We report spin-polarized tunneling density of states measurements of the proximity modulated superconductor-insulator transition in ultrathin Be-Al bilayers. The bilayer samples consisted of a Be film of varying thickness, $d_{\mathrm{Be}}=0.8–4.5\mathrm{nm}$, on which a 1 nm thick capping layer of Al was deposited. Detailed measurements of the Zeeman splitting of the BCS coherence peaks in samples with sheet resistances $R\sim h/4e^2$ revealed a superlinear Zeeman shift near the critical field. Our data suggests that critically disordered samples have a broad distribution of gap energies and that only the higher portion of the distribution survives as the Zeeman critical field is approached. This produces a counterintuitive field dependence in which the gap apparently increases with increasing parallel field.

• Received 7 October 2020
• Revised 4 March 2021
• Accepted 12 May 2021

DOI:https://doi.org/10.1103/PhysRevResearch.3.023141

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Published by the American Physical Society