The key to understanding something is to observe it. This simple approach has driven the fields of femotchemistry and femtophysics in their attempts to understand chemical reactions and phase transitions and has driven new approaches to reach atomic scale dynamics with femtosecond time resolution. However, while in chemistry it may be feasible to track the dynamics of all the atoms in a molecule, it is simply not possible in a solid due to the astronomical number of constituents.
To overcome this difficulty, we need probes that can measure on a range of length scales. In the first part of my talk, I will discuss our progress in using lens-less soft X-ray imaging to observe nanoscale heterogeneity the insulator-metal phase transition in VO2 [1,2]. By applying these methods at the PAL-FEL, we can not only observe nanoscale femtosecond dynamics in heterogenous materials, but perform quantitative spectroscopy of the transient state allowing us to answer key questions about the phase transition [3].
I will then discuss our approach in the hard X-ray region to understand the atomic pathway of the phase transition in VO2. I will show that, by moving to diffuse X-ray scattering, we can observe that the positions of the vanadium ions disorder on the ultrafast timescale, resulting in an incoherent phase transition [4]. I will then discuss our recent, unpublished, work on the mechanisms that drive atomic scale disorder on such a time scale, and how this disorder my enable more energy efficient control of material properties.
[1] L. Vidas et al. Nano Letters 18, 3449 (2018)
[2] A. S. Johnson et al. Science Advances 7, eabf1386 (2021)
[3] S. Wall et al. Science 362, 572 (2018)
[4] A. S. Johnson et al. Nature Physics (in press)
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