1/f flux noise in low- Tc SQUIDs due to superparamagnetic phase transitions in defect clusters

Abstract

It is shown here that 1/f α flux noise in conventional low-Tc 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 randomtelegraphic macro-spin when transitioning to the superparamagnetic phase. This results in 1/f α noise when ensemble averaged over a random distribution of clusters. This model is self-consistent and explains all related experimental results which includes α ∼ 0.8 independent of system-size. The experimental flux-inductancenoise 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. − + + − − − − + + J a insulator metal surface TLS-clusters Figure 1. 1/f α noise model consisting of interacting and fluctuating TLSs in a cluster. The clusters are assumed to form at the metalinsulator interface or on the surface. They are sufficiently far apart so that only spins within a single cluster interact. Number of TLSs within a cluster and the lattice constant a vary.

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