Vortex Dynamics in Superconducting Ratchet in Niobium Thin Film Observed by Lorentz Microscopy

Vortex dynamics in the superconducting ratchet was investigated in a Niobium thin film by direct imaging of Lorentz microscopy. Vortices were field-gradient driven by an ac magnetic field. Microscopic channels for vortex motion with arrow-shaped cages were created by Focused Ion Beam irradiation, where the potential energy distributed in a spatially asymmetric way. With increasing magnetic field, vortices penetrated inside the channel directed toward the thicker part of the sample, while vortices were not willing to intrude the oppositely-oriented channel. On the other hand, with decreasing field, in both channels, large portion of vortices were expelled through the bottle-neck of front cage of the channel toward the outside region. This observation indicated that field-gradient driven vortices were rectified in the asymmetric potential. For further demonstration of vortex motion control, a closed loop of the channel was fabricated and vortex motion was investigated in a two-dimensional circuit.