Tracking the insulator-to-metal phase transition in VO2with few-femtosecond extreme UV transient absorption spectroscopy

Abstract

<p id="d15003293e350">An insulator-to-metal phase transition is a process that changes a solid material from being electrically nonconductive to being conductive. The phase transition in vanadium dioxide is a well-studied example where the process can occur in less than a picosecond, making it exciting for ultrafast electronic switches. This paper measures a record speed for the phase transition of 26 fs into a long-lived excited state of the metal that persists out to &gt;60 ps. The extreme UV absorption spectrum of the material is also measured and (together with the ultrafast timescale) belies a structural mechanism that has long been deliberated. The measured femtosecond timescale provides fundamental insight into the electronic speed limits of these complex phenomena. </p><p class="first" id="d15003293e353">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 (VO ₂) is a canonical example of such correlations. Here, few-femtosecond extreme UV transient absorption spectroscopy (FXTAS) at the vanadium <i>M</i> _2,3 edge is used to track the insulator-to-metal phase transition in VO ₂. 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 VO ₂ absorption spectrum in the extreme UV is developed using atomic cluster model calculations, revealing V ³⁺/d ² 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 d ² 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. </p>