  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.
