Based on the effective four-band model we analyze the spin response in the normal and superconducting states of the Fe-pnictide superconductors. While the normal-state spin excitations are dominated by the continuum of the interorbital antiferromagnetic fluctuations and the intraband spin-density wave fluctuations, the unconventional superconductivity yields different feedback. The resonance peak in the form of the well-defined spin exciton occurs only for the interband scattering at the antiferromagnetic momentum ${\mathbf{Q}}_{\text{AFM}}$ for the $s_{\pm }$ (extended $s$ wave) superconducting order parameter and it disappears rapidly for $\mathbf{q}<{\mathbf{Q}}_{\text{AFM}}$. The resonance feature is extremely weak for the $d_{x^2-y^2}$-wave order parameter due to the specific Fermi-surface topology of these compounds. The essential difference between $s_{\pm }$-wave and $d_{x^2-y^2}$-wave symmetries for the magnetic excitations can be used for experimental determination of the superconducting wave-function symmetry.

• Received 28 September 2008

DOI:https://doi.org/10.1103/PhysRevB.78.140509