Coherence effect in a two-band superconductor: Application to iron pnictides

From a theoretical point of view, we propose an experimental method to determine the pairing symmetry of iron pnictides. We focus on two kinds of pairing symmetries, s_+- and s₊₊, which are strong candidates for the pairing symmetry of iron pnictides. For each of these two symmetries, we calculate both the density and spin response functions by using the two-band BCS model within the one-loop approximation. As a result, a clear difference is found between the s_+- and s₊₊-wave states in the temperature dependence of the response functions at nesting vector Q, which connects the hole and electron Fermi surfaces. We point out that this difference comes from the coherence effect in the twoband superconductor. We suggest that the pairing symmetry could be clarified by observing the temperature dependence of both the density and spin structure factors at the nesting vector Q in neutron scattering measurements.