Metallic bubbles nucleation and growth in VO2 nanofilms: insights from TDDFT+DMFT*

We apply a time-dependent density-functional theory + dynamical mean-field theory (TDDFT+DMFT) approach to model the response of insulating nanofilms of VO2 to perturbations by ultrafast laser pulses. We focus on the spatially-resolved metallization of the systems, and especially on the process of nucleation and time-dependence of the size of the "surface" and "bulk" metallic domains (bubbles) as a function of film width. In particular, we find that the initial universal (parameter-independent) growth of the domains (radius $R\sim t^{1/2}$), changes by the bubbles shrinking ($R\sim t^{-a}$, $a\sim 1$) as a result of Coulomb scattering effects, and eventually by post-femto-second phonon-involved relaxation of the systems to the equilibrium accompanied by infrared photoemission. The time-dependent conductivity obtained from the above results is in a good agreement with available experimental data.[1,2] [1] D.J. Hilton et al., PRL 99, 226401 (2007); [2] T.L. Cocker et al., PRB 85, 155120 (2012).