TALLAHASSEE, Fla. — Two theoretical physics professors in the Magnet Lab's Condensed Matter Theory Group are co-authors of a paper that is attracting considerable attention.
Florida State University Assistant Professor Oskar Vafek and Professor Kun Yang are co-authors of the article "Many-body instability of Coulomb interacting bilayer graphene: Renormalization group approach," which was chosen in January 2010 as an Editor's Suggestion and featured in the American Physical Society journal Physics: spotlighting exceptional research.
A single sheet of graphite (essentially a pencil trace), graphene is made entirely out of a hexagonal array of carbon atoms. Its remarkable electronic properties, which make it a potential candidate for use in technological applications, place graphene at an intellectual frontier of condensed matter physics.
The subtle interference of the electron waves in the presence of the honeycomb potential, in combination with Pauli's exclusion principle, lead to an effective loss of the electron mass in the single atomic layer graphene: Near the Fermi level the electrons disperse as massless Dirac particles in two spatial dimensions. Their velocity, which is therefore not necessarily proportional to their momentum, is experimentally found to be about 300 times smaller than speed of light in vacuum. This ultra-relativistic-like dispersion gives the system a certain degree of robustness with respect to (weak) electron-electron interactions.
A bilayer graphene is a system of two carbon honeycomb lattices stacked in the so called A-B arrangement: atoms in the first layer and belonging to one of the sublattices have atoms directly above them in the second layer, while the atoms of the second sublattice sit below (above) the honeycomb plaquettes. The massless Dirac dispersion in this case is modified and instead of two cones touching, two parabolic bands touch.
Vafek and Yang argue that such a system is unstable even to infinitesimal electron-electron interactions and use renormalization group to identify the most likely broken symmetry ground state. In the parameter regime studied by the authors an interesting new electronic phase, called nematic, was found to have the most divergent susceptibility. This phase is characterized by broken lattice rotational symmetry, but unbroken lattice translational symmetry and the authors propose ways to detect it. Experiments on the bilayers are now underway to further explore this system.