Anomalous phonon redshift in K-doped BaFe2As2 iron pnictides

TL;DR

This study investigates how different dopants affect lattice vibrations in BaFe2As2, revealing an unusual phonon redshift in K-doped samples likely due to coupling with charge or spin, contrasting with other dopants.

Contribution

It uncovers the anomalous phonon redshift in K-doped BaFe2As2 and suggests coupling effects as its origin, which is a novel insight into lattice dynamics in this material.

Findings

Co and P doping cause phonon blue shift due to lattice parameter changes

K doping induces an anomalous phonon red shift, unlike Co and P

Evidence of coupling between lattice vibrations and charge/spin in K-doped samples

Abstract

The effect of K, Co and P dopings on the lattice dynamics in the BaFe$_2$As$_2$ system is studied by infrared spectroscopy. We focus on the phonon at $\sim$ 253 cm$^{-1}$, the highest energy in-plane infrared-active Fe-As mode in BaFe$_2$As$_2$. Our studies show that the Co and P dopings lead to a blue shift of this phonon in frequency, which can be simply interpreted by the change of lattice parameters induced by doping. In sharp contrast, an unusual red shift of the same mode was observed in the K-doped compound, at odds with the above explanation. This anomalous behavior in K-doped BaFe$_2$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.