Phonon anomalies in AEFe₂As₂ (AE = Ca, Sr and Ba)

The infrared vibrational properties of AEFe₂As₂ (AE = Ca, Sr and Ba) have been measured above and below the structural and magnetic transition at T_N = 172, 195 and 138 K [1], and compared to ab initio calculations of the frequency and atomic character at the center of the Brillouin zone. In the Ca compound both of the infrared-active E_u modes split below T_N. However, while the low-frequency mode splits in the Sr compound, the high-frequency mode appears to only have a lower branch which displays an anomalous increase in strength below T_N. In the Ba compound, the high-frequency mode again fails to split and the oscillator strength increases dramatically below T_N [2]; furthermore, there is a net decrease in the strength of the low-frequency modes at low temperature. Ab initio calculations indicate that the nature of the high-frequency mode does not change across this series of compounds. The presence of SDW order along the b axis below T_N suggests the nature of the bonding has been altered due to orbital ordering, possibly explaining the lack of splitting and the increase in strength of the high-frequency mode.

[1] Y. M. Dai et al., Phys. Rev. B 94, 195142 (2016).
[2] A. Akrap et al., Phys. Rev. B 80, 180502(R) (2009).