  Nonequilibrium phase transitions driven by light pulses represent a rapidly
developing field in condensed matter physics. As one of the archetypal strongly
correlated materials, vanadium dioxide (VO2) undergoes a structural phase
transition (SPT) from a monoclinic (M1) to rutile (R) structure and an
insulator-to-metal transition (IMT) either when heated above 340 K or when
excited by an ultrafast laser pulse. Here, we present a tensor network study of
the light-induced phase transitions in VO2 based on a quasi-one-dimensional
model with all the important ingredients -- multi-orbital character,
electron-lattice coupling, and electron-electron correlations -- being
included. We show that this model qualitatively captures the equilibrium
properties of VO2 by calculating the ground state phase diagram and
finite-temperature phase transitions. A hybrid quantum-classical tensor-network
method is used to simulate the dynamics following photoexcitation. We find that
the structure can transform faster than the harmonic phonon modes of M1 phase,
suggesting lattice nonlinearity is key in the SPT. We also find separate
timescales for the evolution of dimerization and tilt distortions in the
lattice dynamics, as well as the loss and subsequent partial restoration
behavior of the displacements, which can provide an explanation for the complex
dynamics observed in recent experiments [C. Brahms et al., arXiv:XXXX.XXXXX].
Moreover, decoupled SPT and IMT dynamics are observed in the numerical
simulations: while the initial M1 structure transforms to the R one in tens of
femtoseconds, the IMT occurs quasi-instantaneously, consistent with recent
experimental findings. Our theoretical studies provide insight into the
light-induced phase transitions of VO2, revealing unexpected non-monotonic
transformation pathways and paving the way for future studies of non-thermal
phase transformations.
