Dispersion of the superconducting spin resonance in underdoped and antiferromagnetic BaFe2As2

Inelastic neutron-scattering measurements have been performed on underdoped Ba(Fe1-xCox)(2)As-2 (x=4.7%) where superconductivity and long-range antiferromagnetic (AFM) order coexist. The magnetic spectrum found in the normal state is strongly damped and develops into a magnetic resonance feature below T-c that has appreciable dispersion along c axis with a bandwidth of 3-4 meV. This is in contrast to the optimally doped x=8.0% composition, with no long-range AFM order, where the resonance exhibits a much weaker dispersion. [see M. D. Lumsden et al., Phys. Rev. Lett. 102, 107005 (2009).] The results suggest that the resonance dispersion arises from interlayer spin correlations present in the AFM ordered state.