Transfer function and thermal noise of YBa2Cu3O7-delta direct current superconducting quantum interference devices operated under large thermal fluctuations

Direct current superconducting quantum interference devices (SQUIDs) made f rom high transition temperature superconductors typically exhibit a still u nexplained higher white noise and smaller transfer function than numerical simulations based on the coupled Langevin equations predict. We present a s ystematic study that compares experimental results with both numerical simu lations and a recently developed analytical approach based on a Fokker-Plan ck equation. To cover a wide range of SQUID parameters, two samples of diff erent inductances were measured at variable temperature. While several pred ictions of the analytical theory could be verified and a good qualitative a greement with both theories is observed, only one sample showed good quanti tative agreement. Furthermore, our simulations performed in the limit of la rge thermal fluctuations show excellent agreement with the analytic theory. (C) 1999 American Institute of Physics. [S0003-6951(99)04515-5].