▎ 摘　　要

Recent experiments have demonstrated that neutral graphene sheets have an insulating ground state in the presence of an external magnetic field. We report on a pi-band tight-binding-model Hartree-Fock calculation which examines the competition between distinct candidate insulating ground states. We conclude that for graphene sheets on substrates the ground state is most likely a field-induced spin-density wave and that a charge-density-wave state is possible for suspended samples. Neither of these density-wave states support gapless edge excitations.