Universal understanding of pressure-induced quantum paraelectric state

为了准确地研究在T=40K以下出现的“量子顺电相”在SrTi^&lt；16&gt；O_3(STO16)中的动力学性质，进行了光散射实验。在此阶段，在低于最低频率横声模频率的频率范围内，出现了一个称为“宽双峰”(BD：~20 GHz)的不同寻常的谱峰。本研究的目的是阐明BD.1的物理起源。我们提出了量子顺电相存在一个长度尺度(在T=27K时为~50 nm)。这个建议是基于对BD.2.SrTi^&lt；18&gt；O_3(STO18：^&lt；lt；16&gt；O替换为^&lt；18&gt；O)在T~29K以下具有铁电相等状态的平均自由程计算。我们发现，在STO18中观察到的部分软化横向声模(STA)的频谱与在T~20K时在STO16中观察到的BD的频谱非常相似，由此建立了一个假设，即STO16中出现的BD的物理起源是STA。我们假设，尽管STO16的基质不是铁电的而是顺电的，但在量子顺电相中存在着STA可以传播的动态铁电区(FERS：~50 nm，T=27K)。此外，我们在Nb^&lt；5+&gt；掺杂的STO16中发现了一个新的光散射峰(NP：~140 GHz)。NP的平均自由程也暗示了量子顺电相中长度尺度的存在。BD和NP的长度尺度表现出相同的温度依赖性和相同的各向异性。由此可见，量子顺电相中存在唯一的长度尺度，该尺度表征了各向异性铁电体的大小。由此可见，对量子顺电相的基本信息还不够充分。因此，我们仍然在大气压下继续进行光散射实验来获得这一信息。

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.