Vortex dynamics of Nb variable-thickness bridges measured by direct flux detection method

Abstract

In order to study the dynamical behavior of vortices moving through superconducting thin film devices, we constructed a probe having a concentric thin-film planar gradiometer coupled to a direct current SQUID. The probe was designed to directly detect the flux generated by a single vortex, based on the monopole approximation. Measurements of static current–voltage (I–V) and flux noise characteristics were made for variable-thickness bridges (VTB) made on epitaxial Nb films. The critical current was unambiguously determined from the onset of vortex motion. Multiple step noise of the flux with a minimum step height of 0.28Φ0 was observed above the critical current in the time traces. This step height is in agreement with the numerical calculation for single vortex motion, confirming the validity of the monopole approximation. It is suggested that the multiple step noise is generated by the successive entry of single vortices into the bridge. VTBs made on polycrystalline Nb films were also measured. The I–V characteristic showed asymmetric voltage increases and a multiple peak structure of the flux noise was observed above the critical current, suggesting the successive depinning of trapped vortices as flux bundles from widely distributed pinning potentials.