Study of subharmonic gap structures in both symmetric and asymmetric superconducting tunnel junctions based on oxidized PbIn alloy electrodes

Abstract

Tunnelling in symmetric superconducting tunnel junctions made of two superconductors with the same energy gaps ( Delta ₁= Delta ₂), as well as in asymmetric junctions with different energy gaps ( Delta ₁ not= Delta ₂), having PbIn alloy as the base electrode in both cases show sharp current steps, the so-called subharmonic gap structures (SGS), in their I-V characteristics. The voltage locations, the changes due to applied microwaves, the RF power dependence, the temperature and magnetic field dependence, the lineshapes, the matrix element dependence, and the ratios of the relative amplitudes of the SGS have been studied to distinguish between two given explanations, namely the self-coupling and the multiparticle tunnelling mechanisms. The author's results on shorted junctions, like many other previous works, support the self-coupling mechanism as a possible explanation for the SGS. On the other hand the results on PbIn alloy base junctions free of any metallic shorts show that, unlike as often stated, the multiparticle tunnelling is a plausible explanation for the SGS in such junctions.