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Temperature dependence of critical current. (a) Normalized measured critical current vs. temperature (dots). Solid line is Ambegaokar-Baratoff prediction fitted to the measured . (b) Normalized measured critical current sensitivity (dots). Solid line obtained by differentiating solid line in (a).
Temperature and critical current noise for three settings of temperature controller. Typical 20 min time traces for the measured (a) temperature and (b) critical current variations at 0.4 K.
Cross-correlation functions of the three temperature and critical current time series shown in Fig. 2.
Cross-spectra and spectral densities. (a) Normalized cross-spectral density (coherence) vs. frequency for data runs A, B, and C. Green line is a single-pole filter with time constant of 3.5 s and asymptotic value of 0.95 at low frequencies. (b) Run C: spectral density of critical current variations and of temperature variations scaled to equivalent critical current variations.
Critical current noise caused by charge-trapping compared to that due to temperature variations. For , we use values from Eqs. (1) and (2), whereas is calculated for Al and Nb assuming (a), (b), and (c). Here, and .
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