Liyuan Zhang and Igor Zaliznyak at the Center for Functional Nanomaterials. Photo: Brookhaven National Laboratory |
By studying three layers of graphene stacked in a particular
way, scientists at the U.S. Department of Energy’s Brookhaven National
Laboratory have discovered a “little universe” populated by a new kind of “quasiparticles”—particle-like excitations of electric charge. Unlike massless
photon-like quasiparticles in single-layer graphene, these new quasiparticles
have mass, which depends on their energy (or velocity), and would become
infinitely massive at rest.
That accumulation of mass at low energies means this trilayer
graphene system, if magnetized by incorporating it into a heterostructure with
magnetic material, could potentially generate a much larger density of
spin-polarized charge carriers than single-layer graphene—making it very
attractive for a new class of devices based on controlling not just electric
charge but also spin, commonly known as spintronics.
“Our research shows that these very unusual quasiparticles,
predicted by theory, actually exist in three-layer graphene, and that they
govern properties such as how the material behaves in a magnetic field—a
property that could be used to control graphene-based electronic devices,” says
Brookhaven physicist Igor Zaliznyak, who led the research team. Their work
measuring properties of tri-layer graphene as a first step toward engineering
such devices was published online in Nature Physics.
Graphene has been the subject of intense research since its
discovery in 2004, in particular because of the unusual behavior of its
electrons, which flow freely across flat, single-layer sheets of the substance.
Stacking layers changes the way electrons flow: Stacking two layers, for
example, provides a “tunable” break in the energy levels the electrons can
occupy, thus giving scientists a way to turn the current on and off. That opens
the possibility of incorporating the inexpensive substance into new types of
electronics.
With three layers, the situation gets more complicated,
scientists have found, but also potentially more powerful.
One important variable is the way the layers are stacked: In “ABA” systems, the carbon atoms making up the honeycomb rings are directly
aligned in the top and bottom layers (A) while those in the middle layer (B)
are offset; in “ABC” variants, the honeycombs in each stacked layer are offset,
stepping upwards layer by layer like a staircase. So far, ABC stacking appears
to give rise to more interesting behaviors—such as those that are the subject of
the current study.
For this study, the scientists created the tri-layer graphene at
the Center for Functional Nanomaterials (CFN) at Brookhaven Lab, peeling it
from graphite. They used microRaman microscopy to map the samples and identify
those with three layers stacked in the ABC arrangement. Then they used the
CFN’s nanolithography tools, including ion-beam milling, to shape the samples
in a particular way so they could be connected to electrodes for measurements.
At the National High Magnetic Field Laboratory (NHMFL) in
Tallahassee, Fla., the scientists then studied the material’s electronic
properties—specifically the effect of an external magnetic field on the
transport of electronic charge as a function of charge carrier density,
magnetic field strength, and temperature.
The measurements provide the first experimental evidence for the
existence of a particular type of quasiparticle, or electronic excitation that
acts like a particle and serves as a charge carrier in the tri-layer graphene
system. These particular quasiparticles, which were predicted by theoretical studies,
have ill-defined mass—that is, they behave as if they have a range of masses—and
those masses diverge as the energy level decreases with quasiparticles becoming
infinitely massive.
Ordinarily such particles would be unstable and couldn’t exist
due to interactions with virtual particle-hole pairs—similar to virtual pairs
of oppositely charged electrons and positrons, which annihilate when they
interact. But a property of the quasiparticles called chirality, which is
related to a special flavor of spin in graphene sytems, keeps the
quasiparticles from being destroyed by these interactions. So these exotic
infinitively massive particles can exist.
“These results provide experimental validation for the large
body of recent theoretical work on graphene, and uncover new exciting
possibilities for future studies aimed at using the exotic properties of these
quasiparticles,” Zaliznyak says.
For example, combining magnetic materials with tri-layer
graphene could align the spins of the charge-carrier quasiparticles. “We
believe that such graphene-magnet heterostructures with spin-polarized charge
carriers could lead to real breakthroughs in the field of spintronics,”
Zaliznyak says.
