Flux noise from superconducting YBa2Cu3O7−x flux transformers

Using a dc superconducting quantum interference device (SQUID) at 4.2 K, we have measured the magnitude and temperature dependence of the flux noise generated by superconducting flux transformers made from YBa2Cu3O7−x. If the transformer loop is opened, the observed noise decreases by a factor of 10, demonstrating that moving vortices generate noise primarily by altering the screening current, rather than by coupling their magnetic flux directly into the SQUID. Model calculations are given for the various noise contributions, and shown to be reasonable predictions of the observed effects.