AI 中文总结
研究稀土铁石榴石中晶体场跃迁与交换自旋动力学的相互作用，利用太赫兹时域光谱，证明了Gd$_{3/2}$Yb$_{1/2}$BiFe$_{5}$O$_{12}$中Yb离子CEF激发与Yb-Fe交换模式杂化，揭示相关现象及机制，突出CEF作用，为太赫兹自旋电子技术奠定基础。
AI 中文摘要
稀土铁石榴石为探索低能激发与复杂的温度依赖磁化动力学之间的相互作用提供了理想平台。在这些系统中，稀土和铁亚晶格之间的交换耦合产生高频集体自旋激发，且局域4$f$电子的强自旋轨道耦合触发太赫兹频率的晶体电场（CEF）跃迁。尽管对石榴石自旋动力学进行了广泛研究，但CEF激发与交换模式之间的相互作用仍未完全明确。利用温度依赖的太赫兹时域光谱，我们证明了Gd$_{3/2}$Yb$_{1/2}$BiFe$_{5}$O$_{12}$中Yb离子CEF激发与Yb-Fe交换模式之间的杂化。这种耦合的特征是随着材料接近其磁化补偿温度，光谱和时间权重发生显著重新分布。值得注意的是，Yb-Fe交换模式在冷却时呈现异常红移，这与通常由交换耦合增加驱动的传统蓝移相反。我们将此现象追溯到Fe交换场与Yb CEF激发相互作用导致的Yb-Fe交换各向异性的改变。这些发现突出了CEF介导的交换耦合在塑造低能自旋动力学中的关键作用，使稀土石榴石成为未来太赫兹自旋电子技术的基石。
英文摘要
Rare-earth iron garnets offer an ideal platform for exploring the interplay of low-energy excitations and the complex temperature-dependent magnetization dynamics. In these systems, exchange coupling between rare-earth and iron sublattices generates high-frequency collective spin excitations. In addition, the robust spin-orbit coupling of localized 4$f$ electrons triggers the crystal-electric-field (CEF) transitions at THz frequencies. Despite extensive research into the garnet spin dynamics, the interplay between CEF excitations and exchange modes has remained largely unmapped. Using temperature-dependent THz time-domain spectroscopy, we demonstrate a hybridization between the Yb-ion CEF excitation and the Yb-Fe exchange mode in Gd$_{3/2}$Yb$_{1/2}$BiFe$_{5}$O$_{12}$. This coupling is characterized by a significant redistribution of spectral and temporal weights as the material approaches its magnetization compensation temperature. Notably, the Yb-Fe exchange mode exhibits an anomalous redshift upon cooling -- a reversal of the conventional blue shift typically driven by increased exchange coupling. We trace this phenomenon to a modification of Yb-Fe exchange anisotropy, driven by the interplay of the Fe exchange field and Yb CEF excitations. These findings highlight the critical role of CEF-mediated exchange coupling in shaping low-energy spin dynamics, positioning rare-earth garnets as a cornerstone for future THz spintronic technologies.
