Dirac nodal pockets in the antiferromagnetic parent phase of FeAs superconductors

We show that previously measured small Fermi-surface pockets within the antiferromagnetic phase of SrFe(2)As(2) and BaFe(2)As(2) are consistent with a Dirac dispersion modulated by interlayer hopping, giving rise to a Dirac point in k space and a cusp in the magnetic field angle-dependent magnetic quantum oscillation frequencies. These findings support the existence of a nodal spin-density wave in these materials, which could play an important role in protecting the metallic state against localization effects. We show that further angle-dependent measurements of the cyclotron effective mass and quantum oscillation amplitude can potentially be used to explore the properties of the quasiparticles close to the Dirac point. The speed of the Dirac fermions in SrFe(2)As(2) and BaFe(2)As(2) is found to be 14-20 times slower than in graphene, suggesting that the pnictides provide a laboratory for exploring quantized Dirac fermions with a much smaller energy scale, making them more likely to be affected by interactions.