Infrared phonon anomaly in BaFe$_2$As$_2$

The detailed in-plane optical properties of single-crystal BaFe$_2$As$_2$ have been determined over a wide frequency range above and below the structural and magnetic transition at $T_N \simeq 138$~K. Both infrared-active $E_u$ modes are observed at 94 and 253~cm$^{-1}$ (11.6 and 31.4~meV) at 295~K. Below $T_N$ the modes are expected to split, $E_u \rightarrow B_{2u} + B_{3u}$. The 94~cm$^{-1}$ mode displays little temperature dependence, but may split at low temperature. In contrast, the 253~cm$^{-1}$ vibration softens discontinuously at $T_N$ but does not split; for $T < T_N$ the frequency of this mode displays almost no temperature dependence, yet it nearly doubles in intensity.\footnote{A. Akrap {\em et al.}, Phys. Rev. B {\bf 80}, 180502(R), (2009).} This anomalous behavior appears to be a consequence of orbital ordering in the Fe-As layers.\footnote{C.-C. Lee, W.-G. Yin, and W. Ku, arXiv:0905.2957.}