Supercurrent in the Quantum Hall Regime: An Examination
This paper presents a significant experimental observation of supercurrent in regimes dominated by quantum Hall (QH) effects, specifically under high magnetic fields. The paper investigates an interaction between superconductivity and QH phenomena in graphene-based Josephson junctions, demonstrating a novel type of supercurrent that extends well into the quantum Hall regime.
Key Observations and Methodology
The researchers used graphene encapsulated in boron nitride with superconducting molybdenum-rhenium alloy contacts. The high-quality heterostructures enabled ballistic transport of charge carriers, evidenced by Fabry-Perot oscillations. The supercurrent demonstrated uniformity in distribution across junction contacts, as indicated by regular Fraunhofer patterns.
Measurements revealed supercurrent persistence in both semiclassical regimes, where cyclotron radii are comparable to junction dimensions, and fully quantized QH regimes, where cyclotron radii are significantly smaller. Notably, supercurrent pockets extended beyond the semiclassical predictions, challenging traditional understandings and indicating unconventional Andreev bound states along QH region perimeters.
Supercurrent Mechanism
The prevailing hypothesis for supercurrent mediation involves chiral hybrid modes. These modes propagate along interfaces between superconducting contacts and the QH region, allowing coupling of spatially separated electron and hole edge states. Such modes form due to Andreev reflections and electron/hole conversions along superconducting boundaries, possibly explaining the apparent crossed Andreev reflections over large scales.
Numerical Results and Implications
The observations emphasized a critical magnetic field periodicity, with the supercurrent oscillating robustly even at elevated fields. The interference patterns exhibited characteristics evocative of phase coherence mediated by hybrid electron-hole states. Temperature dependence studies correlated supercurrent magnitudes with thermal fluctuations, confirming Josephson energy predictions.
This phenomenon opens pathways for creating artificial superconducting hybrids potentially leading to the development of Majorana fermions and parafermions—exotic quasiparticles promising for fault-tolerant quantum computing. Fractional quantum Hall states in graphene, demonstrated at higher fields, are projected to enhance experimental feasibility.
Future Directions
This research lays groundwork for manipulating topological excitations in graphene nanostructures, facilitating their integration into quantum devices. Future development could focus on refining control over edge channels through precise gate applications, fostering advancements in quantum information processing.
Conclusion
The exploration and subsequent demonstration of supercurrent in QH regimes challenge existing paradigms and offer insights into complex interactions of topological phenomena with superconductivity. This work marks a pivotal advancement in the understanding and potential technological application of quantum Hall-superconductor interfaces.
