Manipulation of supercurrent by the magnetic field

Abstract

Superconductor–semiconductor hybrid devices are interesting not only for their known and potential applications but also for the associated novel physical processes. One such example is the proposal for the realization of Majorana zero-modes, which are robust against noise and have applications in quantum information processing. Although the Josephson effect is known for decades, the recent advances in the experimental technologies made it possible only recently to make highly tunable hybrid devices. In this thesis, we study the superconductor–normal-metal–superconductor Josephson junctions and propose new effects or analyze experimental findings. In a Josephson junction, it is difficult to determine whether the flowof supercurrent is ballistic or diffusive. We propose an hourglass-shaped Josephson junction geometry to probe the nature of transport. In this device, the measurement of a critical current as a function of an external magnetic field produces a clear signature of the ballistic supercurrent. In metal-based Josephson junctions, the supercurrent flows uniformly through the scattering region. In contrast, semiconductor-based Josephson junctions allow tunable supercurrent due to the tunable carrier density of the semiconductors. We model a bilayer graphene Josephson junction with a split-top and back gate in the presence of an applied magnetic field to analyze the experimental measurements. The opening of bandgap in bilayer graphene in the gated area by applying tunable electrostatic potential allows spatial manipulation of supercurrent. The magnetic field is then used to probe the supercurrent flow in the device. In general, an applied magnetic field strongly suppresses supercurrent in Josephson junctions because it randomizes the contribution of the individual states. However, we show that graphene Josephson junctions are special and avoid the suppression of critical current under an applied in-plane magnetic field. The critical current as a function of the Zeeman field has a plateau whose size depends on the junction detail. Finally, we study a Josephson junction coupled with a microwave transmission line resonator in collaboration with an experimental group. We model this system to analyze and explain an unexpected experimental result of the system. We show that the unexpected outcome of the experiment is due to the coupling of the higher modes of the transmission line resonator.