Vibrational anomalies in AFe(2)As(2) (A=Ca, Sr, and Ba) single

er and a lower branch; however, the oscillator strengths are quite different. Surprisingly, in both the Sr and Ba materials, below T-N the upper branch appears to be either very weak or totally absent, while the lower branch displays an anomalous increase in strength. The frequencies and atomic characters of the lattice modes at the center of the Brillouin zone have been calculated for the high-temperature phase for each of these materials. The high-frequency Eu mode does not change in position or character across this series of compounds. Below T-N, the E-u modes are predicted to split into features of roughly equal strength. We discuss the possibility that the anomalous increase in the strength of the lower branch of the high-frequency mode below T-N in the Sr and Ba compounds, and the weak (silent) upper branch, may be related to the orbital ordering and a change in the bonding between the Fe and As atoms in the magnetically ordered state.

Complex States