Ballistic Josephson junctions and vertical tunnelling transistors based on graphene heterostructures

• Main Author: Zhu, Mengjian
• Published: University of Manchester 2017
• Subjects: 500
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.728168

In this thesis, we study the transport properties of two different types of graphene devices. The first one is ballistic graphene Josephson junction and the second one is graphene-hexagonal boron nitride-graphene vertical tunnelling transistor. We report on ballistic graphene Josephson junctions with contacts made from type II superconductor, niobium. We observe pronounced Fabry-Pérot oscillations not only in the normal-state resistance but also in the critical current. The proximity effect is mostly suppressed in magnetic fields of B < 10 mT, showing a conventional Fraunhofer pattern. However, some proximity superconductivity survives in fields higher than 1 T which corresponds to more than 1000 flux quanta threading into the junction. We attribute such high-field Josephson effect to individual Andreev bound states that persist near the graphene edges. By studying the Fraunhofer pattern of graphene Josephson junctions, we reconstruct the spatial supercurrent distribution in graphene. The edge-dominated transport is observed only in the case of an energy gap opening in bilayer graphene and graphene/hexagonal boron nitride superlattices, which points to its non-trivial topological origin. Owing to the band structure topology of gapped graphene, the valley-polarized edge modes can extend above the disorders and propagate efficiently for micrometres. By probing the density of states of graphene using graphene tunnelling transistors, we demonstrate a stacking transition in bilayer graphene from incommensurate twisted stacking state to a commensurate AB stacking state by a macroscopic graphene self-rotation. This structural transition is driven by van der Waals energy of two graphene layers and is thermally activated by unpinning the microscopic chemical adsorbents which are then removed by the self-cleaning of graphene. We observe a series of sharp resonant features in the differential conductance of graphene-hexagonal boron nitride tunnelling transistors over a wide range of bias voltages. We attribute them to electron tunnelling assisted by the emission of phonons. The phonon energies corresponding to the resonances are compared with the lattice dispersion curves of graphene-hexagonal boron nitride heterostructure and are close to peaks in the single phonon density of states.