  VO2, of interest for decades due to both its phenomenology and its potential
applications, has a monoclinic distortion of the rutile crystal structure at
ambient temperature that is coupled to its metal-insulator transition (MIT). In
contrast, RuO2 has three electrons more per formula unit, is a metallic
conductor, and has an undistorted rutile structure. Here we report a systematic
study of Ru-doped VO2 (V1-xRuxO2); generally characterizing its crystal
structure, magnetic and electronic properties, and heat capacity. The
composition-dependent Wilson ratio is determined. We find that an unusually
high Ru doping value (x=0.8) is required to achieve a metallic state in
V1-xRuxO2. No superconductivity was observed down to 0.1 K in the metallic
materials. We propose a possible understanding for how the insulating state can
exist in V1-xRuxO2 at high Ru contents.
