Orbital resonance mode in superconducting iron pnictides

We show that the fluctuations associated with ferro-orbital order in the d_xz and d_yz orbitals can develop a sharp resonance mode in the superconducting state with a nodeless gap on the Fermi surface. This orbital resonance mode appears below the particle-hole continuum and is analogous to the magnetic resonance mode found in various unconventional superconductors. If the pairing symmetry is , a dynamical coupling between the orbital ordering and the d-wave sub-dominant pairing channels is present by symmetry. Therefore, the nature of the resonance mode depends on the relative strengths of the fluctuations in these two channels, which could vary significantly for different families of the iron-based superconductors. The application of our theory to a recent observation of a new δ-function-like peak in the B₁g Raman spectrum of Ba_0.6K _0.4Fe₂As₂ is discussed, and we predict that the same orbital resonance mode can be detected in electron energy loss spectroscopy (EELS).