Uniting the order and disorder dynamics in photoexcited VO2

TL;DR

This study uses real-time density functional theory to investigate how laser fluence influences the coherent or disordered dynamics of phase transition in VO2, revealing the roles of photoexcited holes and thermal vibrations.

Contribution

It uncovers how laser fluence controls the order or disorder in VO2's photoinduced phase transition through real-time simulations of ion dynamics.

Findings

Photoexcited holes drive coherent V-V dimer motion.

Weak laser fluence results in disordered phase transition.

Strong laser fluence saturates hole population, limiting transition timescale.

Abstract

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 (rt-TDDFT) 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.