Shearing transition in a superconducting vortex lattice subject to periodic pinning

We have studied the shearing forces in a superconducting vortex system with artificial pinning sites using the Corbino geometry. Current was injected into the center of a Nb disc and propagated radially outward to produce a force in the azimuthal direction on the vortices, with strength proportional to 1/r. We investigated the magnetoresistance properties of the vortex lattice as a function of temperature, current, and pinning lattice configuration. The measurements show steps instead of minima at the matching field positions, indicating an unexpected influence of the pinning array on the motion of the vortex lattice. The results imply that these steps are due to a shearing transition of the vortices.