Isotope effect on the soft-mode dynamics of SrTiO3 studied by Raman scattering -: art. no. 064111

The oxygen isotope effect on the soft-mode dynamics in quantum paraelectric SrTi((OxO1-x)-O-18-O-16)(3) [STO18-x] is investigated by Raman scattering in terms of local symmetry breaking in the paraelectric phase. The exchange rates x of the O-18 atoms are 0.23 [STO18-23] and 0.32 [STO18-32], which are less than the critical value x(c)(=0.33). In STO18-32, whose exchange rate is just below x(c), the soft E-u mode shows hardening behavior in a low-temperature region indicating the onset of local ferroelectric order. On the other hand, the soft E-u mode in STO18-23 does not show any hardening behavior until 0 K. The enhancement of the inhomogeneity in the crystal by the exchange of O-18 is concluded to play an essential role in the soft-mode dynamics of the quantum paraelectric and ferroelectric STO18-x. The relation between the soft-mode dynamics and quantum fluctuation is discussed.