Phonon splitting and anomalous enhancement of infrared-active modes in   BaFe$_2$As$_2$
  We present a comprehensive infrared spectroscopic study of lattice dynamics in the pnictide parent compound BaFe$_2$As$_2$. In the tetragonal structural phase, we observe the two degenerate symmetry-allowed in-plane infrared active phonon modes. Following the structural transition from the tetragonal to orthorhombic phase, we observe splitting into four non-degenerate phonon modes and a significant phonon strength enhancement. These detailed data allow us to provide a physical explanation for the anomalous phonon strength enhancement as the result of anisotropic conductivity due to Hund's coupling. 
A. A. Schafgans, B. C. Pursley, A. D. LaForge, A. S. Sefat, D.   Mandrus, and D. N. Basov
cond-mat.supr-con cond-mat.str-el