  The discovery of high-transition temperature (high-Tc) superconductivity near
antiferromagnetism in iron arsenides raised the possibility of an
unconventional superconducting mechansim1-8. The observation of clear Fermi
surfaces and nodeless superconducting gaps by angle resolved photoemission9-12
suggests that electron pairing in these materials may be mediated by
quasiparticle excitations between sign reversed hole and electron Fermi
pockets5-8. Although the presence of a 'resonance' in the spin excitation
spectrum found by inelastic neutron scattering13-17 is consistent with this
picture18-20, there has been no direct evidence connecting the resonance to the
superconducting gap energy. Here we show that for the optimally electron doped
BaFe1.9Ni0.1As2 (Tc =20 K, Fig. 1c) iron arsenide superconductor, application
of a magnetic field that suppresses the superconductivity and superconducting
gap energy also reduces the intensity and energy of the resonance. These
results suggest that the energy of the resonance is proportional to the
electron pairing energy, and thus indicate that spin fluctuations are
intimately related to the mechanism of superconductivity in iron arsenides.
