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Title: Josephson junctions and SQUIDs based on CVD graphene
Author: Li, Tianyi
ISNI:       0000 0004 7965 1630
Awarding Body: UCL (University College London)
Current Institution: University College London (University of London)
Date of Award: 2019
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Josephson junctions and superconducting quantum interference devices (SQUIDs) with graphene as the weak link between the superconductors have been intensely studied in recent years, with respect to both fundamental physics and potential applications. Since the carrier density and resistivity of graphene are heavily dependent on the Fermi level, Josephson junctions and SQUIDs with graphene as the weak link can have their I-V properties easily tuned by the gate voltage. However, most of the previous work on superconductor-graphene-superconductor (SGS) Josephson junctions and SQUIDs was based on mechanically exfoliated graphene, which is not compatible with wafer-scale production. In this project, we have greatly improved the availability and applicability of graphene-based Josephson junctions and SQUIDs. We developed a method to fabricate Josephson junctions and SQUIDs with graphene grown by chemical vapour deposition (CVD) as the weak link. We demonstrate that very short, wide CVD-graphene-based Josephson junctions with Nb electrodes can work without any undesirable hysteresis in the electrical characteristics from 1.5 K down to a base temperature of 320 mK, and the critical current can be effectively tuned by the gate voltage by up to an order of magnitude. As a result, dc SQUIDs made up of these junctions can have their critical current tuned by both the magnetic field and the gate voltage. We also obtained evidence for ballistic transport in SGS junctions as short as 50 nm. We found that even for junction as wide as 80 µm, the critical current shows an ideal Fraunhofer-like interference pattern in a perpendicular magnetic field, indicating the distribution of supercurrent is uniform. We studied the definition of Josephson penetration depth, and proposed a new formula for 2D coplanar junctions.
Supervisor: Romans, E. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available