Pinning potential, its relativity and dependence on temperature and magnetic field studied on the basis of the I-V characteristics of multifilamentary Nb3Sn superconductors

A series of I-V characteristics of multifilamentary Nb3Sn superconductors measured at various temperatures (2.4-4.2 K) and magnetic fields (6-12 T) was analysed with the help of creep process theory. It was shown that the current-dependent effective pinning potential Uef (I) as well as the temperature- and magnetic field-dependent pinning potential U0(T, B) depend on the choice of the reference current - voltage point in some of the currently used models. Due to this fact, the values of U0(T, B) as well as Uef(I) derived from I-V data have a meaning related to a reference current - voltage point chosen usually in the practically accessible measurement range. In this paper, we used 1 μV cm-1 as the reference voltage. Generalizing some features of creep models we found the way to derive U0(T, B) from experimental data without knowing the precise form of the Uef(I) dependence. The influence of technological parameters (filament composition and heat treatment conditions) on this relative pinning potential U0r(T, B) are discussed. A simple scaling of U0r(T, B) was found for all the conductors under study. Its form is U0r(T, B)=U*(B)f(T/T*), where the values of scaling temperatures T* are very close to the critical temperatures and U*(B) seems to be a maximum value of the relative pinning potential at a given magnetic field in the whole temperature range. (author)