Compositional dependence of the superconducting resonance in underdoped Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$

The low energy magnetic fluctuation spectra of Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ single crystals in the range $0.040 \le x \le 0.055$ were studied in their antiferromagnetically ordered state, above and below their superconducting transition temperature ($T_c$), using inelastic neutron scattering. The normal state excitation spectra are best described by a model of overdamped correlated spin fluctuations characteristic of the paramagnetic phase. Upon entering the superconducting state the excitation spectra is modified by the superconducting resonance. The superconducting resonance energy and its relation to $T_c$, the dispersion of the resonance and its dimensionality, and the absolute spectral weight of the resonance will be discussed in terms of their composition dependence.\\[0pt] Work at the Ames Laboratory was supported by US DOE, Basic Energy Sciences, DMSE under Contract No. DE-AC02-07CH11358. Part of the research conducted at ORNL's High Flux Isotope Reactor was sponsored by the Scientific User Facilities Division, BES, US DOE. Some work at Oak Ridge was supported by US DOE, Basic Energy Sciences, DMSE.