Vortex dynamics in thin superconducting strips observed by scanning SQUID microscopy

We have studied the flux dynamics in superconducting strips patterned from both Nb and weak-pinning amorphous MoGe films using a Scanning SQUID Microscope (SSM). The unparalleled flux sensitivity of the SSM allows us to image the vortices in the strip under a variety of field and cooling conditions with single vortex resolution for low flux density. We are able to apply transport currents while imaging the strip and observe the shift of the vortex distributions due to the Lorentz force. Surface steps etched into the strips significantly alter the flux patterns and introduce asymmetry in the vortex motion under applied transport currents. Both the change in vortex line energy across the step and the screening currents which flow along the step influence the vortex distributions and flux dynamics. We are studying the relationship between the vortex distributions from the SSM images and the transport characteristics of the strips.