Superconductivity in NbGe2 and isostructural C-40 compounds

The superconductivity of NbGe2 and isostructural C-40 compounds has been studied in a quantitative manner. We have ruled out various mechanisms, such as excitonic superconductivity and filamentary superconductivity, which were originally believed to be responsible for the superconductivity in NbGe2. The high transition temperature (T/sub c/ = 16 K) exhibited by sputtered films of NbGe2 are shown to be an intrinsic property of the compound and are attributed to the degree of order produced during the formation process. It has also been shown that different sample preparation techniques lead to different values of the superconducting transition temperature, illustrating that the higher transition temperatures could be associated with a higher degree of order produced in the system. The role of excess germanium and thermal effects are also discussed for NbGe2. A study of the binary compounds of the hexagonal C-40 system consisting of the members CrSi2, VSi2, NbGe2, NbSi2, TaGe2, TaSi2, and HfSn2 shows that four members exhibit superconductivity. Those that showed superconductivity were NbGe2 (T/sub c/ = 16K), NbSi2 (T/sub c/ = 2.9K), TaSi2 (T/sub c/ = 4.4K), and TaGe2 (T/sub c/ = 2.7K). One of the major conclusions established through this study is that a new crystal structure type, namely the hexagonal C-40 system is superconducting. It also appears that the explanation for the appearance of superconductivity in some of these compounds is related to the degree of order present in the system which is brought about by the fabrication technique