Charged topological solitons in zigzag graphene nanoribbons.

Zigzag graphene nanoribbons (ZZGN) have a magnetic ground state (GS) characterized by
edge ferromagnetism with antiferromagnetic coupling between opposite edges. This GS
appears because of a broken symmetry in the spin sector. Therefore, by inverting the spin polarization
of the full system there is another energy degenerate GS. The band structures of the
degenerate GS's are inverted. When connecting two domains with
opposite mass i.e. spin orientation, a symmetry protected zero energy topological state appears at the
interface between the degenerate GS's. These topological states are soliton-like excitations
that carry charge ±e with half electron localized at each edge of the nanoribbon.
The connection between topological defects and electric charge suggests that solitons can be the
relevant charge excitation in ZZGN’s. Then whenever adding
charge to the system an array of solitons can be formed, creating a solitonic phase. By performing
numerical calculations, we find that at low doping, charge added to the system creates magnetic
domains and becomes localized at the domain walls separating opposite degenerate ground states.
Our results show that the topological properties of ZZGNR’s
are generated by electron-electron interactions rather than by spin orbit coupling.