DYNAMICS OF QUANTUM FLUCTUATION STUDIED BY COHERENT PHONON EXCITATION METHOD

重点阐述了量子准电晶体和铁电晶体中量子涨落与宏观有序的关系。样品为量子对电钛酸锶SrTi^<16>O_3（STO16）及其氧同位素交换晶体SrTi(^<18>O_x^<16>O_<1-x>)_3(STO18-x)，其中x=0.23,0.32和0.89。Itch et al.（1999）发现钛酸锶中氧同位素从^<16>O到^<18>O的交换浓度大于33% （x_c）诱导了典型的量子准电态STO16的铁电相。在低至毫开尔文的超低温区观测到了STO16和STO18-x的铁电软Eu模式光谱。在中心对称相的名义拉曼非活性Eu模式光谱被观察为每个晶体温度的函数。得到以下两个结论：(1)在t同位素氧的质量效应完全抑制量子波动的STO-89晶体中，Eu软模在铁电相变点上可以完全软化到零频率（完全凝固），与普通铁电体相比，其阻尼振动更小，其中软模的严重过阻尼谱被报道为中心峰。在STO-89的情况下，量子涨落的作用似乎是通过软模和量子涨落之间的耦合相干地抑制软模振动的阻尼。(2)对于STO-23和STO-32试样，由于两种晶体的浓度均低于临界浓度x_c=0.33，没有发生铁电相变。在这些晶体中，Eu模式的软化行为通过观察到由^<18>O浓度的不均匀性引起的局部对称性破缺区得到证实，尽管它没有表现出任何到零频率的软化。在这种情况下，量子涨落的作用是即使在超低温区域也能防止长程有序的发生。少

We concentrated to elucidate the relation between the quantum fluctuations and the macroscopic order in the quantum paraelectric and ferroelectric crystals. The specimen were the quantum paraelectric strontium titanate SrTi^<16>O_3(STO16) and its oxygen isotope exchanged crystal SrTi(^<18>O_x^<16>O_<1-x>)_3(STO18-x), where x=0.23,0.32 and 0.89. The exchange concentration of the oxygen isotope more than 33% (x_c) from ^<16>O to ^<18>O in strontium titanate induces the ferroelectric phase in the typical quantum paraelectric STO16 as discovered by Itch et al.(1999). The spectra of the ferroelectric soft Eu mode in STO16 and STO18-x have been observed in the ultra-low temperature region down to milli-Kelvin. The nominally Raman inactive Eu mode spectra in the centro-symmetric phase were observed as a function of temperature for each of the crystals.The following two conclusions are obtained ; (1)In STO-89 crystal where the quantum fluctuation is completely depressed by the mass effect of t … More he isotope oxygen, the soft Eu mode can soften completely to zero frequency (complete fireezing) at the ferioelectric phase transition point associated with the less damping vibration than the ordinary other ferroelectrics, where the heavily over-damped spectra of soft mode have been reported as a central peaks. In the case of STO-89, the role of the quantum fluctuations seems to be coherently depression of the damping of the soft mode vibration via the coupling between the soft modes and quantum fluctuations. (2)For the specimen STO-23 and STO-32, the ferroelectric phase transition does not take place because the concentration of both crystals is lower than the critical concentration x_c=0.33. In these crystal, the softening behavior of the Eu mode is confirmed by the observation of the locally symmetry breaking region caused by the inhomogeneity of the concentration of ^<18>O, though it does not show any softening to the zero frequency. In this case, the role of the quantum fluctuation is to prevent the occurrence of the long-range order even in the ultra low temperature region. Less