Suppression of magnetic-flux noise inYBa2Cu3O7−xby a supercurrent

The magnetic-flux noise generated by a patterned film of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ is reversibly suppressed by the application of a supercurrent. The total noise power decreases by an order of magnitude, in quantitative agreement with a model involving thermally activated vortex hopping between pairs of pinning sites of nearly equal pinning energy. The model yields a force constant of 6\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ N/m for a vortex at a typical pinning site, and a hopping distance which increases with temperature from 13 nm at 4.2 K to 64 nm at 60 K. The number density of vortices is at least 0.02 \ensuremath{\mu}${\mathrm{m}}^{\mathrm{\ensuremath{-}}2}$, even in a magnetic field of less than 1 \ensuremath{\mu}T.