AI 中文总结
研究极性电介质中结构相变行为，通过探测顺电STO分离局部场与自发宏观极化贡献。利用线性和二维太赫兹光谱及微观模型，发现局部场启动非微扰晶格动力学不足，自发极化起决定性作用。
AI 中文摘要
离子和电子子系统之间的相互作用决定了极性电介质中结构相变的行为，这种耦合由软光学声子模式介导。在诸如SrTiO₃（STO）等初始铁电体中，强局部场效应可使晶格在相界附近进入非微扰状态。然而，区分局部场与自发宏观极化的不同贡献仍是实验挑战。本文通过在对称相中深入探测顺电STO来分离这些机制，此时宏观自发极化被抑制。线性太赫兹（THz）光谱表明软模具有混合特性，主要由电子极化率驱动。利用二维THz光谱，绘制了潜在的非线性信号，证明系统处于以稳健局部场相干为特征的微扰状态。通过实施由局部场介导的电子和晶格自由度耦合的微观模型，定性再现了这些多维相干特征。研究发现强调，虽然局部场是启动非微扰晶格动力学所必需的，但仅靠它们是不够的。这表明自发极化在决定强相关极性电介质中的软模非线性方面起决定性作用。
英文摘要
The interplay between ionic and electronic subsystems dictates the behavior of structural phase transitions in polar dielectrics, a coupling mediated by soft optical phonon modes. In incipient ferroelectrics such as SrTiO$_3$ (STO), strong local-field effects can drive the lattice into a non-perturbative regime near the phase boundary. However, disentangling the distinct contributions of local fields from those of spontaneous macroscopic polarization remains an experimental challenge. Here, we isolate these mechanisms by probing paraelectric STO deep within its symmetric phase, where macroscopic spontaneous polarization is suppressed. Linear terahertz (THz) spectroscopy reveals that the soft mode exhibits a hybrid character, predominantly driven by electronic polarizability. Utilizing two-dimensional THz spectroscopy, we map the underlying nonlinear signals, demonstrating that the system persists in a perturbative regime characterized by robust local-field coherence. By implementing a microscopic model of coupled electronic and lattice degrees of freedom mediated by local fields, we qualitatively reproduce these multidimensional coherent signatures. Our findings highlight that while local fields are necessary to initiate non-perturbative lattice dynamics, they are insufficient on their own. This reveals that spontaneous polarization plays a deterministic role in dictating soft-mode nonlinearities in strongly correlated polar dielectrics.
