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 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.

Figure 1

Temperature dependence of the real part of the optical conductivity ${\sigma }_1\left(\omega \right)$ in Ba122, focused on the far-infrared frequency range.

Figure 2

The real part of the optical conductivity ${\sigma }_1\left(\omega \right)$, focusing on the phonon modes in Ba122 near 95 ${\text{cm}}^{-1}$ [(a) Ba-Ba] and 257 cm${}^{-1}$ [(b), (c) Fe-As]. We observe both phonon modes to split upon the transition from the HTT to the LTO phases. In (a) and (b) we observe Lorentzian fits to the data, while (c) shows fits of the Fano form [Eq. (1)] to the phonon at 257 cm${}^{-1}$. The fit parameters are shown graphically in (d) (oscillator frequency position, ${\omega }_0$) and (e) (oscillator strength, ${\omega }_p$). Table summarizes the results of our fits.

Figure 3

Temperature evolution of the 257 cm${}^{-1}$ phonon mode in BaFe${}_{1.84}$Co${}_{0.16}$As${}_2$. We find this mode to evolve monotonically with temperature and to be just above the noise floor of our measurement for temperatures at 200 K and below.