标题： 在1$T$-TaSe$_2$电荷密度波态中光谱分辨相干声子的相位和振幅 显示英文标题

标题： Spectrally resolving the phase and amplitude of coherent phonons in the charge density wave state of 1$T$-TaSe$_2$

摘要： 激发和检测相干声子为凝聚态物质提供了独特的见解，特别是在具有强电子-声子耦合的材料中。 我们报告了一项使用瞬态宽带反射光谱学对层状电荷密度波（CDW）化合物1$T$-TaSe$_2$的相干声子进行的研究，在光子能量范围1.75-2.65 eV内进行。 观察到几个强烈且持续时间较长（> 20 ps）的振荡，这些振荡源于CDW超晶格重构，使得能够详细分析其振幅和相位的光谱依赖性。 我们发现，在高于2.4 eV的能量下，涉及Ta d带的跃迁中，CDW振幅模式在2.19 THz处主导了相干响应。 而在较低能量下，其他频率之间出现了拍频现象，其中在2.95 THz处有一个特别强烈的模式。 有趣的是，我们的光谱分析揭示了在2.4 eV处存在一个$\pi$相移。 结果讨论考虑了特定模式与稳态反射率中观察到的光学跃迁所涉及能带的选择性耦合。 这项工作展示了相干声子光谱学如何区分并解析与CDW序强耦合的光学态，并提供通常在传统稳态实验中隐藏的额外信息。 显示英文摘要

摘要： The excitation and detection of coherent phonons has given unique insights into condensed matter, in particular for materials with strong electron-phonon coupling. We report a study of coherent phonons in the layered charge density wave (CDW) compound 1$T$-TaSe$_2$ performed using transient broadband reflectivity spectroscopy, in the photon energy range 1.75-2.65 eV. Several intense and long lasting (> 20 ps) oscillations, arising from the CDW superlattice reconstruction, are observed allowing for detailed analysis of the spectral dependence of their amplitude and phase. We find that for energies above 2.4 eV, where transitions involve Ta d-bands, the CDW amplitude mode at 2.19 THz dominates the coherent response. At lower energies, instead, beating arises between additional frequencies, with a particularly intense mode at 2.95 THz. Interestingly, our spectral analysis reveals a $\pi$ phase shift at 2.4 eV. Results are discussed considering the selective coupling of specific modes to energy bands involved in the optical transitions seen in steady-state reflectivity. The work demonstrates how coherent phonon spectroscopy can distinguish and resolve optical states strongly coupled to the CDW order and provide additional information normally hidden in conventional steady-state experiments.