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 SrFe2As2 and BaFe2As2 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. The speed of the Dirac fermions in SrFe2As2 and BaFe2As2 is found to be 14-20 times slower than in graphene, suggesting that the pnictides provide a laboratory for exploring the effects of strongly interacting Dirac fermions.Comment: 4 page

Geometric Frustration and Dimensional Reduction at a Quantum Critical Point

We show that the spatial dimensionality of the quantum critical point associated with Bose--Einstein condensation at T=0 is reduced when the underlying lattice comprises a set of layers coupled by a frustrating interaction. Our theoretical predictions for the critical temperature as a function of the chemical potential correspond very well with recent measurements in BaCuSi$_{2}$O$_{6}$ [S. E. Sebastian \textit{et al}, Nature \textbf{411}, 617 (2006)].Comment: 5 pages, 2 figure