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.