Superconducting Contacts for Atomically Precise Graphene Nanoribbons

Abstract

This dissertation centers around two topics: graphene nanoribbons (GNRs) and superconductors. The aim of this thesis is to work towards combine these two topics, in order to study how superconducting correlations interact with magnetic correlations within graphene nanoribbons, such as the magnetic edge states present in the zigzag edges of graphene nanoribbons. To introduce superconducting correlations into a graphene nanoribbon, it is important that there is a highly electrically transparent interface between a superconductor and the graphene nanoribbon. This is the main scope of this work.

This work presents research towards using molybdenum rhenium (MoRe) alloy as an electrical contact material for 9 atom wide armchair edge GNRs (9-AGNRs). MoRe electrodes with nanometer-size separations (30 nm and 6 nm) are made and compared with palladium electrodes. Experiments with contacting aerosol gold nanoparticles were performed to confirm that the MoRe electrodes are superconducting and capable of making a clean contact. Beside pure superconducting contacts, Palladium is considered as a contact material, which is made superconducting by the proximity effect. To study the proximity effect, electrical measurements at a base temperature of 30 mK were performed on variable thickness Nb-Au-Nb and Nb-Pd-Nb superconductor-normal metal-superconductor (SNS) junctions made by shadow mask evaporation. A constriction in the Pd layer allows for increasing the junction resistance by feedback-controlled electromigration until a tunnel contact is formed. As the final part of this research, a superconducting diode effect was identified and studied in these SNS junctions.