Magnetic moments in the presence of topological defects in graphene

Instituto de Ciencia de Materiales de Madrid,CSIC, Cantoblanco, E-28049 Madrid, Spain.(Dated: June 21, 2008)We study the influence of pentagons, dislocations and other topological defects breaking thesublattice symmetry on the magnetic properties of a graphene lattice in a Hartree Fock mean fieldscheme. The ground state of the system with a number of vacancies or similar defects belonging tothe same sublattice is known to have total spin equal to the number of uncoordinated atoms in thelattice for any value of the Coulomb repulsion U according to the Lieb theorem. We show that thepresence of a single pentagonal ring in a large lattice is enough to alter this behavior and a criticalvalue of U is needed to get the polarized ground state. Glide dislocations made of a pentagon-heptagon pair induce more dramatic changes on the lattice and the critical value of U needed topolarize the ground state depends on the density and on the relative position of the defects. Wefound a region in parameter space where the polarized and unpolarized ground states coexist.