Dimensional crossover in collective flux pinning

Critical-current experiments on thick ($d=5\mathrm{}-18$ μm) films of amorphous ${\mathrm{Nb}}_3$Ge or ${\mathrm{Mo}}_3$Si reveal a dramatic increase of the pinning force $F_p$ at fields around $0.8B_{c2\mathrm{}}$. The onset of this peak is triggered by a crossover from two- to three-dimensional disorder of the flux-line lattice, which occurs when the longitudinal correlation length $L_c$ is approximately $\frac{d}{2}$. The large pinning force and the unusual nonlinear $V-I$ characteristics show that in the three-dimensional regime $L_c$ is determined by defects of the flux-line lattice which are extremely unstable.