Unraveling the Photoexcited Dual-Threshold Phase Transitions in Niobium Dioxide

Previous femtosecond pump-terahertz probe experiments revealed that the intrinsically coupled insulator-to-metal (IMT) and structural phase (SPT) transitions in niobium dioxide (NbO₂) occur at two distinct fluence thresholds, exhibiting a novel dual-threshold feature. Here, we demonstrate that they are induced by two distinct types of electron-phonon interactions, different from the earlier interpretation of purely electronic IMT at the first fluence threshold (F_th1) and photothermal SPT at the second (F_th2). At F_th1, the photoexcited holes are self-trapped at specific niobium sites, assisted by thermal phonons. Such polaronic distortions drive the IMT into a bad metallic phase, beyond purely electronic effects. At F_th2, besides the IMT that occurs first, coherent phonons induce SPT from the zigzag Nb chains to linear isometric configurations, which enhances the metallicity via a nonthermal pathway rather than thermal effects. Our study provides theoretical insights into potential applications of NbO₂ for multilevel and energy-efficient storage.