利用飞秒X射线光子关联光谱直接观测玻璃形成液体的原子尺度弛豫
Directly observing atomic-scale relaxations of a glass forming liquid using femtosecond X-ray photon correlation spectroscopy
AI总结：
本文利用XFEL飞秒X射线脉冲与分束延迟系统，通过XPCS在亚皮秒时间尺度上直接观测了Ge15Te85玻璃形成液体的原子尺度弛豫与完全退关联，为脆性液体行为提供了直接证据。
摘要：
玻璃形成液体表现出结构弛豫行为，反映了在广泛时间尺度上潜在的原子重排。这些行为在决定许多材料性质方面起着关键作用。然而，由于直接表征无序体系中原子序参量演化关联的实验困难，原子尺度上的弛豫过程尚未被充分理解。在此，以模型体系Ge15Te85为例，我们展示了一种实验方法，利用X射线自由电子激光（XFELs）产生的飞秒级相干X射线脉冲散射来探测弛豫动力学。通过收集由分束延迟系统产生的两个快速相继、几乎相同的X射线脉冲的叠加散斑图样，我们可以提取源自样品动力学的散斑图样衬度衰减，并在亚皮秒时间尺度上观测到局域序的完全退关联。这为Ge15Te85的脆性液体行为提供了直接的原子级证据。我们的结果展示了基于XFEL的X射线光子关联光谱（XPCS）策略，实现了飞秒时间分辨率和原子尺度空间分辨率。这一从同步辐射XPCS的毫秒尺度出发的十二个数量级的扩展，为液体、玻璃及其他高度无序体系中弛豫动力学的实验研究开辟了新途径。
英文摘要：
Glass forming liquids exhibit structural relaxation behaviors, reflecting underlying atomic rearrangements on a wide range of timescales. These behaviors play a crucial role in determining many material properties. However, the relaxation processes on the atomic scale are not well understood due to the experimental difficulties in directly characterizing the evolving correlations of atomic order in disordered systems. Here, taking the model system Ge15Te85, we demonstrate an experimental approach that probes the relaxation dynamics by scattering the coherent X-ray pulses with femtosecond duration produced by X-ray free electron lasers (XFELs). By collecting the summed speckle patterns from two rapidly successive, nearly identical X-ray pulses generated using a split-delay system, we can extract the contrast decay of speckle patterns originating from sample dynamics and observe the full decorrelation of local order on the sub-picosecond timescale. This provides the direct atomic-level evidence of fragile liquid behavior of Ge15Te85. Our results demonstrate the strategy for XFEL-based X-ray photon correlation spectroscopy (XPCS), attaining femtosecond temporal and atomic-scale spatial resolutions. This twelve orders of magnitude extension from the millisecond regime of synchrotron-based XPCS opens a new avenue of experimental studies of relaxation dynamics in liquids, glasses, and other highly disordered systems.
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