Ising-Glauber spin cluster model for temperature-dependent magnetization noise in SQUIDs.

Area of Science:

• Condensed Matter Physics
• Quantum Magnetism
• Superconducting Devices

Background:

• Superconducting Quantum Interference Devices (SQUIDs) exhibit anomalous low-frequency noise, particularly 1/f(α) magnetization noise.
• The microscopic origins of this noise, especially its temperature dependence, have remained unclear, hindering device performance and understanding.
• Farbe+(F(+)) centers at metal-insulator interfaces are hypothesized to play a role in these phenomena.

Purpose of the Study:

• To develop a theoretical model explaining the origin of 1/f(α) magnetization noise in SQUIDs.
• To elucidate the distinct temperature dependencies of flux noise and inductance noise.
• To investigate the role of magnetic ordering and interactions within two-level systems.

Main Methods:

• Development of a novel method for calculating correlation functions.
• Model calculations based on interacting two-level systems at the metal-insulator interface.
• Analysis using three-point correlation functions to establish magnetic ordering.

Main Results:

• A self-consistent model demonstrating that interacting two-level systems lead to 1/f(α) magnetization noise (α(T)≲1).
• Explanation for why inductance noise is temperature-dependent while flux noise is not, despite shared microscopic origins.
• Observed flux-inductance-noise cross correlations are explained by magnetic ordering within the systems.

Conclusions:

• The study provides a unified theoretical framework for understanding complex noise phenomena in SQUIDs.
• Long-range ferromagnetic interactions result in weaker temperature dependence of flux noise compared to short-range interactions.
• The mechanism mediating surface ferromagnetism is unlikely to break time-reversal symmetry in these systems.