Dimensional crossover in collective flux pinning

Critical-current experiments on thick (d=5-18 {μ}m) films of amorphous Nb₃Ge or Mo₃Si reveal a dramatic increase of the pinning force Fₚ at fields around 0.8Bc2. 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 Lc is approximately d/2. The large pinning force and the unusual nonlinear V-I characteristics show that in the three-dimensional regime Lc is determined by defects of the flux-line lattice which are extremely unstable.