Graphene nanosystems and low-dimensional Chern-Simons topological insulators

A graphene nanoribbon is a good candidate for a $(1+1)$ Chern-Simons topological insulator since it obeys particle-hole symmetry. We show that in a finite semiconducting armchair ribbon, which has two zigzag edges and two armchair edges, a $(1+1)$ Chern-Simons topological insulator is indeed realized as the length of the armchair edges becomes large in comparison to that of the zigzag edges. But only a quasi-topological insulator is formed in a metallic armchair ribbon with a pseudogap. In such systems a zigzag edge acts like a domain wall, through which the polarization changes from $0$ to $e/2$, forming a fractional charge of one-half. When the lengths of the zigzag edges and the armchair edges are comparable a rectangular graphene sheet (RGS) is realized, which also possess particle-hole symmetry. We show that it is a $(0+1)$ Chern-Simons topological insulator. We find that the cyclic Berry phase of states of a RGS is quantized as $π$ or $0$ (mod $2π$), and that the Berry phases of the particle-hole conjugate states are equal each other. By applying the Atiyah-Singer index theorem to a rectangular ribbon and a RGS we find that the lower bound on the number of nearly zero energy end states is approximately proportional to the length of the zigzag edges. However, there is a correction to this index theorem due to the effects beyond the effective mass approximation.

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