High T c,structural instabilities and electronic properties of Nb3Ge

Abstract

A number of Nb₃Ge samples with rather high T _c,on (T _c,on ? 22.5 K) were prepared by CVD and characterized by the electrical resistance, X-ray diffraction and scanning electron microscope patterns. It was found that the crystal growth patterns are roughly divided into two categories, (i) a particles-pattern in a nearly single phase of Nb₃Ge and (ii) a network pattern with a fairly large amount of the Nb₅Ge₃ phase but with rather a large resistance ratio and a small resistivity at T ? T _c,on. The latter type of samples shows generally higher T _c,on. It is deduced that the origin of high T _c,on comes mainly from a deformation of the atomic potential at the Nb-site into an anharmonic and rather unstable shape due to the coexistence of the Nb₅Ge₃ phase. The mechanism of the formation of different patterns is discussed on the basis of the theory of non-equilibrium thermodynamics of Prigogine.