Threshold vortex formation and trapping fields in small high-Tc superconducting particles

The Gibbs thermodynamic potential is used to investigate theoretically the possibility of vortex formation in superconducting particles which have different shapes and whose transverse size is smaller than the penetration depth. Expressions are derived for the threshold fields for vortex formation and trapping during cooling with and without an external field. They show that the threshold fields depend on the radius of the particles. This agrees with an experiment performed on powdered high-Tc superconductors with different particle-size distributions. 14 refs., 3 figs