  Quantum spins, referred to the spin operator preserved by full SU(2) symmetry
in the absence of the magnetic anistropy, have been proposed to host exotic
interactions with superconductivity4. However, spin orbit coupling and crystal
field splitting normally cause a significant magnetic anisotropy for d/f-shell
spins on surfaces6,9, breaking SU(2) symmetry and fabricating the spins with
Ising properties10. Recently, magnetic nanographenes have been proven to host
intrinsic quantum magnetism due to their negligible spin orbital coupling and
crystal field splitting. Here, we fabricate three atomically precise
nanographenes with the same magnetic ground state of spin S=1/2 on Pb(111)
through engineering sublattice imbalance in graphene honeycomb lattice.
Scanning tunneling spectroscopy reveals the coexistence of magnetic bound
states and Kondo screening in such hybridized system. Through engineering the
magnetic exchange strength between the unpaired spin in nanographenes and
cooper pairs, quantum phase transition from the singlet to the doublet state
has been observed, in consistent with quantum models of spins on
superconductors. Our work demonstrates delocalized graphene magnetism host
highly tunable magnetic bound states with cooper pairs, which can be further
developed to study the Majorana bound states and other rich quantum physics of
low-dimensional quantum spins on superconductors.
