Second-harmonic-generation microscopic observations of polar state in Li-dopedKTaO3under an electric field
Second-harmonic-generation microscopic observations of polar state in Li-dopedKTaO3under an electric field
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DOI：
10.1103/physrevb.72.144103
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发表时间：
2005-06
影响因子：
3.7
通讯作者：
H. Yokota;T. Ohyama;Y. Uesu
中科院分区：
文献类型：
--
作者：
H. Yokota;T. Ohyama;Y. Uesu
研究了临界浓度为2.8mol%的偏离中心Li杂质的KTaO3铁电体在电场作用下的偶极玻璃态。利用光学二次谐波发生（SHG）显微镜观察了零场冷却（ZFC）、零场加热（ZFH）、零场加热后场加热（FH/ZFC）和场冷却后场加热（FH/FC）过程中SHG强度的显著历史依赖性。在ZFC/ZFH过程中，在低温下观察到弱SHG，而在FH/ZFC过程中，取决于场强，在TF以下的有限温度范围内出现相对高的SHG，并且在FC和FH/FC过程中，SHG表现出铁电性的温度依赖性：它出现在50K的冻结温度下，随温度的降低而增加，并有饱和的趋势。这些实验结果强烈地表明，偶极玻璃态或极性纳米团簇在电场作用下，随着温度的降低而逐渐冻结，转变为半宏观极性态。然而，在足够低的温度下，冻结是如此之强，电场不能扩大极性团簇。这些实验结果表明，类似于弛豫子的极性纳米团簇模型将在掺杂临界浓度的Li的KTaO3更相关。进一步的二次谐波各向异性实验确定了场致极性相的平均对称性为4 × 4mm或4 × 4mm，这也得到了X射线衍射测量的证实。
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.