Ultrafast electron crystallography of the cooperative reaction path in vanadium dioxide
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.
Additional Information© 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Received 6 May 2016; accepted 23 May 2016; published online 6 June 2016. At Caltech, we were grateful to Professor G. R. Rossman for the crystal-cutting equipment, to Professor N.-C. Yeh for the PLD machine, and to L. M. Henling for help with the X-ray measurements. This work was supported by the National Science Foundation, by the Air Force Office of Scientific Research, and by the Gordon and Betty Moore Center for Physical Biology at Caltech. We also thank Yoya Morimoto for help with estimating the mean-free-path lengths of electrons in VO2. D.S.Y. acknowledges support from the R. A. Welch Foundation (Grant No. E-1860) and the University of Houston. P.B. acknowledges support from the European Research Council (project DIVI) and the Munich Centre of Advanced Photonics.
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