Speaker
Description
Controlling phase transitions in correlated quantum materials [1] with ultrafast light pulses offers a powerful route to creating and probing non-equilibrium states of matter. However, identifying the microscopic mechanisms that drive these transformations remains a major challenge. Vanadium dioxide (VO2) provides a particularly compelling example. It undergoes a first-order insulator-to-metal transition from a monoclinic (M1) insulating phase to a rutile (R) metallic phase when heated above 343 K or when excited by an ultrafast optical pulse [2] above a critical fluence. Although this photoinduced transition has been studied extensively, its underlying origin remains controversial, with competing interpretations emphasizing electronic correlations [3], coherent structural dynamics [4], or thermally driven processes [5].
In this work, we demonstrate that temperature-dependent ultrafast pump–probe spectroscopy provides a model-agnostic means to unequivocally distinguish the proposed mechanisms governing the photoinduced insulator-to-metal transition in VO2. We introduce an analytical method based on the dimensionality and statistical properties of the ultrafast heat capacity, extracted from the fluence- and temperature-dependent photoinduced reflectivity dynamics. By examining the associated statistical scaling and dimensionality, we establish a general framework capable of distinguishing between different driving mechanisms, including purely electronic excitation, selective coherent-phonon coupling, and excitation of the full phonon bath through their distinct temperature dependence of the critical fluence.
Our results indicate that a persistent metallic state emerges only when energy is distributed across the complete phonon spectrum, including high-energy oxygen vibrations, highlighting the fundamentally thermal character of the transition. At the same time, we identify that it is possible to trigger an electronic-only transition at short times that disappears rapidly when the lattice is not sufficiently stabilized. These results provide a clearer picture of the interplay between electronic excitation and lattice dynamics in VO2, while establishing ultrafast heat-capacity analysis as a versatile framework for identifying the microscopic origin of light-induced phase transitions. More broadly, this approach can be readily extended to other photoinduced transitions without requiring complex multimodal experimental techniques, providing a versatile framework for understanding and controlling non-equilibrium phase transformations, including those driven by incoherent processes.
References
[1] A. de la Torre et al., “Colloquium: Nonthermal pathways to ultrafast control in quantum materials,” Rev. Mod. Phys. 93, 041002 (2021).
[2] A. Cavalleri et al., “Femtosecond Structural Dynamics in VO2 during an Ultrafast Solid-Solid Phase Transition,” Phys. Rev. Lett. 87, 237401 (2001).
[3] T. V. Slusar et al., “Mott transition in chain structure of strained VO2 films revealed by coherent phonons,” Sci. Rep. 7, 16038 (2017).
[4] S. Wall et al., “Tracking the evolution of electronic and structural properties of VO2 during the ultrafast photoinduced insulator-metal transition,” Phys. Rev. B 87, 115126 (2013).
[5] S. Wall et al., “Ultrafast disordering of vanadium dimers in photoexcited VO2,” Science 362, 572–576 (2018).