Charge dynamics in thermally and doping induced insulator-metal transitions of (Ti$_{1-x}$V$_x$)$_2$O$_3$

Charge dynamics of (Ti$_{1-x}$V$_x$)$_2$O$_3$ with $x=0-0.06$ has been investigated by measurements of charge transport and optical conductivity spectra in a wide temperature range of $2-600$ K with the focus on the thermally and doping induced insulator-metal transitions (IMTs). The optical conductivity peaks for the interband transitions in the 3$d$ $t_{2g}$ manifold are observed in the both insulating and metallic states, while their large variation (by $\sim 0.4$ eV) with change of temperature and doping level scales with that of the Ti-Ti dimer bond length, indicating the weakened singlet bond in the course of IMTs. The thermally and V-doping induced IMTs are driven with the increase in carrier density by band-crossing and hole-doping, respectively, in contrast to the canonical IMT of correlated oxides accompanied by the whole collapse of the Mott gap.