  We combine density-functional theory and the nonequilibrium Green's function
method to study the thermal conductance of graphene nanoribbons with armchair
and zigzag edges. Zigzag ribbons have higher thermal conductance than armchair
ribbons of comparable widths due to an anisotropy in phonon dispersion for
graphene nanoribbons: low-frequency bands in zigzag ribbons are more dispersive
than those in armchair ribbons. The edges with and without hydrogen-passivation
modify the atomic structure and ultimately influence the phonon thermal
transport differently for the two ribbon types.
