Superconducting properties of ultrapure niobium

Abstract: The results of determination of critical magnetic fields, magnetization curves and critical currents of niobium samples of different purity are presented. It is shown that ultrapure niobium near Tc is a superconductor of the first type and becomes a superconductor of the second type with decreasing temperature due to the temperature dependence of the Ginzburg-Landau parameters. The dependence of the hysteresis of the magnetization curves of massive samples on the surface state is investigated. A comparison of the superconducting parameters with the parameters of the electron spectrum of niobium is carried out. The dependence of the critical current of niobium wires on the longitudinal magnetic field agrees with the assumption of a force-free current distribution.