Anomalous Phonon Redshift in K-Doped Bafe2as2 Iron Pnictides

The effect of K, Co, and P dopings on the lattice dynamics in the BaFe2As2 system is studied by infrared spectroscopy. We focus on the phonon at 253 cm- 1, the highest energy in- plane infrared- active Fe- As mode in BaFe2As2. Our studies show that Co and P dopings lead to a blueshift of this phonon in frequency, which can be simply interpreted by the change in lattice parameters induced by doping. In sharp contrast, an unusual redshift of the same mode was observed in the K- doped compound, at odds with the above explanation. This anomalous behavior in K- doped BaFe2As2 is more likely associated with the coupling between lattice vibrations and other channels, such as charge or spin. This coupling scenario is also supported by the asymmetric line shape and intensity growth of the phonon in the K- doped compound.