Critical current density in superconducting niobium films

We have measured the critical current at 4.2 K in Nb-strips of 1 μm thickness and different width, w, prepared by electron-beam vacuum-deposition. The substrate temperature during the deposition was varied between room temperature and 800°C. The width of the Nb strips ranged between 20 and 300 μm. Whereas up to 600°C the different substrate temperatures yielded about the same average critical current density, Jc, (critical current divided by sample cross section) the specimens prepared with 800°C substrate temperature showed a reduction of Jcby a factor of 4-6. In zero applied magnetic field the samples prepared with 400°C substrate temperature or lower showed a decrease of Jcroughly proportional to w^-1/2. Our critical current data obtained in an applied perpendicular field, for the high-field regime, were compared with Kramer´s theory of flux pinning which assumes plastic shearing of the flux-line lattice around individual pinning sites during flux flow.