Universal understanding of pressure-induced quantum paraelectric state

为了精确地研究SrTi^<16>O_3 （STO16）中出现在T=40 K以下的“量子准电相”的动力学性质，进行了光散射实验。在这一阶段，在低于最低频率横向声模频率的频率区域出现了一个不寻常的谱峰，称为“宽双重态”（BD: ~ 20 GHz）。本研究的目的是澄清BD.1的物理起源。我们提出存在表征量子准电相的长度尺度（在T=27 K时~ 50 nm）。这一建议是基于BD.2的平均自由程计算。SrTi^<18>O_3 （STO18: ^<16>O的SrTi^<16>O被^<18>O所取代）在Tc ~ 29 K以下具有铁电相等态。我们发现，在T ~ 20 K时，在STO18中观测到的部分软化的横声模式（STA）的频谱与在STO16中观测到的横声模式（BD）非常相似。我们根据这种相似性建立了一个假设，即在STO16出现的BD的物理起源是STA。我们假设在量子准电相中存在STA可以传播的动态铁电区（在T=27 K时，FERs: ~ 50 nm），尽管STO16的基体不是铁电而是准电。此外，在Nb^<5+>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准电相中长度尺度的存在。BD的长度尺度与NP的长度尺度具有相同的温度依赖性和各向异性。我们可以得出结论，在量子准电相中存在唯一的长度尺度，并且该尺度表征了各向异性光纤传输的大小。如上所述，量子准电相的基本信息尚不充分。因此，我们仍然继续在大气压下进行光散射实验，以获得这些信息。

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.