The metal-insulator transition and unconventional metallic transport in vanadium dioxide $\left(\mathrm{V}{\mathrm{O}}_2\right)$ are investigated with a combination of spectroscopic ellipsometry and reflectance measurements. The data indicate that electronic correlations, not electron-phonon interactions, govern charge dynamics in the metallic state of $\mathrm{V}{\mathrm{O}}_2$. This study focuses on the frequency and temperature dependence of the conductivity in the regime of extremely short mean free path violating the Ioffe-Regel-Mott limit of metallic transport. The standard quasiparticle picture of charge conduction is found to be untenable in metallic $\mathrm{V}{\mathrm{O}}_2$.

• Received 1 September 2006

DOI:https://doi.org/10.1103/PhysRevB.74.205118