Spontaneous quantum Hall effect in quarter-doped Hubbard model on honeycomb lattice and its possible realization in doped graphene system

We show that a magnetic insulating state with nonzero spin chirality is realized in a quarter-doped Hubbard model on honeycomb lattice as a result of the nesting property of the Fermi surface. This state is topological nontrivial and has a quantized Hall conductance of . We find that such a state is robust against next-nearest-neighboring hopping and we propose that it can be realized in a quarter-doped graphene system. We also show that the quarter-doped Hubbard model on honeycomb lattice is equivalent to a 3/4-filled Hubbard model on triangular lattice in the weak coupling limit, in which a similar effect was predicted previously.