利用兆电子伏特超快电子衍射揭示VO2相变的瞬态动力学
Transient dynamics of the phase transition in VO2 revealed by mega electron-volt ultrafast electron diffraction
AI总结：
本研究利用兆电子伏特超快电子衍射揭示准单晶VO2薄膜光致相变中钒二聚体消失与晶体对称性转变不同步的瞬态动力学，为理解其超快相变提供了新信息。
摘要：
二氧化钒（VO2）在室温附近表现出绝缘体-金属转变，并伴随结构转变。这种转变可由超快激光脉冲触发。也曾有研究提出存在奇异的瞬态，例如无结构转变的金属态。这些独特性质使VO2在热可切换器件和光子应用中具有巨大潜力。尽管已付出巨大努力，但光致相变过程中的原子路径仍不清楚。在此，我们合成了自支撑准单晶VO2薄膜，并利用兆电子伏特超快电子衍射研究了其光致结构相变。凭借高信噪比和高时间分辨率，我们观察到钒二聚体和锯齿形链的消失与晶体对称性的转变并不同步。光激发后，初始结构在200飞秒内发生强烈改变，形成无钒二聚体和锯齿形链的瞬态单斜结构。随后，它在约5皮秒内继续演变为最终的正方结构。此外，在我们的准单晶样品中仅观察到一个激光通量阈值，而非多晶样品中所建议的两个阈值。我们的发现为全面理解VO2的光致超快相变提供了重要的新信息。
英文摘要：
Vanadium dioxide (VO2) exhibits an insulator-to-metal transition accompanied by a structural transition near room temperature. This transition can be triggered by an ultrafast laser pulse. Exotic transient states, such as a metallic state without structural transition, were also proposed. These unique characteristics let VO2 have great potential in thermal switchable devices and photonic applications. Although great efforts have been made, the atomic pathway during the photoinduced phase transition is still not clear. Here, we synthesized freestanding quasi-single-crystal VO2 films and examined their photoinduced structural phase transition with mega-electron-volt ultrafast electron diffraction. Leveraging the high signal-to-noise ratio and high temporal resolution, we observe that the disappearance of vanadium dimers and zigzag chains does not coincide with the transformation of crystal symmetry. After photoexcitation, the initial structure is strongly modified within 200 femtoseconds, resulting in a transient monoclinic structure without vanadium dimers and zigzag chains. Then, it continues to evolve to the final tetragonal structure in approximately 5 picoseconds. In addition, only one laser fluence threshold instead of two thresholds suggested in polycrystalline samples was observed in our quasi-single-crystal samples. Our findings provide new essential information for a comprehensive understanding of the photoinduced ultrafast phase transition in VO2.
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