Electronic transport properties of nanographite ribbon junctions

The electronic transport properties through junctions connecting nanographite ribbons of different or same width are investigated by means of the Landauer-Buttiker approach using a tight binding model. Graphite ribbon with zigzag boundary has a single conducting channel of edge states in the low-energy regime. The electrical conductance as a function of the chemical potential shows a rich structure with sharp dips of zero conductance. This perfect reflectivity originates from twofold degenerate resonant levels, i.e., flux states visible in the formation of strong current-current correlation with a Kekule-like vortex pattern. At each energy of conductance-zeros. this degeneracy yields the formation of standing waves in the scattering region of the junctions. The origin of zero-conductance resonances is also discussed by the standard scattering matrix approach, and the similarities between the nanographite ribbon junctions and the asymmetric Aharanov-Bohm ring connected to current leads are pointed out. Since the zero-conductance resonances are connected with the time-reversal symmetry of the system, the application of a magnetic field removes these zero-conductance dips, yielding a pronounced negative magnetoresistance.