Magnetic degeneracy and hidden metallicity of the spin-density-wave state in ferropnictides

Abstract

We analyze spin-density-wave (SDW) order in iron-based superconductors and electronic structure in the SDW phase. We consider an itinerant model for Fe pnictides with two hole bands centered at (0,0) and two electron bands centered at $(0,\ensuremath{\pi})$ and $(\ensuremath{\pi},0)$ in the unfolded Brillouin zone. A SDW order in such a model is generally a combination of two components with momenta $(0,\ensuremath{\pi})$ and $(\ensuremath{\pi},0)$, both yield $(\ensuremath{\pi},\ensuremath{\pi})$ order in the folded zone. Neutron experiments, however, indicate that only one component is present. We show that $(0,\ensuremath{\pi})$ or $(\ensuremath{\pi},0)$ order is selected if we assume that only one hole band is involved in the SDW mixing with electron bands. A SDW order in such three-band model is highly degenerate for a perfect nesting and hole-electron interaction only but we show that ellipticity of electron pockets and interactions between electron bands break the degeneracy and favor the desired $(0,\ensuremath{\pi})$ or $(\ensuremath{\pi},0)$ order. We further show that stripe-ordered system remains a metal for arbitrary coupling. We analyze electronic structure for parameters relevant to the pnictides and argue that the resulting electronic structure is in good agreement with angle-resolved photoemission experiments. We discuss the differences between our model and ${J}_{1}\text{\ensuremath{-}}{J}_{2}$ model of localized spins.