Strong Coupling of the Iron-Quadrupole and Anion-Dipole Polarizations in   Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$
  We use a quantitative convergent beam electron diffraction (CBED) based method to image the valence electron density distribution in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. We show a remarkable increase in both the charge quadrupole of the Fe cations and the charge dipole of the arsenic anions upon Co doping from $x=0$ ($T_c=0$ K) to $x=0.1$ ($T_c=22.5$ K). Our data suggest that an unexpected electronic correlation effect, namely strong coupling of Fe orbital fluctuation and anion electronic polarization, is present in iron-based superconductors. 
Chao Ma, Lijun Wu, Wei-Guo Yin, Huaixin Yang, Honglong Shi, Zhiwei   Wang, Jianqi Li, C. C. Homes, and Yimei Zhu
cond-mat.supr-con