DYNAMICS OF QUANTUM FLUCTUATION STUDIED BY COHERENT PHONON EXCITATION METHOD

我们集中讨论了量子顺电晶体和铁电晶体中的量子涨落与宏观有序之间的关系。样品为量子顺电钛酸锶及其氧同位素交换晶体SrTi(^&lt；18&gt；O_x^&lt；16&gt；O_&lt；1-x&gt；)_3(STO18-x)，，其中x=0.23、0.32和0.89。正如Itch等人(1999)所发现的那样，钛酸锶中氧同位素从~&lt；16&gt；O到~&lt；18&gt；O的交换浓度超过33%(X_C)，导致了典型的量子顺电STO16中的铁电相。观察到了STO16和STO18-x的铁电软Eu模在低至毫开尔文的超低温区的光谱。在中心对称相中观察到标称拉曼非活动Eu模光谱随温度的变化，得到以下两个结论：(1)在STO-89晶体中，t-…的质量效应完全抑制了量子涨落与普通铁电材料相比，由于He同位素氧含量较高，软Eu模能在铁电相变点处完全软化到零频率(完全燃烧)，与其他铁电材料相比，软模的严重过阻尼谱被认为是中心峰。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来相干地抑制软模振动的衰减。(2)对于STO-23和STO-32样品，由于这两种晶体的浓度都低于临界浓度x_c=0.33，所以没有发生铁电相变。在这些晶体中，虽然没有表现出对零频的任何软化，但通过观察到由浓度不均匀引起的局域对称性破碎区，证实了Eu模的软化行为。在这种情况下，量子涨落的作用是即使在超低温区也防止长程有序的发生。较少

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