Ballistic Josephson junctions in edge-contacted graphene

Hybrid graphene-superconductor devices have attracted much attention since the early days of graphene research. So far, these studies have been limited to the case of diffusive transport through graphene with poorly defined and modest quality graphene-superconductor interfaces, usually combined with small critical magnetic fields of the superconducting electrodes. Here we report graphene based Josephson junctions with one-dimensional edge contacts of Molybdenum Rhenium. The contacts exhibit a well defined, transparent interface to the graphene, have a critical magnetic field of 8 Tesla at 4 Kelvin and the graphene has a high quality due to its encapsulation in hexagonal boron nitride. This allows us to study and exploit graphene Josephson junctions in a new regime, characterized by ballistic transport. We find that the critical current oscillates with the carrier density due to phase coherent interference of the electrons and holes that carry the supercurrent caused by the formation of a Fabry-P\'{e}rot cavity. Furthermore, relatively large supercurrents are observed over unprecedented long distances of up to 1.5 $\mu$m. Finally, in the quantum Hall regime we observe broken symmetry states while the contacts remain superconducting. These achievements open up new avenues to exploit the Dirac nature of graphene in interaction with the superconducting state.Comment: Updated version after peer review. Includes supplementary material and ancillary file with source code for tight binding simulation

The study of contact properties in edge-contacted graphene-aluminum Josephson junctions

Transparent contact interfaces in superconductor-graphene hybrid systems are critical for realizing superconducting quantum applications. Here, we examine the effect of the edge-contact fabrication process on the transparency of the superconducting aluminum-graphene junction. We show significant improvement in the transparency of our superconductor-graphene junctions by promoting the chemical component of the edge contact etch process. Our results compare favorably with state-of-the-art graphene Josephson junctions. The findings of our study contribute to advancing the fabrication knowledge of edge-contacted superconductor-graphene junctions

van der Waals contact engineering of graphene field-effect transistors for large-area flexible electronics

Graphene has great potential for high-performance flexible electronics. Although studied for more than a decade, contacting graphene efficiently, especially for large-area, flexible electronics, is still a challenge. Here, by engineering the graphene-metal van der Waals (vdW) contact, we demonstrate that ultra-low contact resistance is achievable via a bottom-contact strategy incorporating a simple transfer process without any harsh thermal treatment (>150°C). The majority of the fabricated devices show contact resistances below 200 Ω·µm with values as low as 65 Ω·µm achievable. This is on a par with the state-of-the-art top- and edge-contacted graphene field-effect transistors (GFETs). Further, our study reveals that these contacts, despite the presumed weak nature of the vdW interaction, are stable under various bending conditions, thus guaranteeing compatibility with flexible electronics with improved performance. This work illustrates the potential of the previously underestimated vdW contact approach for large-area flexible electronics

Extraordinary Magnetoresistance in Encapsulated Graphene Devices

We report a study on the phenomenon of extraordinary magnetoresistance (EMR) in boron nitride encapsulated monolayer graphene devices. Extremely large EMR values–calculated as the change in magnetoresistance, (R(B)–R0)/R0–can be found in these devices due to the vanishingly small resistance values at zero field. In many devices the zero-field resistance can become negative, which enables R0 to be chosen arbitrarily close to zero depending only on measurement precision, resulting in very large EMR. We critically discuss the dependence of EMR on measurement precision and device asymmetry. On the other hand, we also find the largest reported values of the sensitivity to magnetic fields, given by the derivative dR/dB. Moreover, the sensitivity measured in a two-probe configuration is over an order of magnitude larger than in the standard four-probe configuration. Additionally, the gate-voltage-dependent resistance at zero field shows a strong electron-hole asymmetry, which we trace to the nature of the metal-graphene edge contact: as in the well-studied case of metals deposited on graphene, the graphene at one-dimensional edge contacts also appears to be heavily electron-doped leading to the appearance of a resistive pn junction in the neighborhood of the central metallic shunt, when the bulk of graphene is gated to be p-type. We also report the effects of the sizes of the devices and the ratios of metallic disk to graphene on the EMR