Possible vacancy-induced magnetism on a half-filled honeycomb lattice

The effects of impurities on the magnetism of electrons on a half-filled honeycomb lattice are studied using the self-consistent T-matrix approximation and an exact diagonalization analysis. Staggered susceptibility is found to be enhanced by vacancies; it logarithmically diverges as T -> 0 even in the absence of electron-electron interaction. It is found that this strong enhancement of staggered susceptibility is brought about by resonance states at zero energy (zero modes) induced by vacancies. Possible magnetic states induced by vacancies are discussed.