Effect of Co doping on the in-plane anisotropy in the optical spectrum   of underdoped Ba(Fe1-xCox)2As2
  We investigated the anisotropy in the in-plane optical spectra of detwinned Ba(Fe1-xCox)2As2. The optical conductivity spectrum of BaFe2As2 shows appreciable anisotropy in the magnetostructural ordered phase, whereas the dc resistivity is almost isotropic at low temperatures. Upon Co doping, the resistivity becomes highly anisotropic, while the finite-energy intrinsic anisotropy is suppressed. It is found that anisotropy in resistivity arises from anisotropic impurity scattering from doped Co atoms, extrinsic in origin. Intensity of a specific optical phonon mode is also found to show striking anisotropy in the ordered phase. The anisotropy induced by Co impurity and that observed in the optical phonon mode are hallmarks of the highly polarizable electronic state in the ordered phase. 
M. Nakajima, S. Ishida, Y. Tomioka, K. Kihou, C. H. Lee, A. Iyo, T.   Ito, T. Kakeshita, H. Eisaki, and S. Uchida
cond-mat.str-el cond-mat.supr-con