Soft and isotropic phonons in PrFeAsO

PrFeAsOのソフト化した等方的なフォノン

福田 竜生  ; Baron, A. Q. R.*; 中村 博樹  ; 社本 真一  ; 石角 元志; 町田 昌彦  ; 内山 裕士*; 伊豫 彰*; 鬼頭 聖*; 水木 純一郎; 新井 正敏; 永崎 洋*

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*

Phonons in single crystals of PrFeAsO are investigated using high-resolution inelastic X-ray scattering and calculations. Extensive measurements show some changes in phonon spectra with temperature and/or doping. However, we compare our measurements with several calculations and find that the experimentally observed changes are much smaller than differences between the experimental data and the calculations. Agreement is improved if magnetism is included in the calculations, but still large disagreement remains. Several modifications of the calculations are considered and the best agreement is obtained from modifying the non-magnetic model 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 here including those known to have static magnetism.