Tracking the insulator-to-metal phase transition in VO₂ with few-femtosecond extreme UV transient absorption spectroscopy

• Published in: Proceedings of the National Academy of Sciences - PNAS 2017-09, Vol.114 (36), p.9558-9563
• Main Authors: Jager, Marieke F., Ott, Christian, Kraus, Peter M., Kaplan, Christopher J., Pouse, Winston, Marvel, Robert E., Haglund, Richard F., Neumark, Daniel M., Leone, Stephen R.
• Format: Article
• Language: English
• Subjects: CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; extreme UV; insulator-to-metal transition; MATERIALS SCIENCE; Physical Sciences; ultrafast dynamics; vanadium dioxide
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1707602114

Coulomb correlations can manifest in exotic properties in solids, but how these properties can be accessed and ultimately manipulated in real time is not well understood. The insulator-to-metal phase transition in vanadium dioxide (VO₂) is a canonical example of such correlations. Here, few-femtosecond extreme UV transient absorption spectroscopy (FXTAS) at the vanadium M 2,3 edge is used to track the insulator-to-metal phase transition in VO₂. This technique allows observation of the bulk material in real time, follows the photoexcitation process in both the insulating and metallic phases, probes the subsequent relaxation in the metallic phase, and measures the phase-transition dynamics in the insulating phase. An understanding of the VO₂ absorption spectrum in the extreme UV is developed using atomic cluster model calculations, revealing V3+/d² character of the vanadium center. We find that the insulator-to-metal phase transition occurs on a timescale of 26 ± 6 fs and leaves the system in a long-lived excited state of the metallic phase, driven by a change in orbital occupation. Potential interpretations based on electronic screening effects and lattice dynamics are discussed. A Mott–Hubbard-type mechanism is favored, as the observed timescales and d² nature of the vanadium metal centers are inconsistent with a Peierls driving force. The findings provide a combined experimental and theoretical roadmap for using time-resolved extreme UV spectroscopy to investigate nonequilibrium dynamics in strongly correlated materials.