
A team of researchers at the Massachusetts Institute of
Technology (MIT) observed that the electrons display a strange behavior when
five layers of graphene are sandwiched between sheets of boron nitride.
Instead of their usual -1 charge, the electrons display part
charges such as -⅔ or -⅗. This has been dubbed the fractional quantum anomalous
Hall effect (FQAHE) and has physicists around the world excited.
This is not the first time researchers have seen strange
behaviors from electrons. When materials are cooled to absolute zero
temperatures, their resistance becomes quantized.
Electrical resistance is typically observed in the same
direction as the current flow. The material encounters one more resistance,
which runs perpendicular to the current, known as transverse resistance.
The transverse resistance gets quantized and can occur in
multiples of electron charge, such as 1,2,3, etc. These materials maintain the
same traverse resistance as the charge density increases. This is known as the
quantum Hall effect and is analogous to vehicles moving at the same speed on a
highway even when traffic increases.
In other materials, though, there is a larger disorder, and
traverse resistance can also occur when electrons carry fractional charges.
This happens because electrons collectively act like particles with fractional
charges and have earned the term fractional quantum Hall effect (FQHE).
Anomalous Quantum Hall Effect
In the Quantum Hall Effect and FQHE, a strong external
magnetic field is important since it prevents electrons from crashing into each
other. After FQHE was discovered in 1982, physicists stopped looking at
electrons as single particles. They began looking for more clues of quantum
engagement where these atomic constituents behave as part of the act together.
Multiple theories were put forth to explain the phenomenon
of FQHE and predict the behavior of particles displaying it. However,
researchers also wondered if particles could display such behavior without an
external magnetic field. This became known as the anomalous quantum hall effect
or FQAHE if displayed by fractional charges.
It was only in 2012 that researchers at Tsinghua University
in Beijing observed this anomalous behavior in thin ferromagnetic films for the
first time.
Behavior in Moire material
More than a decade later, researchers at the University of
Washington (UW) reported this behavior in specially designed material made from
molybdenum ditelluride (MoTe2). Physicists use the term moire to describe the
pattern of 2D materials that emanate when atom-thin lattices of materials are
stacked on top of each other and then twisted.
The slight changes in atom positions in different layers of
the material result in a shift in electric potential. This acts as a powerful
magnetic field, similar to the ones seen in the quantum Hall effect and FQHE.
When researchers at UW twisted MoTe2 to 1.4 degrees, they
expected to see FQAHE at work. However, it was observed when the twist was
increased to four degrees. Later, researchers at MIT replicated FQAHE when
working with graphene, which was sandwiched between layers of boron nitride.
Although the graphene system needs temperatures of 0.1
Kelvin to work, the material is cleaner and easier to work with. The problem
that physicists have in front of them is to explain how it occurs in the
material.
Researchers at top universities, such as Harvard and
Princeton, have been busy publishing theories on how FQAHE in graphene works.
Understanding fractionally charged particles could help develop new types of
quantum computers.
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