Axial-Parallel Circuit Model for Macroscopic Josephson Junctions in Slug Devices

Droplet-type Superconducting Quantum Interference Devices (SLUGs) mayserve as a promising non-lithographic alternative to conventional Superconducting Quantum Interference Devices (SQUIDs), featuring a simpler fabrication process and superior noise performance at low temperatures. Todeepen the theoretical understanding of SLUG devices, this work validatesand extends the research on SLUGs conducted by Professor Clark, proposing an axis-parallel equivalent circuit model that represents the Josephsonjunction structure of SLUGs as an axially aligned coherent array. Capturingthe interference behavior through distributed phase superposition, the modelachieves a good agreement with the experimental I-Φ data at 4.2K, with anerror margin of less than 3%. Scanning Electron Microscopy (SEM) imagingconfirms the uniform distribution of weak links, verifying the assumptionsof the proposed model. The analysis results demonstrate that SLUGs exhibit excellent geometric and thermal stability, with a noise level as low as0.15µA and a responsivity as high as 1810 µA/Φ0 at 2 K. These findings provide a compact and scalable framework for the optimization of SLUG-basedquantum sensing devices. PACS numbers: 74.50.+r, 85.25.Dq, 07.55.Ge, 84.30.-r