Coherent Phonon Assisted Ultrafast Order-Parameter Reversal and Hidden Metallic State in Ta2NiSe5

The nonequilibrium dynamics during photoinduced insulator-to-metal transition (IMT) in the excitonic insulator (EI) candidate ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ have been investigated, which reproduce the timescale and spectral features of the ultrafast switch and reveal intricate many-body interactions involving multidegrees of freedom. The key role of lattice order parameter (OP) reversal, occurring on a timescale comparable to that of purely electronic processes ($<100\text{ }\text{ }\mathrm{fs}$), is identified. This reversal is enabled by the anharmonic interactions between EI-OP-coupled phonons and the conventional coherent phonons, leading to a modified potential energy landscape and a high-frequency mode up-conversion. The phonon excitation depends on the dynamics of photocarriers distributed around the Fermi level, and thus intertwines with the excitonic quenching and the complete gap collapse. These findings provide a comprehensive understanding of exciton-phonon dynamics in correlated quantum materials.