Spin-polarized tunneling in critically disordered Be-Al bilayers

Abstract

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_{Be}=0.8–4.5nm, 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∼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.