コーテンズ エリック フランス共和国, モンペリエ大学・物性科学研究所, 教授
YAMANAKA Akio Hokkaido Univ.Research Institute for Electronic Science, Inst., 電子科学研究所, 助手 (30182570)
COURTENS Eric Montpellier Univ., Laboratoire des Verres, Prof.
|Budget Amount *help
¥2,900,000 (Direct Cost : ¥2,900,000)
Fiscal Year 1998 : ¥1,300,000 (Direct Cost : ¥1,300,000)
Fiscal Year 1997 : ¥1,600,000 (Direct Cost : ¥1,600,000)
In quantum paraelectrics dielectric constant epsilon increases strongly and saturates at a high value at low temperatures T.They fail to become ferroelectric (FE), which is believed up until now to be caused by the zero-point motion of the soft polar TO phonon. It is current interest that a new type of phase transition could nevertheless be occurring. Given the huge epsilon, it is reasonable that the "new coherent low-T state" called the Muller state below T_q= 37 K in SrTiO_3 where an anomaly was first revealed in EPR, is connected with the development of large polarization entities. So, we studied the hypothetical Muller state, and the nature, if it exists, by hyper-Raman (HR) and inelastic neutron scattering (INS) spectroscopies.
1. The dispersion relations of various phonons in the low q region in SrTiO_3 were determined by INS.It is found that a TA phonon in the specific direction is strongly coupled to the soft polar phonon, which is responsible for some of the anomalies under problem.
2. HR measurements were successfully made with a high resolution for a singly-oriented sample of SrTiO_3. As a result, the frequencies of the relevant TO phonons are found to be highly influenced by strains induced by the structural phase transition at T_q=1O5 K, indicating that the incipient ferroelectric regime (IFR) is primarily caused by the strains, in addition to the quantum effect.
3. Although no anomaly of the T-dependencies of the relevant phonon frequencies has been observed by HR, a few anomalous behaviors are nevertheless found, which include those of the spectral widths, breakdown of the HR selection rules, and unexpected observation of a non-polar Eg phonon that was also observed anomalously in INP.
In summary, we have obtained a few important outcomes in IFR of SrTiO_3, some of which are thought to be related to the Muller state. More studies and analyses are, however, needed to have a decisive conclusion of the existence of the state.