Photoinduced directional propagation of a local structural transition along the dimer chains in NbO 2

Recent studies have demonstrated that photoexcitation can induce structural phase transitions locally under a weak excitation, significantly altering material properties in an ultrahigh-speed and energy-efficient manner. However, the underlying mechanisms cannot be fully explained by the collective atomic motions of photoexcited coherent phonons. In this work, using real-time time-dependent density functional theory (rt-TDDFT), we show that local Nb-Nb dimers are broken in ${\mathrm{NbO}}₂$ due to the atomic driving forces amplified by the trapping of local photoexcited holes. More trapped photoholes are subsequently transferred to adjacent Nb-Nb dimers along the atomic chain direction, and finally forming a conductive channel for carriers, yielding a transition from a semiconductor phase to a metallic phase. Furthermore, we discover that stability of the local phase transition depends on the initial lattice temperature, persisting for a longer timescale in the range of 300 K to 600 K. Our work highlights the directional nature of local phase transitions in photoexcited ${\mathrm{NbO}}₂$ and paves the way for future experimental investigations.