Mesoscopic structural phase progression in photo-excited VO2 revealed by time-resolved x-ray diffraction microscopy

TL;DR

This study uses advanced time-resolved x-ray microscopy to visualize the mesoscopic structural phase progression in VO2 during photoinduced phase transition, revealing localized initiation and growth of lattice structures.

Contribution

It provides the first direct visualization of the spatial and temporal evolution of phase transformation in VO2 at the mesoscopic scale using time-resolved x-ray diffraction microscopy.

Findings

Phase transformation initiates at discrete sites after optical excitation.

The phase front propagates faster than thermal diffusion but slower than sound speed.

The transformation involves a displacive lattice change from monoclinic to rutile phase.

Abstract

Dynamical phase separation during a solid-solid phase transition poses a challenge for understanding the fundamental processes in correlated materials. Critical information underlying a phase transition, such as localized phase competition, is difficult to reveal by measurements that are spatially averaged over many phase separated regions. The ability to simultaneously track the spatial and temporal evolution of such systems is essential to understanding mesoscopic processes during a phase transition. Using state-of-the-art time-resolved hard x-ray diffraction microscopy, we directly visualize the structural phase progression in a VO2 film upon photoexcitation. Following a homogenous in-plane optical excitation, the phase transformation is initiated at discrete sites and completed by the growth of one lattice structure into the other, instead of a simultaneous isotropic lattice symmetry change. The time-dependent x-ray diffraction spatial maps show that the in-plane phase progression in laser-superheated VO2 is via a displacive lattice transformation as a result of relaxation from an excited monoclinic phase into a rutile phase. The speed of the phase front progression is quantitatively measured, and is faster than the process driven by in-plane thermal diffusion but slower than the sound speed in VO2. The direct visualization of localized structural changes in the time domain opens a new avenue to study mesoscopic processes in driven systems.