Inhomogeneity of the ultrafast insulator-to-metal transition dynamics of VO₂

Reviews and Highlights	Quantum Science	Molecular and Soft-matter	Ultrafast Nano-optics and Nanophotonics	Mineralogy and Geochemistry

Brian T. O’Callahan, Andrew C. Jones, Jae Hyung Park, David H. Cobden, Joanna M. Atkin, and Markus B. Raschke
Nature Communications 6, 6849 (2015).
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The insulator-metal transition (IMT) of vanadium dioxide (VO₂) has remained a longstanding challenge in correlated electron physics since its discovery five decades ago. Most interpretations of experimental observations have implicitly assumed a homogeneous material response. Here we reveal inhomogeneous behaviour of even individual VO₂ microcrystals using pump-probe microscopy and nanoimaging. The timescales of the ultrafast IMT vary from 40+/-8 fs, that is, shorter than a suggested phonon bottleneck, to 200+/-20 fs, uncorrelated with crystal size, transition temperature and initial insulating structural phase, with average value similar to results from polycrystalline thin-film studies. In combination with the observed sensitive variations in the thermal nanodomain IMT behaviour, this suggests that the IMT is highly susceptible to local changes in, for example, doping, defects and strain. Our results suggest an electronic mechanism dominating the photoinduced IMT, but also highlight the difficulty to deduce microscopic mechanisms when the true intrinsic material response is yet unclear.