Graphene nanosystems and low-dimensional Chern-Simons topological insulators

TL;DR

This paper investigates the topological properties of graphene nanostructures, demonstrating how different edge configurations realize various Chern-Simons topological insulators and analyzing their edge states and Berry phases.

Contribution

It provides a detailed analysis of how graphene nanoribbons and rectangular sheets exhibit different topological phases and edge states based on their geometry and symmetry, applying the Atiyah-Singer index theorem.

Findings

Finite semiconducting armchair ribbons realize (1+1) topological insulators.

Zigzag edges induce fractional charges of e/2 at domain walls.

The number of zero-energy states scales with zigzag edge length.

Abstract

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 $\pi$ or $0$ (mod $2\pi$), 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.