Raman Scattering Study on Quantum Ferroelectric ¹⁸O-exchanged SrTiO₃

Hiroki Taniguchi, Tokyo Institute of Technology, Japan

Quantum critical behavior has been one of central topics in condensed matter physics from both experimental and theoretical points of view. Among ferroelectric and related materials, SrTiO₃ has been considered to be an important compound because its phase transition nature is dominated by a quantum effect. ¹⁸O-exchanged SrTiO₃ shows quantum ferroelectric phase transition. Initially the ferroelectricity of pure SrTiO₃ is suppressed by quantum fluctuation in a low temperature region near 0 K (quantum paraelectricity). When the ¹⁶O is exchanged by its isotope ¹⁸O, SrTiO₃ becomes to undergo a ferroelectric phase transition at a finite temperature. The exchange rate dependence of Curie temperature is in good agreement with a theoretical prediction for quantum ferroelectricity. However, the mechanism is still unclear due to the lack of sufficient experimental results. The ferroelectric soft mode plays essential role in the system of SrTiO₃. The quantum paraelectricity is understood by the suppression of the softening of the soft mode by zero-point vibration. Therefore, it is significant to investigate an isotope effect on the soft mode dynamics in order to understand the ¹⁸O-exchange induced quantum ferroelectricity of SrTiO₃ correctly. In the present study, we perform Raman scattering experiment on ¹⁸O-exchanged SrTiO₃ as a function of exchange rate. The result indicates that the quantum ferroelectricity is essentially driven by the softening of soft mode enhanced by ¹⁸O-exchange. The remarkable point is that, in the vicinity of quantum critical point (QCP), the softening behavior becomes strongly smeared and the soft mode presents incomplete softening in spite of the underdamped sharp spectral shape. This behavior is in clear contrast to classical incomplete soft mode which is coupled to a relaxational mode and therefore accompanied by a central component. The spectral analysis clarifies that the system goes into a novel paraelectric-ferroelectric phase coexistence state at QCP. Finally, new phase diagram is proposed on the basis of soft mode dynamics. An effect of inhomogeneity on the quantum ferroelectric phase transition is discussed simultaneously.

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