Theory of Multiband Superconductivity in Iron Pnictides

The precise nature of unconventional superconductivity in Iron Pnictides is presently a hotly debated issue. Here, using insights from normal state electronic structure and symmetry arguments, we show how an unconventional SC emerges from the bad metal "normal" state. Short-ranged, multi-band spin- and charge correlations generates nodeless SC in the active planar $d_{xz,yz}$ bands, and an inter-band proximity effect induces out-of-plane gap nodes in the passive $d_{3z^{2}-r^{2}}$ band. While very good quantitative agreement with various key observations in the SC state and reconciliation with NMR and penetration depth data in the same picture are particularly attractive features of our proposal, clinching evidence would be an experimental confirmation of c-axis nodes in future work.Comment: 4 pages, 2 eps figures, submitted to PRL, text modifie

Theory of Magnetic Fluctuations in Iron Pnictides

Magnetic fluctuations in an unconventional superconductor (U-SC) can distinguish between distinct proposals for the symmetry of the order parameter. Motivated thereby, we undertake a study magnetic fluctuations in Iron pnictides, tracking their evolution from the incoherent normal, pseudogapped metal, to the U-SC state. Within our proposal of extended-s-plus s_{xy} inplane gap with proximity-induced out-of-plane line nodes, (i) we describe the evolution of the spin-lattice relaxation rate, from a non-Korringa form in the normal state, to a power-law form in the U-SC in good agreement with experiment, and (ii) we predict a sharp resonance in the U-SC state along (\pi,\pi), but not along (\pi/2,0), along with modulated c-axis intensity in inelastic neutron scattering work as a specific and testable manifestation of our proposal.Comment: 4 pages, 3 figures - to be submitted to PR