We demonstrate that the absorption of femtosecond hard x-ray pulses excites quasispherical, high-amplitude, and high-wave-vector coherent acoustic phonon wave packets using an all hard-x-ray pump-probe scattering experiment. The time- and momentum-resolved diffuse scattering signal is consistent with an ensemble of 3D strain wave packets induced by the rapid electron cascade dynamics following photoionization at uncorrelated excitation centers. We quantify key parameters of this process, including the localization size of the stress field and the photon energy conversion efficiency into elastic energy. The parameters are determined by the photoelectron and Auger electron cascade dynamics, as well as the electron-phonon interaction. In particular, we obtain the localization size of the observed strain wave packet to be 1.5 and 2.5 nm for bulk SrTiO3 and KTaO3 single crystals, respectively. The results provide crucial information on the mechanism of x-ray energy deposition into matter and shed light on the shortest collective length scales accessible to coherent acoustic phonon generation using x-ray excitation.

中文翻译：

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由随机核心级光电离事件产生的纳米级声波包


我们使用全硬 X 射线泵浦探针散射实验证明，飞秒硬 X 射线脉冲的吸收激发准球、高振幅和高波矢量相干声子波包。时间和动量分辨的漫散射信号与在不相关激发中心光电离后快速电子级联动力学诱导的 3D 应变波包集合一致。我们量化了这个过程的关键参数，包括应力场的局域大小和光子能量转化为弹性能的效率。这些参数由光电子和俄歇电子级联动力学以及电子-声子相互作用决定。特别是，对于块体 SrTiO3 和 KTaO3 单晶，我们得到观察到的应变波包的定位尺寸分别为 1.5 和 2.5 nm。这些结果提供了有关 X 射线能量沉积到物质中机制的重要信息，并阐明了使用 X 射线激发产生相干声子可获得的最短集体长度尺度。