Inelastic X-ray scattering studies of the phonon properties on iron-based new superconductors

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

Recently discovered iron-arsenide new superconductor has attracted considerable attraction. Within a few months the transition temperature (Tc) exceeded 50K in some related compounds, and the possible novel physics as well as higher-Tc are expected. In the very early stage calculations of phonon properties appeared and suggested that the superconductivity in these materials are not phonon mediated. However, we carried out inelastic X-ray scattering on some powder as well as single samples, and found pronounced softening of the Fe-As modes compared to LDA-based phonon calculations. Further investigations on superconducting single crystals of PrFeAsO show no significant changes in phonon spectra across Tc at the momentum transfers investigated. However, the spectra are found to agree better with calculations of a magnetic parent material (orthorhombic), though the real superconducting compounds show tetragonal symmetry.