Three-dimensional Resonance in superconducting   BaFe$_{1.9}$Ni$_{0.1}$As$_2$
  We use inelastic neutron scattering to study magnetic excitations of the FeAs-based superconductor BaFe$_{1.9}$Ni$_{0.1}$As$_2$ above and below its superconducting transition temperature $T_c=20$ K. In addition to gradually open a spin gap at the in-plane antiferromagnetic ordering wavevector $(1,0,0)$, the effect of superconductivity is to form a three dimensional resonance with clear dispersion along the c-axis direction. The intensity of the resonance develops like a superconducting order parameter, and the mode occurs at distinctively different energies at $(1,0,0)$ and $(1,0,1)$. If the resonance energy is directly associated with the superconducting gap energy $\Delta$, then $\Delta$ is dependent on the wavevector transfers along the c-axis. These results suggest that one must be careful in interpreting the superconducting gap energies obtained by surface sensitive probes such as scanning tunneling microscopy and angle resolved photoemission. 
Songxue Chi, Astrid Schneidewind, Jun Zhao, Leland W. Harriger, Linjun   Li, Yongkang Luo, Guanghan Cao, Zhu'an Xu, Micheal ...