Spin-Polarized Tunneling in Critically Disordered Be-Al Bilayers

TL;DR

This study investigates spin-polarized tunneling in ultra-thin Be-Al bilayers near the superconductor-insulator transition, revealing a broad distribution of gap energies and an unusual increase in the superconducting gap with magnetic field.

Contribution

It provides new insights into the Zeeman splitting and gap distribution in critically disordered superconductor-insulator bilayers, a previously unexplored regime.

Findings

Super-linear Zeeman shift observed near critical field

Broad distribution of gap energies in disordered samples

Gap appears to increase with increasing magnetic field

Abstract

We report spin-polarized tunneling density of states measurements of the proximity modulated superconductor-insulator transition in ultra thin Be-Al bilayers. The bilayer samples consisted of a Be film of varying thickness, $d_\mathrm{Be}=\,$0.8-4.5 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 super-linear 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 counter-intuitive field dependence in which the gap apparently increases with increasing parallel field.