Gate-defined superconducting nanostructures in bilayer graphene weak links

Abstract

State-of-the-art edge-connected graphene/hexagonal boron nitride van der Waals heterostructures provide low contact resistivity, high charge carrier mobilities as well as a large mean free path. In combination with their high device geometry flexibility they appear thus to be predestined for realizing high-quality tunable weak links in Josephson junctions, which can be readily implemented into superconducting circuits for quantum technological applications. However, designing gate-controlled nanostructures in monolayer graphene remains a serious challenge due to its lack of a band gap which hinders the confinement of charge carriers. The present thesis aims to address this shortcoming by establishing bilayer graphene as a suitable alternative. Unlike the single-layer relative, bilayer graphene offers the opportunity to open an electronic band gap by breaking the layer symmetry which is possible with the ease of exposing electric displacement fields across the two layers. In this regard, employing the combination of locally defined back and top gate architectures allows to design electrostatically induced nanostructures based on spatial band structure engineering. In this thesis, at first the realization of a gate-tunable charge carrier confinement is presented. The formation of the constriction is demonstrated by means of superconducting magneto-interferometry measurements. Building on the successfully induced electrostatic confinement and in combination with a more sophisticated double top gate structure, a fully operable quantum point contact is implemented within the bilayer graphene weak link. When the junction is measured in the normal state, quantized conductance is observed due to the formation of one-dimensional subbands. Though, unlike in other material systems we here explore the complexity of the degeneracy of spin, valley and unusual mini-valley quantum degrees of freedom. In final measurements, the quantum point contact is probed in the superconducting state. The measured critical current through the junction displays a discrete variation directly correlated to the quantized steps in the normal state conductance. These results pave the way towards the study of individual Andreev bound levels through this superconducting quantum point contact. In conclusion, the presented work demonstrates the implementation of electrostatically tunable superconducting nanostructures in bilayer graphene weak links which serves as a platform for the design of more complex electronic circuits.