Research work · 2006
Perfect Softening of the Ferroelectric Mode in the Isotope-Exchanged Strontium Titanate of SrTiO318 Studied by Light Scattering
Masaki Takesada, Mitsuru Itoh, Tetsuya Yagi
Physical Review Letters
Abstract
The ${E}_{u}$ mode of the isotope-induced ferroelectric strontium titanate ${\mathrm{SrTi}}^{18}{\mathrm{O}}_{3}$ shows a perfect softening at the ferroelectric phase transition temperature ${T}_{c}$, where the frequency of the underdamped mode approaches completely to zero within the instrumental resolution. The spectra of the Raman inactive soft ${E}_{u}$ mode have been successfully observed owing to local symmetry breaking and by long-term accumulation of the spectral intensity with a high resolution technique. The mechanism of the phase transition is concluded to be an ideal displacive-type accompanied with perfect softening of the Slater-type polar ${E}_{u}$ mode. The difference between the soft mode behavior of $\mathrm{SrTi}^{16}\mathrm{O}_{3}$ and $\mathrm{SrTi}^{18}\mathrm{O}_{3}$ indicates that the origin of the quantum paraelectric state of $\mathrm{SrTi}^{16}\mathrm{O}_{3}$ lies in the quantum fluctuation of the oxide octahedron in the perovskite structure.