Saturation to nonsaturation transition of macroscopic pinning force in superconductors

A new flux-pinning mechanism is proposed for moderately strong pins showing a saturation of macroscopic pinning force density F_p at high fields. For moderately strong pins, a shear correlation length in a pinned fluxoid lattice decreases to the order of lattice spacing at high fields. It is shown that a phase separation between an amorphous-like, dense-dislocation phase and a crystal-like, dilute-dislocation phase occurs to minimize the free energy of the pinned fluxoid lattice. In this case, the derived expression for F_p at high fields shows a saturation tendency which is almost insensitive to an increase of the elementary pinning force, f_p. As f_p becomes stronger, the volume fraction of the dense-dislocation phase increases and, finally, the whole fluxoid lattice is occupied only by the amorphous-like, dense-dislocation phase. A nonsaturation of F_p then appears even at high fields.