Elementary and collective excitations as probes for order parameter symmetry in Fe-based superconductors

Abstract

We show that the superconducting quasiparticle energy of Fe-based superconductors would be different for a sign-changing order parameter than that for a non–sign-changing order parameter. This particularly happens in the low-doping region when the electron and hole Fermi surfaces nest and superconductivity coexists with either magnetic or orbital or both density waves. This difference in the nature of superconducting quasiparticles is also detectable from collective mode excitations. We further show that while the magnetic spin density wave order competes with superconductivity, the orbital density wave grows when the superconductivity order parameter changes sign accross the electron-hole Fermi surface. However, when the superconducting order parameter does not change sign both the density waves compete with superconductivity. Irrespective of the nature of the superconducting order parameter the spin and orbital density waves are concurrent indicating simultaneous magnetic and structural transition. The significance of the various results is discussed comparing the various experimental observations.