The order-disorder phase transition in VO2 revealed by an x-ray free electron laser*

Femtosecond laser pulses provide a promising strategy to control quantum phases of matter with potential to realize “properties on demand” [1]. While laser pulses can distort the lattice into novel structures with exotic properties not accessible in equilibrium, probing those transient structures remains a challenge. X-ray Free Electron Lasers (XFELs) hold promise to visualize the atomic and electronic structure of these transient states as they transform. I will present XFEL results on the ultrafast monoclinic (M 1 ) to rutile (R) transition of VO 2 . We used femtosecond x-ray diffuse scattering [2,3] at the Linac Coherent light Source (LCLS) at SLAC to study the structural dynamics of the transition at all length-scales. Contrary to the displacive transitions common among charge density waves (CDWs) where the dynamics occur in the vicinity of the CDW wavevector, the M 1 to R transformation in VO 2 proceeds by an ultrafast increase in the x-ray diffuse intensity spanning the Brillouin zone. The intensity momentum distribution shows that photoexcitation induces a prompt uncorrelated disordering of the vanadium dimers of the M 1 phase that reach a rutile quasi-equilibrium distribution in ~ 150 fs. Our results suggest that the pump induces a highly anharmonic potential that enables fast disordering of the lattice, which is intimately related with the high phonon entropy of the R phase.

References
[1] Basov, D. N., Averitt, R. D. & Hsieh, Nat. Mater. 16, 1077–1088 (2017).
[2] S. Wall et al, Science 362, 572 (2018).
[3] M. Trigo et al, Nat. Phys. 9, 790 (2013).