Nonlocal exchange effects in zigzag-edge magnetism of neutral graphene nanoribbons

We study the role of nonlocality of exchange in a neutral zigzag graphene nanoribbon within the pi-orbital unrestricted Hartree-Fock approximation. Within this theory we find that the magnetic features are further stabilized for both the intraedge and interedge exchange as compared to mean-field theories of zigzag ribbons based on local exchange (such as the Hubbard model or the ab initio local density approximation). The interedge exchange produces an enhancement of the band gap of the magnetic ground-state solutions. The effect of this enhanced exchange on the edge states cannot be satisfactorily achieved by a local interaction with renormalized parameters.