On the applicability ranges of T_c formulas for proximity-coupled thin SN and SS bilayers

Abstract This Brief Note revisits the well-established T c formulas for proximity-coupled thin superconductor-normal conductor (SN) and superconductor-superconductor (SS) bilayers. While several formulas exist, their interrelations and ranges of validity are not well documented, which can often lead to misapplication and potential misinterpretation of experimental data. To address this issue, the note clarifies these relationships and applicability ranges, offering guidance for their proper use in experimental and applied settings. McMillan's formula is recommended for SN bilayers due to its broad applicability, with its SS counterpart providing similar reliability.