Two-dimensional collective flux pinning, defects, and structural relaxation in amorphous superconducting films

Abstract

The existence of two-dimensional collective pinning has been experimentally confirmed by measuring the pinning force ${F}_{p}$ in superconducting amorphous films of transition-metal---metalloid alloys (${\mathrm{Nb}}_{3}$Ge, ${\mathrm{Nb}}_{3}$Si, ${\mathrm{Mo}}_{3}$Si) as a function of perpendicular field, temperature, and thickness of the specimens. The field and temperature dependence of ${F}_{p}$ can be well explained if it is assumed that the pinning defects in these amorphous superconductors are quasidislocation loops of sizes comparable to the superconducting coherence length. Structural relaxation studies in ${\mathrm{Nb}}_{3}$Ge show that these defects are stable against annealing at temperatures up to at least 0.8 of the recrystallization temperature. The possible effect of randomly distributed pinning centers (random field) on the phase transitions of a two-dimensional flux-line lattice is briefly discussed.