High-temperature superconductivity in iron pnictides occurs when electrons are doped into their antiferromagnetic (AF) parent compounds. In addition to inducing superconductivity, electron-doping also changes the static commensurate AF order in the undoped parent compounds into short-range incommensurate AF order near optimal superconductivity. Here we use neutron scattering to demonstrate that the incommensurate AF order in BaFe2−xNixAs2 is not a spin-density-wave arising from the itinerant electrons in nested Fermi surfaces, but consistent with a cluster spin glass in the matrix of the superconducting phase. Therefore, optimal superconductivity in iron pnictides coexists and competes with a mesoscopically separated cluster spin glass phase, much different from the homogeneous coexisting AF and superconducting phases in the underdoped regime.
@article{arxiv.1407.0568,
title = {Short-range cluster spin glass near optimal superconductivity in BaFe$_{2-x}$Ni$_{x}$As$_{2}$},
author = {Xingye Lu and David W. Tam and Chenglin Zhang and Huiqian Luo and Meng Wang and Rui Zhang and Leland W. Harriger and T. Keller and B. Keimer and L. -P. Regnault and Thomas A. Maier and Pengcheng Dai},
journal= {arXiv preprint arXiv:1407.0568},
year = {2014}
}