Low-frequency critical current noise in Josephson junctions induced by temperature fluctuations

Magnetic Flux Noise in dc SQUIDs: Temperature and Geometry Dependence

“…The spectra are 1=f-like, with a slope that flattens at higher frequencies as the white noise from the shunt resistors becomes significant. At low frequencies (f & 10 À1 Hz) and high temperatures (T * 1:2 K), fluctuations in the critical current are significant, thereby increasing the slope of the spectra [26]. Consequently, in our fits to the spectra, we disregard noise from this region.…”

“…For all data represented here, we biased the SQUID where dI=dÈ-determined by measuring the response to a small oscillating flux-was a maximum. The temperature T, measured using calibrated Ge (mixing chamber) and RuOx (sample box) resistance thermometers, was stabilized with feedback from the Ge thermometer to better than 1 part in 10 4 during data acquisition [26]. Nyquist noise from R c yielded a temperature within AE5% of that of the thermometers.…”

The role of measurement time on the universal crossover from $$1/f$$ 1 / f to non- $$1/f$$ 1 / f noise behavior

“…There is not much evidence for the existence of this elusive minimum frequency, perhaps because of the experimental challenge to resolve such low frequencies. Nevertheless, as we have already mentioned, a handful of experiments have reported a change in the 1/ω functional form [11,[26][27][28][29][30][31] that we argue are proof of its existence. The second potential solution put forward the idea of acknowledging the non-stationary character of the process yielding flicker noise [34].…”

“…As frequency is decreased to the new ω obs the power spectral density now exhibits a crossover from 1/f to a different functional form. Indeed, as already mentioned a handful of experiments may have already shown such crossover [11,[26][27][28][29][30][31], which we here claim must be a universal feature of any stochastic process.…”

“…This crossover appears again in cases that could not be farther from each other, e.g., voltage fluctuations in superconducting thin films [26], fluctuations in the membrane potential of nerve cells [27,28], voltage fluctuations due to transport of magnetic flux vortices in type-II superconductors [29,30], critical-current fluctuations in Josephson junctions [31], and fluctuations in the inter-beat interval of the human heart [11]. However, it does not seem to be present in the majority of experiments.…”