Abstract
Time-resolved electron diffraction with atomic-scale spatial and temporal resolution was used to unravel the transformation pathway in the photoinduced structural phase transition of vanadium dioxide. Results from bulk crystals and single-crystalline thin-films reveal a common, stepwise mechanism: First, there is a femtosecond V−V bond dilation within 300 fs, second, an intracell adjustment in picoseconds and, third, a nanoscale shear motion within tens of picoseconds. Experiments at different ambient temperatures and pump laser fluences reveal a temperature-dependent excitation threshold required to trigger the transitional reaction path of the atomic motions.
Highlights
via photoexcitation above the bandgap,4 the structural phase transition has been resolved on the atomic spatiotemporal scale to proceed along a stepwise reaction path
where the atoms move from the initial
transitional to final conformations with a hierarchy of time scales ranging from femtoseconds
Summary
Vanadium dioxide is a strongly correlated material with a first-order thermodynamic transition at around 340 K between an insulating phase of monoclinic crystallographic symmetry and a metallic phase with a tetragonal (rutile) crystal structure. Dynamically, via photoexcitation above the bandgap, the structural phase transition has been resolved on the atomic spatiotemporal scale to proceed along a stepwise reaction path, where the atoms move from the initial over transitional to final conformations with a hierarchy of time scales ranging from femtoseconds (fs) to hundreds of picoseconds (ps). Different aspects of this path require different degrees of cooperation, evident from the distinct energy thresholds below which the structural phase transition does not fully proceed. Spectroscopic methods using lasers or terahertz radiation have revealed such a threshold for the electronic and phononic aspects of the transition, and related time scales. Overall, there is a cooperative, sequential reaction path in which the structural and electronic degrees of freedom are intimately related.. Via photoexcitation above the bandgap, the structural phase transition has been resolved on the atomic spatiotemporal scale to proceed along a stepwise reaction path, where the atoms move from the initial over transitional to final conformations with a hierarchy of time scales ranging from femtoseconds (fs) to hundreds of picoseconds (ps).5 Different aspects of this path require different degrees of cooperation, evident from the distinct energy thresholds below which the structural phase transition does not fully proceed.. The results indicate, for both the bulk and the thin-film crystals, that a temperature-dependent laser fluence threshold is required to properly initiate the structural pathway This observation for the structural transformation matches earlier findings on the metal-insulator transition by spectroscopy and supports the initial conjecture that the laser-induced and thermal pathways are intimately related
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