A study on the metal-insulator transition in V$_{1-x}$W$_{x}$O$_{2}$ by optical spectroscopy

A strong Coulomb repulsion between electrons in a system with half filling makes a Mott insulator, although the band theory predicts that the system should be metallic. Carrier doping of a Mott insulator leads to an anomalous change in the electronic structure such as the transfer of spectral weight. VO$_{2}$ having a 3$d^{1}$ electron configuration has attracted considerable attention for its classification into a Mott system or a Peierls band insulator because of the metal-insulator transition accompanying structural change. We investigated the metal-insulator transition in thin films of tungsten-doped vanadium oxide: V$_{1-x}$W$_{x}$O$_{2}$, focusing on the lightly doped regime. Optical gap excitation at 2.5 eV for a film of $x$ = 0.012 is substantially suppressed, which coincides with the appearance of electronic spectral weight in the mid-infrared region. In films with x=0.04 we observed a strong Drude spectrum indicative of the metallic state. Our results are difficult to reconcile with the dominant role of the Peierls picture in the metal to insulator transition.