We use scanning tunneling microscopy to investigate the doping dependence of quasiparticle interference (QPI) in NaFe1-xCoxAs iron-based superconductors. The goal is to study the relation between nematic fluctuations and Cooper pairing. In the parent and underdoped compounds, where four-fold rotational symmetry is broken macroscopically, the QPI patterns reveal strong rotational anisotropy. At optimal doping, however, the QPI patterns are always four-fold symmetric. We argue this implies small nematic susceptibility and hence insignificant nematic fluctuation in optimally doped iron pnictides. Since Tc is the highest this suggests nematic fluctuation is not a prerequistite for strong Cooper pairing.
@article{arxiv.1403.0090,
title = {Doping dependence of the anisotropic quasiparticle interference in NaFe1-xCoxAs iron-based superconductors},
author = {Peng Cai and Wei Ruan and Xiaodong Zhou and Cun Ye and Aifeng Wang and Xianhui Chen and Dung-Hai Lee and Yayu Wang},
journal= {arXiv preprint arXiv:1403.0090},
year = {2014}
}