Enhancement of the Josephson Current in a Quantum Dot Connected to Majorana Nanowires

We investigate the behavior of the Josephson current in a system composed of a quantum dot (QD) sandwiched between two nanowires by using the nonequilibrium Green's function technique. We consider that the nanowires are in proximity to s-wave superconducror substrates, and Majorana bound states (MBSs) are induced at their ends. It is also assumed that the two nanowires are not aligned in the same orientation, but form a bent angle with respect to each other. It is found that when only one spin state on the QD is coupled to the left nanowire, the Josephson current is the typical sinusoidal function of the phase difference between the two nanowires. If both spin states hybridize to the MBSs with equal coupling strengths, the Josephson current then is not a sinusoidal function of the phase difference. In particular, when the bent angle between the two nanowires is p/2 and the two modes of the MBSs in each nanowire are decoupled from each other, the Josephson current is enhanced by about twenty times in magnitude as compared to the former case. Moreover, the simultaneously enhanced currents of the two spin directions are of the same magnitude but flow in opposite directions and they induce a large pure spin current. Our results also show that this abnormally enhanced Josephson current will be suppressed by a vertical magnetic field applied to the QD.

Automated summary

We explore the behavior of Josephson current in a system consisting of a quantum dot (QD) sandwiched between two nanowires using nonequilibrium Green's function technique. It is observed that the Josephson current is a sinusoidal function when only one spin state on the QD is coupled to the left nanowire. However, if both spin states hybridize equally to the Majorana bound states (MBSs), the Josephson current is no longer a sinusoidal function. Moreover, a bent angle between the nanowires enhances the Josephson current significantly, and a vertical magnetic field applied to the QD can suppress this enhancement.