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arXiv 1902.05264cond-mat.mtrl-sci
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光激发金属/绝缘体异质结中微观非平衡能量转移动力学

Microscopic non-equilibrium energy transfer dynamics in a photoexcited metal/insulator heterostructure

N. Rothenbach, M. E. Gruner, K. Ollefs, C. Schmitz-Antoniak, S. Salamon, P. Zhou, R. Li, M. Mo, S. Park, X. Shen, S. Weathersby, J. Yang, X. J. Wang, R. Pentche… 展开作者

N. Rothenbach, M. E. Gruner, K. Ollefs, C. Schmitz-Antoniak, S. Salamon, P. Zhou, R. Li, M. Mo, S. Park, X. Shen, S. Weathersby, J. Yang, X. J. Wang, R. Pentcheva, H. Wende, U. Bovensiepen, K. Sokolowski-Tinten, A. Eschenlohr

摘要:

软X射线光谱的元素特异性使其成为分析金属-绝缘体异质结构中局域光激发引起的超快动力学微观起源的理想工具。使用[Fe/MgO] $_n$作为模型系统,我们进行紫外泵浦/软X射线探测实验,这是敏感的所有成分,这些异质结构,探测电子和晶格激发。互补的超快电子衍射实验独立地分析了Fe组分的晶格动力学,并与从头计算一起产生全面的洞察微观过程,导致局部弛豫在一个单一的成分或两个成分之间的非局部弛豫。除了在Fe L $_3$吸收边监测的Fe中的电子激发和通过电子-声子耦合在1 ps内弛豫之外,软X射线分析确定了在MgO层的氧K吸收边发生在0.5 ps内的变化。这种跨越Fe-MgO界面的超快能量转移是由高频界面振动模式介导的,所述高频界面振动模式由Fe中的热电子激发并以模式选择性非热方式耦合到MgO中的振动。第二,较慢的时间尺度被确定在氧K前边缘和铁L $_3$边缘。较慢的过程代表了声学声子的能量转移,并有助于整个异质结构的热化。因此,我们发现界面能量转移与声子系统中的非平衡行为有关。由于我们的实验缺乏跨界面电荷转移的特征,因此我们得出结论,在这些非局域、非平衡动力学中,声子介导的过程主导了电子和晶格激发的竞争。

英文摘要:

The element specificity of soft X-ray spectroscopy makes it an ideal tool for analyzing the microscopic origin of ultrafast dynamics induced by localized optical excitation in metal-insulator heterostructures. Using [Fe/MgO]$_n$ as a model system, we perform ultraviolet pump/soft X-ray probe experiments, which are sensitive to all constituents of these heterostructures, to probe both electronic and lattice excitations. Complementary ultrafast electron diffraction experiments independently analyze the lattice dynamics of the Fe constituent, and together with ab initio calculations yield comprehensive insight into the microscopic processes leading to local relaxation within a single constituent or non-local relaxation between two constituents. Besides electronic excitations in Fe, which are monitored at the Fe L$_3$ absorption edge and relax within 1 ps by electron-phonon coupling, soft X-ray analysis identifies a change at the oxygen K absorption edge of the MgO layers which occurs within 0.5 ps. This ultrafast energy transfer across the Fe-MgO interface is mediated by high-frequency, interface vibrational modes, which are excited by hot electrons in Fe and couple to vibrations in MgO in a mode-selective, non-thermal manner. A second, slower timescale is identified at the oxygen K pre-edge and the Fe L$_3$ edge. The slower process represents energy transfer by acoustic phonons and contributes to thermalization of the entire heterostructure. We thus find that the interfacial energy transfer is associated with non-equilibrium behavior in the phonon system. Because our experiments lack signatures of charge transfer across the interface, we conclude that phonon-mediated processes dominate the competition of electronic and lattice excitations in these non-local, non-equilibrium dynamics.

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