Doping dependence of the anisotropic quasiparticle interference in   NaFe1-xCoxAs iron-based superconductors
  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. 
Peng Cai, Wei Ruan, Xiaodong Zhou, Cun Ye, Aifeng Wang, Xianhui Chen,   Dung-Hai Lee, and Yayu Wang
cond-mat.supr-con cond-mat.str-el