Sub-THz Raman Response and Soft Phonon in Tetragonal BaTiO3

The sub-THz polarized Raman response was measured along the [100] direction in the tetragonal BaTiO3 single crystal. The soft phonon was detected as a resonance peak for the first time by Raman spectroscopy, thanks to the use of notch filters that suppressed elastic scattering. No relaxation peak was observed and we demonstrated that, contrary to the situation along the ferroelectric c-axis, the soft phonon is sufficient to describe the dielectric response of BaTiO3 along the a-axis, except in close vicinity of the tetragonal-cubic phase transition.