Abstract
It is shown here that flux noise in conventional low- SQUIDs is a result of low temperature superparamagnetic phase transitions in small clusters of strongly correlated color center defects. The spins in each cluster interact via long-range ferromagnetic interactions. Due to its small size, the cluster behaves like a random-telegraphic macrospin when transitioning to the superparamagnetic phase. This results in noise when ensemble averaged over a random distribution of clusters. This model is self-consistent and explains all related experimental results which includes independent of system size. The experimental flux-inductance-noise spectrum is explained through three-point correlation calculations and time-reversal symmetry-breaking arguments. Also, unlike the flux noise, it is shown why the second-spectrum inductance noise is inherently temperature dependent due to the fluctuation-dissipation theorem. A correlation-function calculation methodology using Ising-Glauber dynamics was key for obtaining these results.
7 More- Received 27 November 2017
- Revised 18 November 2018
DOI:https://doi.org/10.1103/PhysRevB.99.024305
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