▎ 摘　　要

Using density functional calculations we have investigated the local spin moment formation and lattice deformation in graphene when an isolated vacancy is created. We predict two competing equilibrium structures: a ground-state planar configuration with a saturated local moment of 1.5 mu(B) and a metastable nonplanar configuration with a vanishing magnetic moment, at a modest energy expense of 50 meV. Though nonplanarity relieves the lattice of vacancy-induced strain, the planar state is energetically favored due to maximally localized defect states (v sigma, v pi). In the planar configuration, charge transfer from itinerant (Dirac) states weakens the spin polarization of v pi yielding a fractional moment, which is aligned parallel to the unpaired v sigma electron through Hund's coupling. As a by-product, the Dirac states (d pi) of the two sublattices undergo a minor spin polarization and couple antiferromagnetically. In the nonplanar configuration, the absence of orthogonal symmetry allows interaction between v sigma and local d pi states, to form a hybridized v sigma' state. The nonorthogonality also destabilizes the Hund's coupling, and an antiparallel alignment between v sigma and v pi lowers the energy. The gradual spin reversal of v pi with increasing nonplanarity opens up the possibility of an intermediate structure with a balanced v pi spin population. If such a structure is realized under external perturbations, diluted vacancy concentration may lead to v sigma-based spin-1/2 paramagnetism. Carrier doping, electron or hole, does not alter the structural stability. However, the doping proportionately changes the occupancy of v pi state and hence the net magnetic moment.