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

Area of Science:

• Materials Science
• Condensed Matter Physics
• Nanoscale Science

Background:

• Understanding solid-solid phase transitions in correlated materials is hindered by dynamical phase separation.
• Spatially averaged measurements obscure critical mesoscopic processes like localized phase competition.
• Simultaneous spatial and temporal tracking is essential for elucidating phase transition dynamics.

Purpose of the Study:

• To directly visualize the structural phase progression in Vanadium Dioxide (VO2) during photoexcitation.
• To investigate the mesoscopic mechanisms governing phase transitions in driven systems.
• To overcome limitations of spatially averaged techniques in studying phase separation.

Main Methods:

• Utilized state-of-the-art time-resolved hard X-ray diffraction microscopy.
• Employed homogenous in-plane optical excitation of a VO2 film.
• Acquired time-dependent X-ray diffraction spatial maps to track structural evolution.

Main Results:

• Phase transformation initiated at discrete sites, proceeding via lattice structure growth, not isotropic symmetry change.
• In-plane phase progression in laser-superheated VO2 observed as a displacive lattice transformation (monoclinic to rutile).
• Phase front progression speed measured: faster than thermal diffusion, slower than sound speed in VO2.

Conclusions:

• Direct visualization of localized structural changes in the time domain is achieved.
• Revealed a novel mechanism for phase progression in VO2 driven by photoexcitation.
• Opens new avenues for studying mesoscopic processes in dynamic, driven material systems.