Nonlinear flux-line dynamics in vanadium films with square lattices of submicron holes

Commensurability effects between the superconducting flux-line lattice and a square lattice (period $d=1\mathrm{}\mu \mathrm{m}$) of submicron holes (diameter $D=0.4\mathrm{}\mu \mathrm{m}$) in 1500 Å vanadium films were studied by atomic-force microscopy, dc magnetization, ac susceptibility, magnetoresistivity, and $I-V$ measurements. Peaks in the susceptibility and critical current at matching fields are found to depend nonlinearly upon the value of external ac field or current, as well as the inferred symmetry of the flux-line lattice.