Electron doping evolution of the neutron spin resonance in   NaFe$_{1-x}$Co$_{x}$As
  Neutron spin resonance, a collective magnetic excitation coupled to superconductivity, is one of the most prominent features shared by a broad family of unconventional superconductors including copper oxides, iron pnictides, and heavy fermions. In this work, we study the doping evolution of the resonances in NaFe$_{1-x}$Co$_x$As covering the entire superconducting dome. For the underdoped compositions, two resonance modes coexist. As doping increases, the low-energy resonance gradually loses its spectral weight to the high-energy one but remains at the same energy. By contrast, in the overdoped regime we only find one single resonance, which acquires a broader width in both energy and momentum, but retains approximately the same peak position even when $T_c$ drops by nearly a half compared to optimal doping. These results suggest that the energy of the resonance in electron overdoped NaFe$_{1-x}$Co$_x$As is neither simply proportional to $T_c$ nor the superconducting gap, but is controlled by the multi-orbital character of the system and doped impurity scattering effect. 
Chenglin Zhang, Weicheng Lv, Guotai Tan, Yu Song, Scott V. Carr,   Songxue Chi, M. Matsuda, A. D. Christianson, J. A. Fernan...
cond-mat.supr-con cond-mat.str-el