Soft phonon mode coupled with antiferromagnetic order in incipient-ferroelectric Mott insulators SrBaMnO

Sakai, Hideaki*; Fujioka, Jun*; Fukuda, Tatsuo  ; Bahramy, M. S.*; Okuyama, Daisuke*; Arita, Ryotaro*; Arima, Takahisa*; Baron, A. Q. R.*; Taguchi, Yasujiro*; Tokura, Yoshinori*

Infrared optical and inelastic X-ray scattering spectra have been systematically investigated in combination with first-principles calculations for paraelectric and antiferromagnetic perovskite SrBaMnO single crystals, which are close to a ferroelectric transition arising from off-center displacement of magnetic Mn ions. One optical phonon rapidly softens toward zero frequency at room temperature with increasing . This soft-mode behavior is also reproduced by the first-principles calculations, from which we have predicted the vibration mode of all the optical phonons. Below the antiferromagnetic-transition temperature, the soft mode hardens with decreasing temperature and then resoftens toward the lowest temperature.