Two-dimensional resonant magnetic excitation in BaFe1.84Co0.16As2
  Inelastic neutron scattering measurements on single crystals of superconducting BaFe1.84Co0.16As2 reveal a magnetic excitation located at wavevectors (1/2 1/2 L) in tetragonal notation. On cooling below TC, a clear resonance peak is observed at this wavevector with an energy of 8.6(0.5) meV, corresponding to 4.5(0.3) kBTC. This is in good agreement with the canonical value of 5 kBTC observed in the cuprates. The spectrum shows strong dispersion in the tetragonal plane but very weak dispersion along the c-axis, indicating that the magnetic fluctuations are two-dimensional in nature. This is in sharp contrast to the anisotropic three dimensional spin excitations seen in the undoped parent compounds. 
M. D. Lumsden, A. D. Christianson, D. Parshall, M. B. Stone, S. E.   Nagler, G.J. MacDougall, H. A. Mook, K. Lokshin, T. Ega...