Thermopower analysis of the electronic structure around the metal-insulator transition inV1−xWxO2

The electronic structure across the metal-insulator (MI) transition of electron-doped ${\mathrm{V}}_{1-x}{\mathrm{W}}_x{\mathrm{O}}_2$ epitaxial films ($x=0-0.06$) grown on $\alpha \text{−}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ substrates was studied by means of thermopower ($S$) measurements. Significant increase of $\left|S\right|$ values accompanied by MI transition was observed, and the transition temperatures of $S$ ($T_S$) decreased with $x$ in a good linear relation with MI transition temperatures. $\left|S\right|$ values of ${\mathrm{V}}_{1-x}{\mathrm{W}}_x{\mathrm{O}}_2$ films at $T>T_S$ were constant at low values of $23\mu \mathrm{V}{\mathrm{K}}^{-1}$ independently of $x$, which reflects a metallic electronic structure, whereas those at $T<T_S$ almost linearly decreased with logarithmic W concentrations. The gradient of $-213\mu \mathrm{V}{\mathrm{K}}^{-1}$ agrees well with $-k_B/e\ln 10(-198\mu \mathrm{V}{\mathrm{K}}^{-1})$, suggesting that ${\mathrm{V}}_{1-x}{\mathrm{W}}_x{\mathrm{O}}_2$ films have insulating electronic structures with a parabolic density of state around the conduction band bottom.