Collective-Flux-Pinning Phenomena in Amorphous Superconductors

The pinning force $F_p$ in amorphous ${\mathrm{Nb}}_3$Ge films with $T_c\sim 4$ K has been measured as a function of perpendicular field, temperature, and thickness of the samples. As a result of the large $\kappa$ and low $J_c$ characteristic of amorphous superconductors the pinning force for the first time is found to be in good agreement with the theory of Larkin and Ovchinnikov for collective pinning extended to the two-dimensional case. Pinning-mediated renormalization of the shear modulus leads to a sharp peak in $F_p$ near the upper critical field.