Study of New Condensed State in Quantum Paraelectric Regime

在量子顺电体中，介电常数ε在低温T下强烈增加并在高值处饱和。它们不能成为铁电体（FE），这被认为是由软极性TO声子的零点运动引起的。目前的兴趣是，一种新型的相变可能正在发生。由于SrTiO_3中存在着巨大的电子自旋共振（EPR），所以在T_q= 37K以下，SrTiO_3中出现了一个称为Muller态的“新的相干低T态”，它与大极化体的形成有关。因此，我们研究了假设的Muller态，如果它存在的话，通过超拉曼（HR）和非弹性中子散射（INS）光谱来研究它的性质。利用惯性导航系统（INS）测量了SrTiO_3中低q区各声子的色散关系，发现在特定方向上的TA声子与软极性声子强耦合，这是导致问题中某些反常现象的原因.对SrTiO_3单晶样品进行了高分辨率的HR测量。结果表明，在T_q= 105 K时，结构相变引起的应变对相关TO声子的频率有很大影响，表明初始铁电区（IFR）除了量子效应外，还主要由应变引起.虽然HR没有观察到相关声子频率的T依赖性的异常，但仍然发现了一些异常行为，包括谱宽的异常，HR选择规则的破坏，以及在INP中意外观察到的非极性Eg声子。总之，我们在SrTiO_3的IFR中获得了一些重要的结果，其中一些被认为与穆勒状态有关。然而，还需要更多的研究和分析，才能对国家的存在得出决定性的结论。

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.