Image shows a bilayer graphene schematic. The blue beads represent carbon atoms. Image: Lau laboratory, UC Riverside. |
A research team led by physicists at the University of California,
Riverside has
identified a property of bilayer graphene (BLG) that the researchers say is
analogous to finding the Higgs boson in particle physics.
Graphene, nature’s thinnest elastic material,
is a one-atom thick sheet of carbon atoms arranged in a hexagonal lattice.
Because of graphene’s planar and chicken wire-like structure, sheets of it lend
themselves well to stacking.
BLG is formed when two graphene sheets are
stacked in a special manner. Like graphene, BLG has high current-carrying
capacity, also known as high electron conductivity. The high current-carrying
capacity results from the extremely high velocities that electrons can acquire
in a graphene sheet.
The physicists report online in Nature
Nanotechnology that in investigating BLG’s properties they found that when
the number of electrons on the BLG sheet is close to 0, the material becomes
insulating (that is, it resists flow of electrical current)—a finding that has
implications for the use of graphene as an electronic material in the
semiconductor and electronics industries.
“BLG becomes insulating because its
electrons spontaneously organize themselves when their number is small,” said Chun
Ning (Jeanie) Lau, an associate professor of physics and astronomy and the lead
author of the research paper. “Instead of moving around randomly, the electrons
move in an orderly fashion. This is called ‘spontaneous symmetry breaking’ in
physics, and is a very important concept since it is the same principle that ‘endows’ mass for particles in high energy physics.”
Lau explained that a typical conductor has
a huge number of electrons, which move around randomly, rather like a party
with ten thousand guests with no assigned seats at dining tables. If the party
only has four guests, however, then the guests will have to interact with each
other and sit down at a table. Similarly, when BLG has only a few electrons the
interactions cause the electrons to behave in an orderly manner.
New quantum particle
Allan MacDonald, the Sid W. Richardson Foundation Regents Chair in the
Department of Physics at The University of Texas at Austin and a coauthor on
the research paper, noted that team has measured the mass of a new type of
massive quantum particle that can be found only inside BLG crystals.
“The physics which gives these particles
their mass is closely analogous to the physics which makes the mass of a proton
inside an atomic nucleus very much larger than the mass of the quarks from
which it is formed,” he said. “Our team’s particle is made of electrons,
however, not quarks.”
MacDonald explained that the experiment the
research team conducted was motivated by theoretical work which anticipated
that new particles would emerge from the electron sea of a BLG crystal.
“Now that the eagerly anticipated particles
have been found, future experiments will help settle an ongoing theoretical
debate on their properties,” he said.
Practical applications
An important finding of the research team is that the intrinsic “energy gap” in
BLG grows with increasing magnetic field.
In solid state physics, an energy gap (or
band gap) refers to an energy range in a solid where no electron states can
exist. Generally, the size of the energy gap of a material determines whether
it is a metal (no gap), semiconductor (small gap), or insulator (large gap).
The presence of an energy gap in silicon is critical to the semiconductor
industry since, for digital applications, engineers need to turn the device ‘on’ or conductive, and ‘off’ or insulating.
Single-layer graphene (SLG) is gapless,
however, and cannot be completely turned off because regardless of the number
of electrons on SLG, it always remains metallic and a conductor.
“This is terribly disadvantageous from an
electronics point of view,” said Lau, a member of UC Riverside’s Center for
Nanoscale Science and Engineering. “BLG, on the other hand, can in fact be
turned off. Our research is in the initial phase, and, presently, the band gap
is still too small for practical applications. What is tremendously exciting
though is that this work suggests a promising route—trilayer graphene and
tetralayer graphene, which are likely to have much larger energy gaps that can
be used for digital and infrared technologies. We already have begun working
with these materials.”