Constructive Quantum Interference in Isolated Armchair Graphene Nanoribbons?

It is demonstrated that in wider armchair graphene nanoribbons (AGNRs), and in particular the 13-AGNRs, there is a critical length Lc where two (or more) pairs of topological end-stats of opposite parities and nearly equal energies can develop in a form of a phase transition, giving rise to a sudden jump in the nominal conductivity by a factor of about 2.5, which is interpreted as a signature of constructive quantum interference (CQI). The “phase transition” is also associated with an abrupt increase in the energy gap(s) of about the same magnitude. Such unusual CQI, in which the role of “anchors” can be considered to be effectively played by the zigzag-end region, persists for lengths Lc ≈ 40Å up to L≈ 80Å and is unique for the 13-AGNRs, in contrast to other wide AGNRs (11-, 15-) studied here, in which the “phase transition” occurs without CQI. This is attributed to the aromaticity of 13-AGNRs which is characterized by “migration of sextets”, based on the resonance of two different aromatic Clar sextets, similarly to Coronene.