Anisotropic Pinning and Guided Motion of Vortices in Type-II Superconductors

Guided motion of vortices in type-II superconductors is studied by measuring transverse voltages. The experiments were performed mainly on a Nb–Ta alloy for various magnetic fields, transport currents, and temperatures. The experiments show that in general, the onset of apparent resistance in the mixed state is coupled with fully guided motion of the vortices. With increasing magnetic field, the ratio of transverse and longitudinal voltages decreases. However, this ratio goes through a maximum in materials where the apparent resistivity goes through a minimum. A model is discussed with a guiding force becoming independent of current at higher current densities. The voltages calculated with this model are in agreement with those measured on a Nb–Ta alloy and on a Pb–In alloy. The model is also applied to the situation where a true Hall effect is measured. The conclusion is that pinning and guiding can reduce the Hall angle when the current density is small. The large Hall angles measured in the mixed state cannot be related to guided motion with the existing models of flux flow.