Lattice and electronic anomalies of ${\text{CaFe}}_2{\text{As}}_2$ studied by Raman spectroscopy

We report inelastic light-scattering experiments on ${\text{CaFe}}_2{\text{As}}_2$ in the temperature range of 4–290 K. In in-plane polarizations two Raman-active phonon modes are observed at 189 and $211{\text{cm}}^{-1}$, displaying $A_{1g}$ and $B_{1g}$ symmetries, respectively. Upon heating through the tetragonal-to-orthorhombic transition at about $T_S=173\text{K}$, the $B_{1g}$ phonon undergoes a discontinuous drop of frequency by $4{\text{cm}}^{-1}$, whereas the $A_{1g}$ phonon shows a suppression of the integrated intensity. Their linewidth increases strongly with increasing temperature and saturates above $T_S$. This suggests (i) a first-order structural phase transition and (ii) a drastic change in charge distribution within the FeAs plane through $T_S$.

Figure 1

(Color online) Raman spectra in $\left(xx\right)$ and $\left(xu\right)$ polarizations at 4 and 290 K, respectively. The solid lines denote the $\left(xu\right)$ polarization where the incident light is parallel to the $x$ axis and the scattered light is unpolarized. For the $\left(xx\right)$ polarization, the incident and scattered lights are polarized parallel to the $x$ axis.

Figure 2

(Color online) Comparison of Raman spectra using three different incident laser lines $\lambda =632$, 532, and 488 nm at 4 K.

Figure 3

(Color online) Temperature dependence of Raman spectra. The dashed vertical bar indicates the position of the $211{\text{cm}}^{-1}$ mode at $T=4\text{K}$.

Figure 4

(Left) Temperature dependence of the peak position and linewidth of the $211{\text{cm}}^{-1}$ $B_{1g}$ mode. (Right) Temperature dependence of the peak position and linewidth of the $189{\text{cm}}^{-1}$ $A_{1g}$ mode. The full square represents the blue line data and the open triangle represents the green ones.