Correlated metallic state of vanadium dioxide
  The metal-insulator transition and unconventional metallic transport in vanadium dioxide (VO$_2$) are investigated with a combination of spectroscopic ellipsometry and reflectance measurements. The data indicates that electronic correlations, not electron-phonon interactions, govern charge dynamics in the metallic state of VO$_2$. 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 VO$_2$. 
M. M. Qazilbash, K. S. Burch, D. Whisler, D. Shrekenhamer, B. G. Chae,   H. T. Kim, and D. N. Basov