Induced chiral Dirac fermions in graphene by a periodically modulated magnetic field

The effect of a modulated magnetic field on the electronic structure of neutral graphene is examined in this paper. It is found that application of a small staggered modulated magnetic field does not destroy the Dirac-cone structure of graphene and so preserves its fourfold zero-energy degeneracy. The original Dirac points (DPs) are just shifted to other positions in k space. By varying the staggered field gradually, new DPs with exactly the same electron-hole crossing energy as that of the original DPs are generated, and both the new and original DPs are moving continuously. Once two DPs are shifted to the same position, they annihilate each other and vanish. The process of generation and evolution of these DPs with the staggered field is found to have a very interesting pattern, which is examined carefully. Generally, there exists a corresponding branch of anisotropic massless fermions for each pair of DPs, with the result that each Landau level (LL) is still fourfold degenerate except the zeroth LL which has a robust 4n(t)-fold degeneracy with n(t) the total number of pairs of DPs. As a result, the Hall conductivity sigma(xy) shows a step of size 4n(t)e(2)/h across zero energy.