Two-Fluid Description for Iron-Based Superconductors

We present a two-fluid description for iron-based superconductors, which contains an itinerant electron Fermi-liquid and a local moment spin-liquid, coupled together via an effective Hund's rule interaction. We examine the low-energy collective behavior of such a system. We find that an electron-spinon composite mode emerges in the intermediate coupling regime, which may account for the hump-dip behavior observed in the resent scanning tunneling spectroscopy experiments. The superconductivity and spin-density-wave phases are consistently described within the same framework. Possible experimental test is also proposed.