Universal understanding of pressure-induced quantum paraelectric state

为了精确地研究SrTi_2O_3（STO_（16））在T=40 K以下出现的“量子顺电相”的动力学性质，进行了光散射实验<16>。在此阶段，在低于最低频率横向声学模式频率的频率区域出现称为“宽双重峰”（BD：~ 20 GHz）的不寻常光谱峰。本研究的目的是澄清BD的物理起源。1.我们提出了存在一个长度尺度（T=27 K时约为50 nm）来表征量子顺电相。SrTi ^<18>O 3（STO 18：<16>SrTi^ O中的^ O<16>被^ O取代<18>）在Tc低于29 K时具有铁电相的状态。我们发现在20 K时，在STO 18中观察到的部分软化的横声模（STA）的频谱与在STO 16中观察到的BD的频谱非常相似。我们建立了一个假设，从这种相似性，在STO 16中出现的BD的物理起源是STA。我们假设STO_（16）的基体不是铁电体而是顺电体，但STO_（16）的动态铁电区（FERs：~ 50 nm，T=27 K）存在于量子顺电相中，STA可以在其中传播. NP的平均自由程也意味着在量子顺电相位中存在长度尺度。BD和NP的长度尺度显示出相同的温度依赖性和相同的各向异性性质。我们可以得出结论，在量子顺电相位中存在唯一的长度尺度，并且该尺度表征了各向异性FER的大小。因此，我们仍然在大气压下继续进行光散射实验以获得该信息。

A light scattering experiment has been performed in order to investigate precisely the dynamical nature of "quantum paraelectric phase" appearing below T=40 K in SrTi^<16>O_3 (STO16). In this phase, an unusual spectral peak called "broad doublet" (BD:〜20 GHz) emerges in the frequency region lower than the frequency of the transverse acoustic mode of lowest frequency. The purpose of present study is to clarify the physical origin of the BD.1.We have proposed that there exists a length scale (〜50 nm at T=27 K) characterizing the quantum paraelectric phase. This proposal is based on the mean free path calculation for the BD.2.SrTi^<18>O_3 (STO18: ^<16>O of SrTi^<16>O is substituted by ^<18>O) has a state such as ferroelectric phase below Tc〜29 K. We found that the spectrum of a partially softened transverse acoustic mode (STA) observed in STO18 was quite similar to that of the BD observed in STO16 at T〜20 K. We built up a hypothesis from this similarity that the physical origin of the BD emerging in STO16 is the STA. We assumed here that the dynamical ferroelectric regions (FERs: 〜50 nm at T=27 K), in which STA can propagate, exist in the quantum paraelectric phase, though matrix of STO16 is not ferroelectric but paraelectric.3.Furthermore, we found a new light scattering peak (NP:〜140 GHz) in a Nb^<5+> doped STO16. The mean free path of the NP also implies the existence of a length scale in quantum paraelectric phase. The length scale of BD and that of NP show the same temperature dependence and the same anisotropic nature. We can conclude that the unique length scale exists in the quantum paraelectric phase and that the scale characterizes the size of anisotropic FER.As known from above, the basic information on quantum paraelectric phase is not enough yet. Thus, we still continue the light scattering experiment under the atmospheric pressure to obtain that information.