Effect of magnetism on lattice dynamics of SrFeAs via high-resolution inelastic X-ray scattering

Murai, Naoki  ; Fukuda, Tatsuo  ; Kobayashi, Tatsuya*; Nakajima, Masamichi*; Uchiyama, Hiroshi*; Ishikawa, Daisuke*; Tsutsui, Satoshi*; Nakamura, Hiroki  ; Machida, Masahiko  ; Miyasaka, Shigeki*; Tajima, Setsuko*

We report the first IXS study of single crystals of SrFeAs that have been detwinned via application of in-plane uniaxial stress. Our results clearly show anisotropy in phonon structure below = characterized by mode splitting at tetragonally-equivalent momentum transfers. To the best of our knowledge, this is the first observation of phonon anisotropy in iron-pnictides at finite momentum transfers. We obtain good agreement with our results using calculations that are modified to account for a reduced magnetic moment and fluctuating magnetism. Previous efforts using either non-magnetic calculations or static magnetism tend to either get the dispersion wrong from non-magnetic calculations or strongly over-estimate the phonon mode splitting when static magnetism is included. Our model for the response below incorporates a phenomenological reduction in force constant anisotropy, by about 65%, that can be attributed to residual fast magnetic fluctuations. Above , all fluctuations are assumed to be fast and the response becomes the average of the results from the static magnetism. This serves as a starting point for a general model of phonons in iron-pnictides applicable to both non-magnetic and magnetic phases.