Temperature-dependent infrared spectrum of Nb_xV_1-xO₂ single crystals at the phase transition

Vanadium dioxide (VO₂) is famous for the reversible metal-to-insulator transition - from an insulating monoclinic phase to a metallic rutile phase - slightly above room temperature which allows many photonics applications such as optical switching and modulators. Since the metal-to-insulator transition temperature is sensitive to the concentration of substituents, lower or higher the transition temperature by doping chemical substitution expands VO₂ application to the lower energy scale. Here, we investigate an optical property of Nb_xV_1-xO₂ single crystals (x = 0.4, 0.11, 0.15, 0.24, 0.35, and 0.88) across the metal-to-insulator transition. Although VO₂ and NbO₂ are isoelectronic and show the same type of metal-to-insulator transition, the transition temperature of NbO₂ is much higher than that of VO₂. Nb_xV_1-xO₂ alloy will help in resolving the structural and electronic connection among crystal phases. We measured a broadband reflection spectrum from visible to long-wave mid-infrared at various low temperatures. We obtained an optical conductivity of Nb_xV_1-xO₂ extracted by using Kramers-Kronig analysis. To evaluate the metal-to-insulator or the metal-to-semiconductor transition, we applied an extended Drude model to fit the optical conductivity.