Ultimate flux-pinning force in high-field superconductors

Abstract

Conservative forces cannot produce the highest possible pinning because, once flux is moving, they average to zero. It is necessary to look at dissipative mechanisms. A simple example is a superconducting ring moving through a magnetic field inhomogeneity. As the pinning limit we postulate the loss of the free-energy difference between the flux line structure and the normal state, for a flux motion by the distance of the average fluxoid spacing. A pinning structure approaching a three-dimensional chequer-board pattern can ultimately reach perhaps a third of the pinning limit at the field B _opt, where the fluxoid spacing agrees with the pinning structure spacing. In reviewing the highest reported critical current measurements it is found that rapidly quenched and judiciously recrystallized A15 samples come close to ultimate pinning, as do some NbN films. According to this model NbTi can, in principle, still be improved by a factor three to five. For Y-Ba-Cu-O the situation is unclear because of the anisotropy, but the highest intragrain currents indicate perhaps 10% of the ultimate.