Inelastic Neutron-Scattering Measurements of a Three-Dimensional Spin Resonance in the FeAs-Based BaFe1.9Ni0.1As2 Superconductor

We use inelastic neutron scattering to study magnetic excitations of the FeAs-based superconductor BaFe1.9Ni0.1As2 above and below its T-c (=20 K). In addition to gradually open a spin gap at the in-plane antiferromagnetic ordering wave vector (1, 0, 0), the effect of superconductivity is to form a three-dimensional resonance with clear dispersion along the c axis. 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 associated with the superconducting gap energy Delta, then Delta is dependent on the wave vector 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.