Existence of Orbital Order and its Fluctuation in   Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ Revealed by X-ray Absorption Spectroscopy
  We performed temperature dependent X-ray linear dichroism (XLD) experiments on an iron pnictide system, Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ with $x$=0.00 and 0.05, to experimentally verify existence of orbital ordering (OO). We observed clear XLD in polarization dependent X-ray absorption spectra of Fe $L$ edges. By exploiting the difference in the temperature dependent behaviors, we were able to separate OO and structure contributions to XLD. The observed OO signal indicates different occupation numbers for $d_{yz}$ and $d_{zx}$ orbitals and supports existence of a ferro-OO. The results are also consistent with the theoretical prediction. Moreover, we find substantial OO signal above the transition temperature, which suggests that OO fluctuation exists well above the transition temperature. 
Y. K. Kim, W. S. Jung, G. R. Han, K.-Y. Choi, K.-H. Kim, C.-C. Chen,   T. P. Devereaux, A. Chainani, J. Miyawaki, Y. Takata,...
cond-mat.supr-con