Lattice and electronic anomalies of CaFe2 As2 studied by Raman spectroscopy

We report inelastic light-scattering experiments on CaFe2 As2 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 A1g and B1g symmetries, respectively. Upon heating through the tetragonal-to-orthorhombic transition at about TS =173□K, the B1g phonon undergoes a discontinuous drop of frequency by 4□ cm-1, whereas the A1g phonon shows a suppression of the integrated intensity. Their linewidth increases strongly with increasing temperature and saturates above TS. This suggests (i) a first-order structural phase transition and (ii) a drastic change in charge distribution within the FeAs plane through TS.