SHG microscopic observations of polar state in Li-doped KTaO3 under electric field

TL;DR

This study uses SHG microscopy to investigate the polar states and dipole glass behavior in Li-doped KTaO3 under electric fields, revealing the transformation from glassy to polar states and the symmetry of the induced phase.

Contribution

It provides new insights into the electric field-induced polar states and the nature of dipole glass in Li-doped KTaO3 using SHG microscopy and diffraction techniques.

Findings

Dipole glass or nano-clusters freeze with decreasing temperature.

Electric field induces a semi-macroscopic polar state.

Polar phase symmetry is tetragonal 4mm or 4.

Abstract

Incipient ferroelectric KTaO3 with off-center Li impurity of the critical concentration of 2.8 mol% was investigated in order to clarify the dipole glass state under electric field. Using optical second-harmonic generation (SHG) microscope, we observed a marked history dependence of SHG intensity through zero-field cooling (ZFC), zero-field heating (ZFH), field heating after ZFC (FH/ZFC) and FH after field cooling (FH/FC). These show different paths with respect to temperature: In the ZFC/ZFH process, weak SHG was observed at low temperature, while in the FH/ZFC process, relatively high SHG appears in a limited temperature range below TF depending on the field strength, and in the FC and FH/FC processes, the SHG exhibits ferroelectric-like temperature dependence: it appears at the freezing temperature of 50K, increases with decreasing temperature and has a tendency of saturation. These experimental results strongly suggest that dipole glass state or polar nano-clusters which gradually freezes with decreasing temperature is transformed into semi-macroscopic polar state under the electric field. However at sufficiently low temperature, the freezing is so strong that the electric field cannot enlarge the polar clusters. These experimental results show that the polar nano-cluster model similar to relaxors would be more relevant in KTaO3 doped with the critical concentration of Li. Further experiments on the anisotropy of SHG determine that the average symmetry of the field-induced polar phase is tetragonal 4mm or 4, which is also confirmed by the X-ray diffraction measurement.