Photoinduced structural dynamics and transformation pathway of sapphire-supported ultrathin vanadium dioxide*

The complex phase-transition behaviors of vanadium dioxide (VO$_{\mathrm{2}})$ have drawn continual attention because of their strong dependence on a wide range of factors including temperature, strain, doping, photoexcitation, and voltage. Dynamically, ultrafast electron diffraction (UED) with atomic-scale spatial and temporal resolution has been used to unravel the transformation pathway in the photoinduced structural phase transition of VO$_{\mathrm{2}}$, using bulk and nearly strain-free specimens. In this presentation, we report the UED results from 10-nm-thick crystalline VO$_{\mathrm{2}}$ supported on Al$_{\mathrm{2}}$O$_{\mathrm{3}}$(0001) and examine the influence of surface strain on the ultrafast structural transformation. It becomes clear that a complete conversion from the monoclinic phase to the tetragonal lattice does not take place on the time scale of few hundred picoseconds unless a fluence multiple times higher than the thermodynamic enthalpy threshold is used. For some crystalline domains, structural transformation may not be seen even after 1 ns following an intense photoexcitation. The implications of the present and previous observations as well as the transformation pathway will be discussed.