Effect of Current on Collective Motion of Flux Lines in a Bulk Y1Ba2Cu3O7-δ Superconductor

When a permanent magnet, placed above a high temperature superconductor (HTSC), is moved by an external force, the flux lines in the HTSC will follow the magnet and perform a collective motion due to the interaction between the flux lines and the magnet. Here, the collective motion of flux lines in an Y1Ba2Cu3O7-δ (Y123) HTSC is studied by a mechanical pendulum method for various transport currents I through the sample. The magnet is set in low frequency (typically 1 Hz) oscillations with a mechanical pendulum, and the current I is flowing perpendicular to the direction of the magnet motion. The experiments show that the shift of the stiffness (corresponding the shift of the frequency squared) exhibits a cross-over phenomenon: from independence to power law-like dependence for increasing currents. The results are compared with the theories of collective flux creep in HTSCs.