A recent study by Igor Zaliznyak and others at the Brookhaven National Laboratory proved empirically that exotic particle-like phenomena predicted in theory actually exist in layered sheets of carbon atoms. The study, using practical and replicable methods, verified the predicted electrical properties of this type of carbon configuration.
Graphene is a flat, two-dimensional sheet of carbon atoms bonded in a hexagonal lattice. The graphite commonly used as pencil lead is made up of stacks of graphene.
Isolated single layers of graphene are cheaper and 100 times more conductive than silicon. For these reasons many people think that graphene will replace silicon as the conductive material of the future. Multiple layered graphene has unique conductive properties that might make it preferable to single layer graphene for electronics applications.
The band gap is a property of atoms that determines how freely electrons move across a material. If the band gap is large, the material is an insulator; it does not conduct electricity. In good conductors there is no band gap; electrons are free to move.
Single layer graphene is like a superconductor in that it has a very small band gap. In multi-layer graphene, the band gap can be adjusted by a magnetic field. This means that multi-layer graphene, unlike the usual superconductors, can be changed from a good conductor to an insulator without changing the temperature. In electronics applications, this means that multi-layer graphene can conduct a high current, which can be turned on and off.
Tri-layer graphene was used in these experiments. Three sheets of carbon were stacked on one another in an ABC configuration where each sheet (A, B and C) is offset from the one below it.
To obtain samples of this kind, the team used mechanical exfoliation, peeling off a mixture of different graphene samples from graphite using an adhesive tape and identifying which was tri-layer ABC. They verified the number of layers using Raman microscopy by shining a laser on the sample and measuring reflectivity.
They then tested the magnetoresistance, the changing electric properties of the samples, in the presence of magnetic fields. The Landau level quantization refers to the discrete energies that electrons can obtain. In a magnetic field the Landau levels change. From the magnetoresistance, they found that the Landau level quantization that occurs in ABC tri-layer graphene is atypical.
This phenomenon is explained by the presence of exotic quasi-particles, interactions of electrons that can be treated as particles. Fermions are a class of subatomic particles that includes protons, neutrons and electrons, among others. Dirac fermions are stable particles because they are not their own antiparticle. Physicists know that ‘spin' is a property of fermions that is conserved when they interact.
The quasi-particles found in graphene are fermions, such as protons or electrons. In monolayer graphene, the quasi-particles are massless; in bilayer graphene they are massive, but in ABC tri-layer graphene the quasi-particles' masses depend on their energies. When the energy is very low and the particles are at rest, the mass of the quasi-particles goes to infinity.
Electron interactions would make these quasi-particles unstable, but their spin characteristics prevent them from decaying and, therefore, they are stable; they obey the Dirac equation. The unique quasi-particles present in ABC tri-layer graphene and its resistive variability, when they are better understood, could make graphene an even more powerful conductor.
As research on graphene continues, the rigorous techniques used in this experiment may make it easier to collect graphene samples and test electronic properties of the material.
