ESA Science & Technology - Modelling the energy gap in transition metal/aluminium bilayers

Publication date: 16 July 2000

Authors: Brammertz, G., et al.

Journal: Proceedings of the 6th Twente Workshop on Superconducting Elements
Year: 2000

We present an application of the generalised proximity effect theory. The theory has been used to determine the energy gap (Delta g) in proximised transition metal - aluminium bilayer structures such as Nb/Al, Ta/Al, V/Al and Mo/Al. These bilayers have different film thicknesses ranging from 5 to 260 nm. For the cases of Nb/Al, Ta/Al and V/Al bilayers, the interface parameters Gamma and Gamma_BN (here we define Gamma as the ratio of the products of normal state resistivity and coherence length in each film of the bilayer while Gamma_BN is the ratio of the boundary resistance between film 1 and 2 to the product of the resistivity and coherence length in the second film), which were used as input parameters to the model, were inferred experimentally from an existing bilayer of each kind and then suitably modified for different film thicknesses. This experimental assessment is therefore based on a comparison of measurements of the critical temperature and the energy gap at 300 mK with the predictions from the model for various values of Gamma, Gamma_BN. The energy gap of the bilayer was experimentally determined by using symmetrical Superconducting Tunnel Junctions (STJs) of the form S-Al-AlOx-Al-S, where each electrode corresponds to a proximised bilayer. However for the case of Mo/Al bilayers the interface parameters were determined theoretically since currently no STJ data for this configuration are available. The results for the Nb/Al, Ta/Al and V/Al bilayers have also then been compared to experimentally determined energy gaps found for a series of STJs with different film thicknesses. The correspondence between experiment and theory is very good.