Ultrafast dynamics of atomic correlated disordering in photoinduced VO$_2$

TL;DR

This study uses real-time density functional theory to reveal how atomic disordering in VO₂ during photoinduced phase transitions depends on temperature and atomic motion directions, highlighting anisotropic disordering mechanisms.

Contribution

It uncovers the temperature-dependent dynamics and anisotropic nature of atomic disordering in VO₂ during photoinduced phase transitions using rt-TDDFT.

Findings

Higher lattice temperature accelerates atomic disordering at lower excitation levels.

Transition timescales become temperature-independent above a certain threshold.

Atomic disordering is more prominent along the x direction due to V-V dimer motion.

Abstract

Recent experiments suggest that atomic disordering dynamics are more universal than conventional coherent processes in photoinduced phase transitions (PIPTs), yet its mechanism remains unclear. Using real-time time-dependent density functional theory (rt-TDDFT), we find that, at lower photoexcitation, higher lattice temperature accelerates atomic disordering, which thereby lowers the threshold for phase transition, by thermally exciting more phonons to randomize the lattice vibrations in VO$_2$. Above this threshold, however, we observe that the transition timescale and atomic disordering become temperature-independent since thermally excited lattice vibrations induce a similar evolution of photoexcited holes. Additionally, we show that photoexcitation initially elongates the V-V dimers followed by a rotation with tangential displacements (along the z-axis) mediated by O atoms, resulting in strongly correlated motion along the z-axis. Consequently, atomic disordering is more dominant along the x direction, attributed to the relatively unrestricted motion of V-V dimers along this direction. The motion of V atoms along the z-axis is more constrained, leading to less disorder along the z-axis, which results in a "correlated disorder" phenomenon. This anisotropic disordering in VO$_2$ offers new insights into PIPTs mechanisms, guiding future studies on photoinduced disordered transitions.