Polar order in quantum paraelectric SrTi16O3 and SrTi18O3 at low temperature

Optical second-harmonic generation (SHG) in (SrTiO3)-O-16 (STO16) and (SrTiO3)-O-18 (STO18) was investigated using the SHG microscope. While no biased STO16 exhibits weak and almost temperature-independent SHG signals, a marked SHG is observed under the electric field in the quantum paraelectric region. In STO18, strong SHG signals appear spontaneously below 36 K. However, neutron and X-ray diffraction analyses indicate that no structural change appears at low temperature in STO18, or STO16 under the electric field. By taking into account the fact that the SHG is sensitive to the local polar order, the combined studies reveal that the long-range order of the polar phase does not develop on either crystal and is frozen in local regions.