Title: Core-level signature of long-range density-wave order and short-range excitonic correlations probed by attosecond broadband spectroscopy Show Chinese title

Title: 飞秒宽带光谱探测的长程密度波序和短程激子关联的核心级特征

Abstract: Advances in attosecond core-level spectroscopies have successfully unlocked the fastest dynamics involving high-energy electrons. Yet, these techniques are not conventionally regarded as an appropriate probe for low-energy quasiparticle interactions that govern the ground state of quantum materials, nor for studying long-range order because of their limited sensitivity to local charge environments. Here, by employing a unique cryogenic attosecond beamline, we identified clear core-level signatures of long-range charge-density-wave (CDW) formation in a quasi-2D excitonic insulator candidate, even though equilibrium photoemission and absorption measurements of the same core levels showed no spectroscopic singularity at the phase transition. Leveraging the high time resolution and intrinsic sensitivity to short-range charge excitations in attosecond core-level absorption, we observed compelling time-domain evidence for excitonic correlations in the normal-state of the material, whose presence has been subjected to a long-standing debate in equilibrium experiments because of interfering phonon fluctuations in a similar part of the phase space. Our findings support the scenario that short-range excitonic fluctuations prelude long-range order formation in the ground state, providing important insights in the mechanism of exciton condensation in a quasi-low-dimensional system. These results further demonstrate the importance of a simultaneous access to long- and short-range order with underlying dynamical processes spanning a multitude of time- and energy-scales, making attosecond spectroscopy an indispensable tool for both understanding the equilibrium phase diagram and for discovering novel, nonequilibrium states in strongly correlated materials. Show Chinese abstract

Abstract: 阿秒核心能级光谱技术的进步成功揭示了涉及高能电子的最快速动力学。 然而，由于这些技术对局域电荷环境的敏感性有限，它们通常不被视为探测低能准粒子相互作用（控制量子材料基态）或研究长程有序性的合适探针。 在这里，通过采用独特的低温阿秒束线，我们在一个准二维激子绝缘体候选材料中识别出了长程电荷密度波（CDW）形成的清晰核心能级特征，尽管对该相同核心能级的平衡光发射和吸收测量在相变时未显示出光谱上的奇异现象。 利用阿秒核心能级吸收在高时间分辨率和对短程电荷激发固有敏感性的优势，我们观察到了该材料正常态中激子关联的令人信服的时间域证据，而这些关联在平衡实验中由于相空间中相似部分的声子涨落干扰而长期存在争议。 我们的发现支持这样一种情景：即在基态中，短程激子涨先于长程有序形成，为理解准低维系统中激子凝聚的机制提供了重要见解。 这些结果进一步证明了同时访问长程和短程有序性以及潜在的动力学过程的重要性，这些过程跨越了多种时间和能量尺度，使阿秒光谱成为理解和探索强关联材料平衡相图及新型非平衡态不可或缺的工具。