Probing spin fluctuations in graphene nanoribbons with STM tunnelling spectroscopy*

We address the question of how scanning electron tunnelling spectroscopy can be used to probe spin excitations that emerge in localized states in atomically precise graphene ribbons. We consider both sequential and cotunnelling regimes, and discuss how the dI/dV curves provide information about the fluctuating local moments that are expected to occur in zigzag edges and other zero modes that occur in graphene ribbons. We model the graphene nanoribbons with a Hubbard model and we obtain the relevant electronic states by numerical diagonalization in a reduced active space. We conclude that inelastic electron tunnelling spectroscopy provides an image, with atomic scale resolution, of the spectral function of the low energy spin excitations in nanographene, and should provide unambiguous evidence of the emergence of local moments in this class of systems.