Characterization of electronic structure around metal–insulator transition in V_1−xW_xO₂ thin films by thermopower measurement

Abstract

Electronic structure across the metal–insulator (MI) transition of electron-doped V_1−xW_xO₂ epitaxial films (x = 0–0.06) grown on α-Al₂O₃ substrates was studied by means of thermopower (S) measurements. Significant increase of |S|-values accompanied by MI transition was observed, and the transition temperatures of S (T_S) decreased with x in good linear relation with MI transition temperatures. |S| values of V_1−xW_xO₂ films at T > T_S were constant at low values of 23 µV 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 µV K⁻¹ agrees well with −k_B/e·ln10 (−198 µV K⁻¹), suggesting that V_1−xW_xO₂ films have insulating electronic structures with a parabolic density of state around the conduction band bottom.