Vortex noise in superconductors is due to the motion of quantized flux (vortices) across the surface of the superconductor. This changing magnetic flux gives rise to a voltage which we detect as a noise signal. Previous measurements show that the vortex noise is related to the interaction of the vortices with intrinsic pinning sites which limit vortex motion. These studies show that the noise signal is composed of voltage bursts related to the vortices moving in fits and starts due to the random location and strength of the intrinsic vortex pinning sites. Our samples are composed of a periodic triangular array of holes that serve as vortex pinning sites. These samples show periodic minima in resistance when the vortex/pinning site densities are commensurate. We find a vortex noise signal that is field, current and temperature dependent, arising from the vortices being depinned. We measure the vortex noise to determine if there is a change from uncorrelated vortex depinning below the first matching field where unoccupied pinning sites exist, to highly correlated vortex depinning at the first matching field when the vortexes are commensurate with the pinning lattice. Our measurements show that there is no field dependence in the vortex noise signal, indicating that the vortex depinning is correlated as the lattice is driven by a DC current. We explain this result in terms of the long range order of our pinning lattice and the strength of the vortex-vortex interaction in our 100 nm pinning lattice. Our findings suggest that the vortex noise is dominated by the vortex-vortex not vortex-pinning site interactions.
University of Minnesota Ph.D. dissertation. September 2012. Major: Physics. Advisor: E. Dan Dahlberg. 1 computer file (PDF); vii, 74 pages, appendices A-B.
Schulz, Tanner Franz.
Superconducting vortex noise measurements on niobium films with periodic pinning sites.
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