Magnetic Resonant Mode in the Low-Energy Spin-Excitation Spectrum of   Superconducting Rb2Fe4Se5 Single Crystals
  We have studied the low-energy spin-excitation spectrum of the single-crystalline Rb2Fe4Se5 superconductor (Tc = 32 K) by means of inelastic neutron scattering. In the superconducting state, we observe a magnetic resonant mode centered at an energy of 14 meV and at the (0.5 0.25 0.5) wave vector (unfolded Fe-sublattice notation), which differs from the ones characterizing magnetic resonant modes in other iron-based superconductors. Our finding suggests that the 245-iron-selenides are unconventional superconductors with a sign-changing order parameter, in which bulk superconductivity coexists with the sqrt(5) x sqrt(5) magnetic superstructure. The estimated ratios of the resonance energy to Tc and the superconducting gap indicate moderate pairing strength in this compound, similar to that in optimally doped 1111- and 122-pnictides. 
J. T. Park, G. Friemel, Yuan Li, J.-H. Kim, V. Tsurkan, J.   Deisenhofer, H.-A. Krug von Nidda, A. Loidl, A. Ivanov, B. Keim...
cond-mat.supr-con cond-mat.str-el