Lattice and electronic anomalies of CaFe₂As₂ studied by Raman spectroscopy

We report inelastic light-scattering experiments on CaFe₂As₂ in the temperature range of 4–290 K. In in-plane polarizations two Raman-active phonon modes are observed at 189 and 211 cm⁻¹, displaying A_1g and B_1g symmetries, respectively. Upon heating through the tetragonal-to-orthorhombic transition at about T_S=173 K, the B_1g phonon undergoes a discontinuous drop of frequency by 4 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.