Li, Tianyi;
(2019)
Josephson Junctions and SQUIDs Based on CVD Graphene.
Doctoral thesis (Ph.D), UCL (University College London).
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Abstract
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.
| Type: | Thesis (Doctoral) |
|---|---|
| Qualification: | Ph.D |
| Title: | Josephson Junctions and SQUIDs Based on CVD Graphene |
| Open access status: | An open access version is available from UCL Discovery |
| Language: | English |
| Additional information: | Copyright © The Author 2019. Original content in this thesis is licensed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0) Licence (https://creativecommons.org/licenses/by/ 4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request. - Some third party copyright material has been removed from this e-thesis. |
| UCL classification: | UCL UCL > Provost and Vice Provost Offices UCL > Provost and Vice Provost Offices > UCL BEAMS UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science |
| URI: | https://discovery.ucl.ac.uk/id/eprint/10076037 |
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