DYNAMICS OF QUANTUM FLUCTUATION STUDIED BY COHERENT PHONON EXCITATION METHOD

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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