Protected nodal electron pocket from multiple-Q ordering in underdoped high temperature superconductors

A multiple wavevector (Q) reconstruction of the Fermi surface is shown to yield a profoundly different electronic structure to that characteristic of single wavevector reconstruction, despite their proximity in energy. We consider the specific case in which ordering is generated by Qx = [2{\pi}a, 0] and Qy = [0,2{\pi}b] (in which a = b = 1/4) - similar to those identified in neutron diffraction and scanning tunneling microscopy experiments, and more generally show that an isolated pocket adjacent to the nodal point knodal = [\pm {\pi}/2, \pm {\pi}/2] is a protected feature of such a multiple-Q model, potentially corresponding to the nodal 'Fermi arcs' observed in photoemission and the small size of the electronic heat capacity found in high magnetic fields - importantly, containing electron carriers which can yield negative Hall and Seebeck coefficients observed in high magnetic fields.Comment: 4 page

Who is failing abused and neglected children?

This is a response to an article by Nigel Speight and Jane Wynne, ‘Is the Children Act failing severely abused and neglected children?’, published in this journal in March 2000.1 Overall, we consider the article to be polemical and inadequately argued. Many of the points made are unsubstantiated and there are errors of fact. Where does evidence based practice go if senior practitioners prefer anecdotes and personal belief to research findings? Restrictions on space preclude an exhaustive reply to all the points Speight and Wynne raise, so we have confined ourselves to addressing those considered most significant

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

Spin-density wave Fermi surface reconstruction in underdoped YBa2Cu3O6+x

We consider the reconstruction expected for the Fermi surface of underdoped YBa2Cu3O6+x in the case of a collinear spin-density wave with a characteristic vector Q=(pi[1+/-2 delta],pi), assuming an incommensurability delta~0.06 similar to that found in recent neutron scattering experiments. A Fermi surface possibly consistent with the multiple observed quantum oscillation frequencies is obtained. From the low band masses expected using this model as compared with experiment, a uniform enhancement of the quasiparticle effective mass over the Fermi surface by a factor of ~7 is indicated. Further predictions of the Fermi surface topology are made, which may potentially be tested by experiment to indicate the relevance of this model to underdoped YBa2Cu3O6+x.Comment: