  Graphene nanostructures are a promising platform for engineering electronic
states with tailored magnetic and quantum properties. Synthesis strategies on
metallic substrates have made it possible to manufacture atomically precise
nanographenes with controlled size, shape and edge geometry. In these
nanographenes, finite spin magnetic moment can arise as a result of many-body
interactions in molecular orbitals with $\pi$-conjugated character and subject
to strong spatial confinement, for example at the zig-zag edges. However, owing
to the mixing of the molecular orbitals and metallic states from the catalysing
substrate, most of their expected quantum phenomenology is severely hindered.
The use of in-situ ultra-thin decoupling layers can impede nanographene-metal
hybridization and facilitate the expression of predicted properties. Here we
show that the edges of narrow chiral graphene nanoribbons (GNRs) over MgO
monolayers on Ag(001) can host integer charge and spin-1/2 frontier states. The
electron occupation varies with the GNR length, which alternates even or odd
number of electrons, thus resulting correspondingly in a non-magnetic
closed-shell state or an open-shell paramagnetic system. For the latter, we
found the spectral fingerprint of a narrow Coulomb correlation gap. Charged
states, up to 19 additional electrons, were identified by comparing mean-field
Hubbard (MFH) simulations of the density of states with experimental maps of
the discretized molecular orbitals acquired with a scanning tunnelling
microscope (STM). In consideration of the length-dependent magnetic moment and
the discrete nature of the electronic structure, we envisage that GNRs
supported by thin insulating films can be used as tailor-made active elements
in quantum sensing and quantum information processing.
