Estimation of the noise temperature of Y-Ba-Cu-O grain boundary Josephson junctions

We examine the extent to which low-frequency critical current fluctuations contribute to increased noise temperatures in HTS grain boundary junctions. The effective Josephson linewidth is first estimated (a) by fitting of the noise-rounded DC I(V) characteristic, and (b) by fitting of the noise-rounded Shapiro steps. The expected Linewidth due to the contribution of the low-frequency voltage noise spectral density is then calculated. Values of noise temperature T-n similar to 170 +/- 10 K as found by the three methods are mutually consistent, indicating that the low-frequency noise is indeed responsible for the observed noise temperatures. This result is not inconsistent with previous measurements of the linewidth of the Josephson radiation, when the bandwidth of the measurement system is taken into account. Implications of these results for the reliability of HTS grain-boundary junctions in metrology applications such as Josephson noise thermometry are discussed. (C) 1997 Elsevier Science B.V.