Limitations on critical currents in high temperature superconductors

Abstract

The critical current denisty, Jc, of high temperature superconducting oxides such as YBa2Cu3O7−δ can vary by more than four orders of magnitude, depending on whether the sample is in the form of an epitaxial film, a single crystal or a polycrystal. As a function of increasing reduced magnetic field, Jc in high temperature superconductors generally decreases more readily than that of conventional superconductors such as Nb3Sn. These experimental observations are considered in terms of various critical current limiting factors which include: 1) depairing, 2) flux creep, 3) weak link effects. In the limit of depairing, the zero field Jc, is expected to be in the range of 108–9 A/cm2 at 4.2 K and in the range of 107–8 A/cm2 at 77 K. The highest experimental value for Jc, which was observed in [001] single crystal epitaxial films is at least an order of magnitude lower than the depairing limit. In these films, Jc is found to be limited by flux creep. With the aid of the techniques of epitaxial film growth and ...