Spin-orbit coupling in Fe-based superconductors

The recently discovered iron-based superconductors have attracted considerable attention mainly for their unconventional pairing state. In connection with the determination of pairing symmetry, the resonance peak observed in neutron scattering experiments [1] agrees well with predicted results for the extended s-wave ($s_\pm$) gap symmetry [2]. However, recent neutron measurements shows that there is anisotropy in the spin resonance [3]. In particular, $S^zS^z$ component is different from $S^+S^-$ component of the dynamical spin susceptibility. Such breaking of the spin-rotational invariance in the spin-liquid phase without long-range order can occur due to spin-orbit (SO) coupling. We study the role of the SO coupling in the multiorbital model for Fe-pnictides, and discuss how it influences spin resonance feature and the relation to SC pairing. \\[4pt] [1]. A.D. Christianson \textit{et al.}, Nature \textbf{456}, 930 (2008). [2]. M.M. Korshunov and I. Eremin, Phys. Rev. B \textbf{78}, 140509(R) (2008); T.A. Maier and D.J. Scalapino, \textit{ibid}, 020514(R) (2008). [3]. O.J. Lipscombe \textit{et al.}, Phys. Rev. B \textbf{82}, 064515 (2010).