  The insulator-to-metal transition (IMT) in vanadium dioxide (VO2) has
garnered extensive attention for its potential applications in ultrafast
switches, neuronal network architectures, and storage technologies. However, a
significant controversy persists regarding the formation of the IMT,
specifically concerning whether a complete structural phase transition from
monoclinic (M1) to rutile (R) phase is necessary. Here we employ the real-time
time-dependent density functional theory (rt-TDDFT) to track the dynamic
evolution of atomic and electronic structures in photoexcited VO2, revealing
the emergence of a long-lived monoclinic metal phase (MM) under low electronic
excitation. The emergence of the metal phase in the monoclinic structure
originates from the dissociation of the local V-V dimer, driven by the
self-trapped and self-amplified dynamics of photoexcited holes, rather than by
a pure electron-electron correction. On the other hand, the M1-to-R phase
transition does appear at higher electronic excitation. Our findings validate
the existence of MM phase and provide a comprehensive picture of the IMT in
photoexcited VO2.
