Anomalous phonon redshift in K-dopedBaFe2As2iron pnictides

The effect of K, Co, and P dopings on the lattice dynamics in the ${{\mathrm{BaFe}}_{2}\mathrm{As}}_{2}$ system is studied by infrared spectroscopy. We focus on the phonon at $\ensuremath{\sim}253\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, the highest energy in-plane infrared-active Fe-As mode in ${{\mathrm{BaFe}}_{2}\mathrm{As}}_{2}$. 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 ${{\mathrm{BaFe}}_{2}\mathrm{As}}_{2}$ 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.