Boundary-Induced Systematic Bias in Josephson Junction Critical Current — A Phase-Preserving Correction Beyond the Ambegaokar–Baratoff Theory

This work reports a systematic and reproducible bias in the prediction of the Josephson junction critical current using the standard Ambegaokar–Baratoff (AB) relation. Across multiple materials and operating conditions, the AB formula consistently overestimates the critical current by approximately 8–10%, a discrepancy commonly attributed to experimental noise or environmental effects. In this study, we demonstrate that this deviation is neither random nor sample-specific, but instead represents a structural bias inherent in the idealized assumptions of the AB theory. We introduce a compact correction framework based on boundary-induced suppression, in which the AB prediction is modified by a fixed multiplicative factor. Crucially, this correction does not rely on device-specific fitting parameters and remains valid across different superconducting materials, including aluminum and niobium junctions. Using identical experimental inputs, the proposed model achieves approximately 99.4% agreement with measured critical currents, compared to 88–90% accuracy for the uncorrected AB relation. The results indicate that the commonly observed discrepancy should be interpreted as a design-level theoretical bias rather than experimental noise. From a practical perspective, this correction directly reduces design margins and iteration costs in superconducting and quantum circuit development. This work does not propose new superconducting physics, but instead provides a boundary-aware reinterpretation of existing theory. A browser-based supplementary tool is provided separately to enable independent verification and reproducibility of the proposed correction. This work is released under the Creative Commons Attribution–NonCommercial 4.0 International License (CC BY-NC 4.0). Commercial use requires explicit permission from the author.