Orthorhombic fluctuations in tetragonal Fe2As ( = Sr and Eu)

Kobayashi, Hisao*; Ikeda, Shugo*; Yoda, Yoshitaka*; Nakamura, Hiroki  ; Machida, Masahiko  

The structural and magnetic phase transition in FeAs ( = Sr and Eu) was investigated from the viewpoint of the Fe-specific phonon density of states, (), utilizing Fe nuclear resonant inelastic-scattering and ab initio phonon calculations. The Fe optical phonon branches in () around 25 meV show no anomalies across the phase transition temperature, , with decreasing temperature. Meanwhile, the peak intensity around 32 meV decreases and increases at 1.25 and , respectively, which originates from phonon branches involving Fe and As atomic displacements in the tetragonal structure. This decrease is related to the phonon branches involving atomic displacements in the ab plane. Furthermore, the average sound velocity below 8 meV shows no temperature dependence down to , suggesting softening in some of the sound waves. Our results indicate the presence of orthorhombic fluctuations in tetragonal FeAs.