Vanadium dioxide (VO₂) is an exemplary strongly correlated material known for its dramatic insulator-to-metal transition around room temperature (Fig. 1(a)). The phase transition can also be triggered by external stimuli such as electric gating and light irradiation. VO₂ thus provides an interesting platform for novel functionalities. By using time-resolved photoemission spectroscopy (TrPES) implemented by a high-harmonic generation method, we investigated the ultrafast photo-induced transition of VO₂, and provide insights into the non-equilibrium dynamics governed by strong correlations.

Fig. 1. Insulator-to-metal transition of VO₂. (a) Structural change from monoclinic insulating phase to rutile metal phase upon the thermal transition. (b) Time-resolve spectrum at delay = 0.95 ps (F ~ 9 mJ/cm², 270 K) compared with a static photoemission spectrum of a rutile metallic state recorded at 350 K. (c) Time evolution of spectral weight over [-0.2, 0.2 eV]. The inset shows the unoccupied side of the spectra for delay = 0.95, 4.05, 9.07 ps in the logarithmic scale.

The valence-band spectrum of a VO₂ thin film was monitored in a pump-and-probe TrPES configuration. A metallic state emerged when the sample was impinged by a 170-fs pulse above a critical fluence F = 6 mJ/cm², which was directly evidenced by the ultrafast increase of the spectral weight at the Fermi level (E_F). The transition was accompanied by a spectral-weight transfer on a 1-eV scale characteristic of strong electron correlations. The photo-induced metallic state exhibited a unique spectrum that tails up to ~0.4 eV above E_F. This indicates that the emergent metallic state is different from the rutile state (Fig. 1(b)). After the impulsive impact, the spectral weight at E_F further increased until ~3 ps (Fig. 1(c), inset), indicating that the metallic state grew proliferatively. The ~0.4-eV tail also persisted for > 3 ps, the behavior of which cannot be explained by the photo-response of typical Mott insulators that relaxes within ~1 ps. These observations emphasize the importance of non-electronic degrees of freedom in the photo-induced dynamics of VO₂.

References

• [1] R. Yoshida, T. Yamamoto, Y. Ishida, H. Nagao, T. Otsuka, K. Saeki, Y. Muraoka, R. Eguchi, K. Ishizaka, T. Kiss, S. Watanabe, T. Kanai, 
J. Itatani, and S. Shin, Phys. Rev. B 89, 205114 (2014).

Authors

• R. Yoshida, T. Yamamoto, Y. Ishida, H. Nagao^a, T. Otsuka^a, K. Saeki^a, Y. Muraoka^a, R. Eguchi^a, K. Ishizaka, T. Kiss, S. Watanabe, T. Kanai, J. Itatani, and S. Shin
• ^aOkayama University