Ultrafast Nanoscale Imaging of Phase Transitions in Quantum Materials Using Femtosecond X-ray Pulses

Quantum materials, those with strongly coupled electronic, lattice, and spin degrees of freedom, can give rise to a wealth of different emergent phases like superconductivity, whose properties are highly desirable for applications. Recently ultrafast laser excitation has emerged as a powerful new way of generation new phases out of equilibrium, giving rise to the idea of "properties on demand". However, our understanding of how these light-induced phases emerge and stabilize is still in it's infancy. One particular complication comes from the fact that, even in equilibrium, quantum materials often exhibit a nanoscale breakup into different phases because of the competition between long and short range forces. While these effects are often invoked in explanations of light-induced phases, it has not been possible to directly image nanoscale phase dynamics at the ultrafast time scale.

Recently we have overcome this limitation and performed full nanoscale phase imaging of a light-induced phase transition in the prototypical quantum material vanadium dioxide. Using femtosecond coherent X-ray imaging at an X-ray free electron laser we obtained a full video of the domain structure with 150 fs temporal and 50 nm spatial resolution following ultrafast photoexcitation [1,2]. As I will show, we find ultrafast signatures long associated with nanoscale phase dynamics can actually be explained by simple ultrafast strain waves, and challenge claims of a novel emergent metallic phase in this material. I will also show new work towards imaging superconductivity and phase fluctuations at the ultrafast timescale.

1.A.S. Johnson et al., Science Advances 7, eabf1386 (2021).
2. A.S. Johnson et al., Nature Physics 19 (2), 297-297 (2023)
3. M.R. Otto et al., PNAS 116, 450-455 (2019).