Abstract The phase transition of VO2 from a monoclinic insulator to a rutile metal, which occurs thermally at T C = 340 K, can also be driven by strong photoexcitation. The ultrafast dynamics during this photoinduced phase transition (PIPT) have attracted great scientific attention for decades, as this approach promises to answer the question of whether the insulator-to-metal (IMT) transition is caused by electronic or crystallographic processes through disentanglement of the different contributions in the time domain. We review our recent results achieved by femtosecond time-resolved photoelectron, optical, and coherent phonon spectroscopy and discuss them within the framework of a selection of latest, complementary studies of the ultrafast PIPT in VO2. We show that the population change of electrons and holes caused by photoexcitation launches a highly non-equilibrium plasma phase characterized by enhanced screening due to quasi-free carriers and followed by two branches of non-equilibrium dynamics: (i) an instantaneous (within the time resolution) collapse of the insulating gap that precedes charge carrier relaxation and significant ionic motion and (ii) an instantaneous lattice potential symmetry change that represents the onset of the crystallographic phase transition through ionic motion on longer timescales. We discuss the interconnection between these two non-thermal pathways with particular focus on the meaning of the critical fluence of the PIPT in different types of experiments. Based on this, we conclude that the PIPT threshold identified in optical experiments is most probably determined by the excitation density required to drive the lattice potential change rather than the IMT. These considerations suggest that the IMT can be driven by weaker excitation, predicting a transiently metallic, monoclinic state of VO2 that is not stabilized by the non-thermal structural transition and, thus, decays on ultrafast timescales.

中文翻译：

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VO 2 光致相变过程中的超快动力学

摘要 VO2 从单斜绝缘体到金红石金属的相变在 TC = 340 K 时发生，也可由强光激发驱动。几十年来，这种光致相变 (PIPT) 期间的超快动力学引起了极大的科学关注，因为这种方法有望回答绝缘体到金属 (IMT) 的转变是由电子或晶体过程通过解缠结引起的问题。时域中的不同贡献。我们回顾了我们最近通过飞秒时间分辨光电子、光学和相干声子光谱取得的成果，并在 VO2 中超快 PIPT 的一系列最新、补充研究的框架内讨论它们。我们表明，由光激发引起的电子和空穴的数量变化启动了一个高度非平衡的等离子体相，其特征是由于准自由载流子而增强了屏蔽，然后是非平衡动力学的两个分支：（i）瞬时（在时间分辨率）在电荷载流子弛豫和显着离子运动之前的绝缘间隙的坍塌，以及（ii）瞬时晶格电位对称性变化，表示通过离子运动在更长的时间尺度上开始结晶相变。我们讨论了这两个非热通路之间的相互联系，特别关注 PIPT 在不同类型实验中的临界通量的含义。基于此，我们得出结论，在光学实验中确定的 PIPT 阈值很可能由驱动晶格电位变化所需的激发密度而不是 IMT 决定。这些考虑表明 IMT 可以由较弱的激发驱动，预测 VO2 的瞬态金属单斜状态，该状态不受非热结构转变的稳定，因此在超快时间尺度上衰减。