Josephson-like effects in coherent motion of vortices in a periodic potential

Abstract

Resonant quasi-Josephson effects induced by coherent vortex motion in artificial reversible periodic potential structures have been investigated. Periodic potential has been imposed by application of a magnetic tape containing a prerecorded harmonic signal to the surface of a high- T c thin film strip. Motion of current driven vortices in the film with applied tape is highly coherent and leads to the appearance of a series of self-resonant DC current steps on the I– V characteristic of the sample. The presented model attributes these steps to locking of the vortex nucleation frequency to the internal self-frequencies of the system; the latter being set by the time of flight of vortex bundles across the sample width and across the spatial period of the applied potential. Voltages of the current steps have been found to scale consistently with the proposed model. The velocity of vortex motion inferred from the voltages of the first quasi-Josephson steps was found to be extremely large. The possible explanation of this fact in terms of the huge size of vortex bundles is discussed and confronted with independent experimental evaluations of the bundle size from the flux-flow noise spectra and vortex induced random telegraph voltages in high- T c superconductors.