Current-driven vortex dynamics in a periodic potential

Quasi-Josephson effects due to coherent vortex motion in artificial reversible periodic potential structures in high-T_csuperconducting thin films have been investigated. Periodic pinning conditions have been created by applying a magnetic tape containing a prerecorded harmonic signal to the surface of high quality Y₁Ba₂Cu₃O_7-δthin films. Application of the periodic pinning enforces coherence in current-driven motion of Abrikosov vortices in wide and short macrobridges and leads to the appearance of Josephson-like effects manifesting themselves in series of self-induced current steps on the current-voltage characteristics. The equation of motion for vortices flowing across periodic potential structures is analogous to the phase equation for low-capacitance classical Josephson junctions. The perturbation solutions of this equation contain resonant Shapiro-like self-steps resulting from the locking of the frequency at which vortices are created at the sample borders to the resonant frequencies of the vortex system. Self-resonant frequencies are set by the characteristic time of flight across the sample width and across the period of the applied potential. Voltages of the self-induced current steps have been found to scale with inverse of the characteristic length corresponding to the magnetic period and/or to the sample half-width, consistently with the theoretically derived relations. Experimental data indicate that vortices move in large bundles containing several thousands of flux quanta. The temperature dependence of the step voltages can be ascribed to changes in vortex velocity due to the temperature-dependent viscosity factor.