Supergap and subgap enhanced currents in asymmetric S1/F/S2 Josephson junctions
Mohammad Alidoust and Klaus Halterman
Phys. Rev. B 102, 224504 (2020). [PDF]
We have theoretically studied the supercurrent profiles in three-dimensional normal metal and ferromagnetic Josephson configurations, where the magnitude of the superconducting gaps in the superconducting leads are unequal, i.e., Δ1 ≠ Δ2 , creating asymmetric S1/N/S2 and S1/F/S2 systems. Our results reveal that by increasing the ratio of the superconducting gaps Δ2/Δ1, the critical supercurrent in a ballistic S1/N/S2 system can be enhanced by more than 100 % and reaches a saturation point, or decays away, depending on the junction thickness, magnetization strength, and chemical potential. The total critical current in a diffusive S1/N/S/2 system was found to be enhanced by more than 50 % parabolically and reaches saturation by increasing one of the superconducting gaps. In a uniform ferromagnetic junction, the supercurrent undergoes reversal by increasing Δ2/Δ1>1. Through decomposing the total supercurrent into its supergap and subgap components, our results illustrate their crucial relative contributions to the Josephson current flow. It was found that the competition of subgap and supergap currents in a S1/F/S2 junction results in the emergence of second harmonics in the current-phase relation. In contrast to a diffusive asymmetric Josephson configuration, the behavior of the supercurrent in a ballistic system with Δ2/Δ1=1 can be properly described by the subgap current component only, in a wide range of parameter sets, including Fermi level mismatch, magnetization strength, and junction thickness. Interestingly, when Δ2/Δ1>1, our results have found multiple parameter sets where the total supercurrent is driven by the supergap component. Therefore, our comprehensive study highlights the importance of subgap and supergap supercurrent components in both the ballistic and diffusive regimes. We focus on experimentally accessible material and geometric parameters that can lead to advancements in cryogenic devices based on Josephson junction architectures that utilize supergap currents, which are less sensitive to temperature compared to the subgap current.
