Phonon splitting and anomalous enhancement of infrared-active modes in BaFe2As2

We present a comprehensive infrared spectroscopic study of lattice dynamics in the pnictide parent compound BaFe2As2. 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 the orthorhombic phase, we observe a splitting into four nondegenerate 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 being the result of anisotropic conductivity due to Hund's coupling.