Electronic and magnetic properties of graphene quantum dots with two charged vacancies

Electronic and magnetic properties of a system of two charged vacancies in hexagonal shaped graphene quantum dots are investigated using a mean-field Hubbard model as a function of the Coulomb potential strength beta of the charge impurities and the distance R between them. For beta = 0, the magnetic properties of the vacancies are dictated by Lieb's rules where the opposite (same) sublattice vacancies are coupled antiferromagnetically (ferromagnetically) and exhibit Fermi oscillations. Here, we demonstrate the emergence of a non-magnetic regime within the subcritical region: as the Coulomb potential strength is increased to beta similar to 0.1, before reaching the frustrated atomic collapse regime, the magnetization is strongly suppressed and the ground state total spin projection is given by S-z = 0 both for opposite and same sublattice vacancy configurations. When long-range electron-electron interactions are included within extended mean-field Hubbard model, the critical value for the frustrated collapse increases from beta(cf) similar to 0.28 to beta(cf) similar to 0.36 for R < 27 angstrom.