铁电体中极化长程涨落的光子关联谱实验研究∗

摘要

从极化团簇的随机涨落出发，利用维纳过程模型，推导了铁电体中极化长程涨落的弛豫规律以及光强自相关函数所可能的表现形式.阐述了微观极化团簇的弛豫过程与宏观测量弛豫规律之间的联系.通过对原有氦氖激光光子关联谱实验装置进行改进，观测了BaTiO3和0.71Pb (Mg1/3Nb2/3)O3-0.29PbTiO3单晶中极化团簇长程涨落在居里点和立方到四方相变点附近的弛豫过程.在BaTiO3中发现极化团簇长程涨落在居里点之上4 K 存在双弛豫现象，此现象与其有序无序相变机理相联系.在Pb(Mg1/3Nb2/3)O3-0.29PbTiO3中发现极化团簇长程涨落在相变点两侧都存在双弛豫现象.利用推导的理论结果很好地拟合了实验结果并提取了极化团簇长程涨落的弛豫时间.两种样品中极化团簇长程涨落的弛豫时间都在相变点出现突变，并呈现临界慢化现象.%Based on the theory of random fluctuation of polarization clusters and the model of Wiener random process, the relaxation law of long-range fluctuation of polarization and the possible forms of light intensity autocorrelation function g2(τ) measured from photon correlation spectroscopy (PCS) experiments have been derived. The relationship between relaxation mechanisms of microscopic polarization clusters and macro relaxation laws is expounded. This research supplies a theoretical model for the application of PCS in researching the relaxation process of polarization clusters in ferroelectrics. Based on the improved He-Ne laser PCS experimental set-up, the relaxation process of long-range fluctuation of polarization clusters in BaTiO3 and 0.71Pb (Mg1/3Nb2/3) O3-0.29PbTiO3 single crystals near phase transition temperature is studied. As for BaTiO3, the dual relaxation processes of long-range fluctuation of polarization clusters are observed at temperatures above T C+4 K, which may be related to its order-disorder mechanism of phase transition. For 0.71Pb (Mg1/3Nb2/3) O3-0.29PbTiO3, the dual relaxation processes exist on both sides of the cubic-tetragonal phase transition temperature. The PCS experimental results are fitted well by the derived theoretical model, and the characteristic relaxation times of long-range fluctuation of polarization clusters are extracted. Two relaxation times,τs andτl corresponding to short and long relaxation time, respectively, are initially observed, whereτs is several microseconds, and τl is tens of microseconds. The abrupt increase of relaxation times at phase transition temperature and the phenomenon of critical slowing down can be observed in the two samples.