Coherent vibrational dynamics of NbO2 film

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The high thermodynamic transition temperature (\ensuremath{\approx}1080 K) of niobium dioxide (${\mathrm{NbO}}_{2}$) makes it advantageous in differentiating the effects of electronic excitation and heat accumulation in the photoinduced phase transition. Recently, a photoinduced nonthermal metallization of ${\mathrm{NbO}}_{2}$ film has been claimed [R. Rana, J. M. Klopf, J. Grenzer, H. Schneider, M. Helm, and A. Pashkin, Phys. Rev. B 99, 041102(R) (2019)], but the simultaneous lattice evolution still remains blank. In this paper, photoinduced evolution in lattice vibrations of a crystalline ${\mathrm{NbO}}_{2}$ film has been investigated using broadband coherent phonon spectroscopy. Signatures of two optical phonons at 155 and $185\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\text{--}1}$ are identified, and the corresponding lattice vibrations are found to be associated with the motions of Nb-Nb dimers. Pump fluence-dependent measurements reveal an electronic phase transition at a threshold fluence of $\ensuremath{\approx}10\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}/\mathrm{c}{\mathrm{m}}^{2}$, while the lattice structure is preserved. A transient disordering in the lattice vibration is simultaneously observed, but lattice transformation is prevented probably by the large enthalpy barrier of the strong Peierls insulation. The accompanied drastic modulation in optical properties without atomic rearrangement also suggests the potential of ${\mathrm{NbO}}_{2}$ in improving the operation speed of related devices.