Selective phonon-mode excitation revealed by tracing the pathway of photo-induced structural-phase-transition in single-crystal VO₂

The Metal-Insulator (M-I) transition in VO₂ that is associated with a monoclinic-rutile (M-R) structure phase transition has been extensively studied due to the potential application of its fast electrical/optical switch properties. It remains a big challenge to reveal the transition pathway to fully understand the underlying mechanism. In this study, we use MeV ultrafast electron diffraction to monitor the structure evolution in single-crystal VO₂ under femtosecond laser excitation. The M-R phase transition was observed at a high sample temperature (T = 314 K) under a specific pump fluence of 16 mJ/cm². Crystallographic structural analysis reveals that the atomic motions have a two-step dynamics. During the first a few ps, both stretching and rotation A_g phonon modes are involved in the collective vanadium motions. Then only rotation A_g phonon mode is continuously excited in the following hundreds of ps. Moreover, such the M-R phase transition cannot be observed at a much lower sample base temperature (T = 77 K), indicating a certain role of thermal fluctuation in this photo-induced phenomenon.