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 NaFe1−xCoxAs 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 Tc drops by nearly a half compared to optimal doping. These results suggest that the energy of the resonance in electron overdoped NaFe1−xCoxAs is neither simply proportional to Tc nor the superconducting gap, but is controlled by the multi-orbital character of the system and doped impurity scattering effect.
@article{arxiv.1605.06890,
title = {Electron doping evolution of the neutron spin resonance in NaFe$_{1-x}$Co$_{x}$As},
author = {Chenglin Zhang and Weicheng Lv and Guotai Tan and Yu Song and Scott V. Carr and Songxue Chi and M. Matsuda and A. D. Christianson and J. A. Fernandez-Baca and L. W. Harriger and Pengcheng Dai},
journal= {arXiv preprint arXiv:1605.06890},
year = {2016}
}