Evidence for a structurally-driven insulator-to-metal transition in ${\mathrm{VO}}_2$: A view from the ultrafast timescale

We apply ultrafast spectroscopy to establish a time-domain hierarchy between structural and electronic effects in a strongly correlated electron system. We discuss the case of the model system ${\mathrm{VO}}_2$, a prototypical nonmagnetic compound that exhibits cell doubling, charge localization, and a metal-insulator transition below 340 K. We initiate the formation of the metallic phase by prompt hole photo-doping into the valence band of the low-$T$ insulator. The insulator-to-metal transition is, however, delayed with respect to hole injection, exhibiting a bottleneck time scale, associated with the phonon connecting the two crystallographic phases. This structural bottleneck is observed despite faster depletion of the $d$ bands and is indicative of important bandlike character for this controversial insulator.

Figure 1

Structural and electronic phases of ${\mathrm{VO}}_2$. The high-$T$ phase is rutile, with the ${\mathrm{V}}^{4+}$ ions approximately at the center of ${\mathrm{O}}^{2-}$ octhahedra (not shown). The low-$T$ phase is derived after pairing and tilting along the $c$ axis, i.e., it has a unit cell of doubled size and has distortion of the oxygen cages. The $3d^3{s}^2$ vanadium atoms contribute four electrons to fill the valence band, leaving one electron in the conduction band. In the high-$T$ phase, the bands close to the Fermi level are the $\mathrm{V}3d$ bands, composed by a purely $d\Vert$ band (oriented along the $c$ axis) and by $3d\pi$-hybridized bands, mixed with $p$ orbitals of the oxygen ligands. In the low-$T$ phase, dimerization splits the $d\Vert$ band, with contributions from both structural and electronic correlations. Distortion of V-O bonds lifts the $3d_{\pi }$ band above the Fermi level.

Figure 2

Reflectivity and/or transmission responses of free-standing ${\mathrm{VO}}_2\text{−}{\mathrm{Si}}_3{\mathrm{N}}_4$ structures for 100-fs excitation pulses at 790-nm wavelength and $50\mathrm{mJ}∕{\mathrm{cm}}^2$. The pump-probe experiments were performed at near-normal incidence.

Figure 3

Pump-probe reflectivity experiments of the photoinduced phase transition. The experiments are performed in the nonetched structure with variable pulse durations between 1.5 ps and 15 fs, as measured at the sample position. White light was amplified in 1-mm, 32°-cut BBO, pumped with 400-nm pulses crossing the seed light at 3.5°. Pulse compression in a pair of prisms was used to minimize the duration of the pump-probe autocorrelation at the sample position. The experiments were conducted using pulses of 100-nm bandwidth full width at half maximum centered around 650 nm.

Figure 4

(a) Time-resolved evolution of the reflectivity, measured using a $\mathrm{Ti}:\mathrm{Sa}$ oscillator emitting 40-fs pulses at 850 nm. The inset displays the oscillatory part of the signal, obtained by subtracting the background. (b) Unpolarized, continuous-wave Raman spectra (dashed curve), acquired in backscattering geometry using 532-nm cw excitation. The modes at 5.85 THz and 6.75 THz are fully symmetric $A_g$ modes, while the lower mode is of $B_g$ symmetry (Ref. 25). The continuous curve: Fourier transform of the time-resolved oscillations, revealing the coherent excitation of the $A_g$ modes only.