Revisiting Andreev processes in superconductor-graphene-superconductor (SGS) Josephson junctions: comparison with experimental results

Abstract

In view of the recent progress in experiments on charge transport through various Josephson junctions made out of graphene, we have made a careful comparison between the theory and some of the available experimental results. Within the framework of a transfer matrix approach, we have first analytically derived the spectrum of Andreev bound states (ABS) in a superconductor -graphene-superconductor (SGS) junction for a wide range of experimentally relevant parameters. We have particularly considered the case of monolayer graphene (MLG). The theoretical results can account for both the retro Andreev reflection (RAR) and the specular Andreev reflection (SAR) in the relevant parameter range. Using the ABS spectrum we have evaluated the current through such junctions and the junction conductance from the analytically derived expressions at different bias voltages for a range of other system parameters directly taken from the experimental works. These theoretical results have then been compared with experimental results. Evaluated current and the conductance show scaling behaviour with change in the junction length and agree well with the experimental results. In the relevant parameter regime where the SAR process is dominant, the calculated values of the current and the conductivity have been found much lower than the corresponding values observed when the RAR process is dominant.