The Instability of High Tc States in Nb3Ge

Most of high Tc A–15 type Nb3Ge samples (Tc=23~24 K) synthesized by us using CVD method contained a large quantity of the tetragonal Nb5Ge3 phase. These high Tc samples were unstable against thermal cycles and Tc's decreased down to ~20 K. The coexisting Nb5Ge3 phase must play important roles to keep the high Tc state of Nb3Ge phase by stabilizing the A–15 type crystal structure and also to raise the magnetic flux pinning force density as pinning centers. It is suggested that the appearance of an anharmonic type potential acting on Nb atoms is closely related with high Tc of Nb3Ge phase.