Influence of magnetism on phonons in ${\text{CaFe}}_2{\text{As}}_2$ as seen via inelastic x-ray scattering

In the iron pnictides, the strong sensitivity of the iron magnetic moment to the arsenic position suggests a significant relationship between phonons and magnetism. We measured the phonon dispersion of several branches in the high-temperature tetragonal phase of ${\text{CaFe}}_2{\text{As}}_2$ using inelastic x-ray scattering on single-crystal samples. These measurements were compared to ab initio calculations of the phonons. Spin-polarized calculations imposing the antiferromagnetic order present in the low-temperature orthorhombic phase dramatically improve agreement between theory and experiment. This is discussed in terms of the strong antiferromagnetic correlations that are known to persist in the tetragonal phase.

Figure 1

(Color online) (a) Constant-$Q$ line scan at $Q=\left(0,0,5.75\right)$. (b) Constant-$Q$ line scan at $Q=\left(1.5,0,3.0\right)$. Experimental data are given by points and pseudo-Voigt fits by the solid line. The dotted (red) and dashed (blue) lines correspond to calculations of the phonon structure factor for spin-polarized and nonmagnetic calculations, respectively.

Figure 2

(Color online) Phonon dispersion weighted by the structure factor along $\left(00L\right)$ (a) without and (b) with antiferromagnetic order. The white dots are experimental data points.

Figure 3

(Color online) Phonon dispersion weighted by the structure factor along $\left(H03\right)$ (a) without and (b) with antiferromagnetic order. The white dots are experimental data points.