Unifying the order and disorder dynamics in photoexcited VO2

Photoinduced phase transition (PIPT) is always treated as a coherent process, but ultrafast disordering in PIPT is observed in recent experiments. Utilizing the real-time time-dependent density functional theory method, here we track the motion of individual vanadium (V) ions during PIPT in VO2 and uncover that their coherent or disordered dynamics can be manipulated by tuning the laser fluence. We find that the photoexcited holes generate a force on each V-V dimer to drive their collective coherent motion, in competing with the thermal-induced vibrations. If the laser fluence is so weak that the photoexcited hole density is too low to drive the phase transition alone, the PIPT is a disordered process due to the interference of thermal phonons. We also reveal that the photoexcited holes populated by the V-V dimerized bonding states will become saturated if the laser fluence is too strong, limiting the timescale of photoinduced phase transition.

Automated summary

This study utilizes the real-time time-dependent density functional theory method to track the motion of individual vanadium ions in VO2 during photoinduced phase transition (PIPT). The results reveal that the coherent or disordered dynamics of the vanadium ions can be manipulated by tuning the laser fluence. The competing forces of photoexcited holes and thermal-induced vibrations determine the nature of the PIPT process. The interference of thermal phonons leads to a disordered PIPT process when the laser fluence is weak, while the saturation of photoexcited holes limits the timescale of PIPT when the laser fluence is strong.