Phonon spectra of PrFeAsO via inelastic X-ray scattering

Fukuda, Tatsuo  ; Baron, A. Q. R.*; Nakamura, Hiroki  ; Shamoto, Shinichi  ; Ishikado, Motoyuki; Machida, Masahiko  ; Uchiyama, Hiroshi*; Iyo, Akira*; Kito, Hijiri*; Mizuki, Junichiro; Arai, Masatoshi; Eisaki, Hiroshi*

We have investigated phonons of PrFeAsO using high-resolution inelastic X-ray scattering and calculations. Extensive measurements show that there are some changes in phonon spectra with temperature and/or doping. However, we compare our data with several calculations and find that the experimentally observed changes are much smaller than differences between the experiment. data and the calculations. Agreement is improved if magnetism is included in the calculations. However, still in disagreement even in the parent sample below T. We consider several modifications of the calculations. The best agreement is obtained from modifying the nonmagnetic calculation. by softening only the in-plane nearest-neighbor Fe-As force constant. Fluctuating magnetism may be a partial explanation for the failure of the calculations, but seems incomplete in the face of the similarity of the measured phonon response in all the systems investigated including those known to have static magnetism.