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

Direct current superconducting quantum interference devices (SQUIDs) made from high transition temperature superconductors typically exhibit a still unexplained higher white noise and smaller transfer function than numerical simulations based on the coupled Langevin equations predict. We present a systematic study that compares experimental results with both numerical simulations and a recently developed analytical approach based on a Fokker-Planck equation. To cover a wide range of SQUID parameters, two samples of different inductances were measured at variable temperature. While several predictions of the analytical theory could be verified and a good qualitative agreement with both theories is observed, only one sample showed good quantitative agreement. Furthermore, our simulations performed in the limit of large thermal fluctuations show excellent agreement with the analytic theory. (C) 1999 American Institute of Physics. [S0003-6951(99)04515-5].