  The design and fabrication of robust metallic states in graphene nanoribbons
(GNRs) is a significant challenge since lateral quantum confinement and
many-electron interactions tend to induce electronic band gaps when graphene is
patterned at nanometer length scales. Recent developments in bottom-up
synthesis have enabled the design and characterization of atomically-precise
GNRs, but strategies for realizing GNR metallicity have been elusive. Here we
demonstrate a general technique for inducing metallicity in GNRs by inserting a
symmetric superlattice of zero-energy modes into otherwise semiconducting GNRs.
We verify the resulting metallicity using scanning tunneling spectroscopy as
well as first-principles density-functional theory and tight binding
calculations. Our results reveal that the metallic bandwidth in GNRs can be
tuned over a wide range by controlling the overlap of zero-mode wavefunctions
through intentional sublattice symmetry-breaking.
