September 26, 2011 — Scientists at the US Brookhaven National Laboratory discovered a new kind of quasiparticles (excitations of electric charge that resemble particles) in three-layer graphene sheets. These quasiparticles have mass dependent on their velocity, unlike mass-less quasiparticles known to exist in single-layer graphene. If the quasiparticles were at rest, they would become "infinitely massive."

Combine the tri-layer graphene quasiparticles with a heterostructure with magnetic material, and a much larger density of spin-polarized charge carriers than single-layer graphene could be generated. Spintronics devices could be designed to control both electric charge and spin, aligning the charge-carrier quasiparticle spins. The "very unusual quasiparticles" govern how tri-layer graphene behaves in a magnetic field, and other behaviors, said Igor Zaliznyak, a Brookhaven physicist who led the research team.

Figure. Tri-layer graphene stacked in an ABC pattern creates three offset layers like stair steps. SOURCE: Brookhaven Lab.

Graphene’s electrons flow freely across honeycomb-lattice-like flat, single-layer sheets. When layers are stacked, graphene electrons can be tuned, allowing current control like that in electronic devices. The stacking pattern is important. For this research, the honeycomb lattice in each stacked layer were offset, creating a "staircase" for electron flow.

The tri-layer graphene was made at the Center for Functional Nanomaterials (CFN) at Brookhaven Lab by peeling layers from graphite and mapping samples with microRaman microscopy. The CFN’s nanolithography tools, including ion-beam milling, were used to shape the samples for electrode connections, enabling measurements.

At the National High Magnetic Field Laboratory (NHMFL) in Tallahassee, FL, the scientists then studied the effect of an external magnetic field on the transport of electronic charge as a function of charge carrier density, temperature, and magnetic field strength.

These measurements confirmed theoretical work on the unique quasiparticles in the tri-layer graphene system. The quasiparticles behave as if they have a range of masses, diverging as the energy level decreases with quasiparticles becoming infinitely massive. Chirality, a spin-related property, protects these quasiparticles from being destroyed by virtual particle-hole pairs.

The work was published online in Nature Physics on September 25, 2011. Access the article here: http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2104.html

It was funded primarily by the US Department of Energy (DOE) Office of Science (BES); work at the NHMFL was funded by the National Science Foundation (NSF) and the State of Florida. A great deal of research was carried out by Liyuan Zhang, research associate at Brookhaven and Yan Zhang, a graduate student from Stony Brook University.

The Center for Functional Nanomaterials at Brookhaven National Laboratory is one of the five DOE Nanoscale Science Research Centers (NSRCs), premier national user facilities for interdisciplinary research at the nanoscale. For more information about the DOE NSRCs, please visit http://nano.energy.gov.

Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers.

Story courtesy of Karen McNulty Walsh, Brookhaven National Laboratory